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title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
ASP.NET Core Blazor event handling	guardrex	Learn about Blazor's event handling features, including event argument types, event callbacks, and managing default browser events.	>= aspnetcore-3.1	riande	mvc	11/08/2022	blazor/components/event-handling

ASP.NET Core Blazor event handling

This article explains Blazor's event handling features, including event argument types, event callbacks, and managing default browser events.

:::moniker range=">= aspnetcore-7.0"

Specify delegate event handlers in Razor component markup with @on{DOM EVENT}="{DELEGATE}" Razor syntax:

The {DOM EVENT} placeholder is a Document Object Model (DOM) event (for example, click).
The {DELEGATE} placeholder is the C# delegate event handler.

For event handling:

Asynchronous delegate event handlers that return a xref:System.Threading.Tasks.Task are supported.
Delegate event handlers automatically trigger a UI render, so there's no need to manually call StateHasChanged.
Exceptions are logged.

The following code:

Calls the UpdateHeading method when the button is selected in the UI.
Calls the CheckChanged method when the checkbox is changed in the UI.

Pages/EventHandlerExample1.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample1.razor" highlight="10,17,27-30,32-35":::

In the following example, UpdateHeading:

Is called asynchronously when the button is selected.
Waits two seconds before updating the heading.

Pages/EventHandlerExample2.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample2.razor" highlight="10,19-24":::

Event arguments

Built-in event arguments

For events that support an event argument type, specifying an event parameter in the event method definition is only necessary if the event type is used in the method. In the following example, xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs is used in the ReportPointerLocation method to set message text that reports the mouse coordinates when the user selects a button in the UI.

Pages/EventHandlerExample3.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample3.razor" highlight="17-20":::

Supported xref:System.EventArgs are shown in the following table.

Event	Class	Document Object Model (DOM) notes
Clipboard	xref:Microsoft.AspNetCore.Components.Web.ClipboardEventArgs
Drag	xref:Microsoft.AspNetCore.Components.Web.DragEventArgs	xref:Microsoft.AspNetCore.Components.Web.DataTransfer and xref:Microsoft.AspNetCore.Components.Web.DataTransferItem hold dragged item data. Implement drag and drop in Blazor apps using JS interop with HTML Drag and Drop API.
Error	xref:Microsoft.AspNetCore.Components.Web.ErrorEventArgs
Event	xref:System.EventArgs	xref:Microsoft.AspNetCore.Components.Web.EventHandlers holds attributes to configure the mappings between event names and event argument types.
Focus	xref:Microsoft.AspNetCore.Components.Web.FocusEventArgs	Doesn't include support for `relatedTarget`.
Input	xref:Microsoft.AspNetCore.Components.ChangeEventArgs
Keyboard	xref:Microsoft.AspNetCore.Components.Web.KeyboardEventArgs
Mouse	xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs
Mouse pointer	xref:Microsoft.AspNetCore.Components.Web.PointerEventArgs
Mouse wheel	xref:Microsoft.AspNetCore.Components.Web.WheelEventArgs
Progress	xref:Microsoft.AspNetCore.Components.Web.ProgressEventArgs
Touch	xref:Microsoft.AspNetCore.Components.Web.TouchEventArgs	xref:Microsoft.AspNetCore.Components.Web.TouchPoint represents a single contact point on a touch-sensitive device.

For more information, see the following resources:

EventArgs classes in the ASP.NET Core reference source (dotnet/aspnetcore main branch)

[!INCLUDE]
xref:Microsoft.AspNetCore.Components.Web.EventHandlers holds attributes to configure the mappings between event names and event argument types.

Custom event arguments

Blazor supports custom event arguments, which enable you to pass arbitrary data to .NET event handlers with custom events.

General configuration

Custom events with custom event arguments are generally enabled with the following steps.

In JavaScript, define a function for building the custom event argument object from the source event:

function eventArgsCreator(event) { 
  return {
    customProperty1: 'any value for property 1',
    customProperty2: event.srcElement.value
  };
}

Register the custom event with the preceding handler in wwwroot/index.html (Blazor WebAssembly) or Pages/_Host.cshtml (Blazor Server) immediately after the Blazor <script>:
```
<script>
  Blazor.registerCustomEventType('customevent', {
    createEventArgs: eventArgsCreator
  });
</script>
```
[!NOTE] The call to registerCustomEventType is performed in a script only once per event.

Define a class for the event arguments:

public class CustomEventArgs : EventArgs
{
    public string? CustomProperty1 {get; set;}
    public string? CustomProperty2 {get; set;}
}

Wire up the custom event with the event arguments by adding an xref:Microsoft.AspNetCore.Components.EventHandlerAttribute attribute annotation for the custom event. The class doesn't require members. Note that the class must be called EventHandlers in order to be found by the Razor compiler, but you should put it in a namespace specific to your app:
```
[EventHandler("oncustomevent", typeof(CustomEventArgs), enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}
```

Register the event handler on one or more HTML elements. Access the data that was passed in from JavaScript in the delegate handler method:

<button @oncustomevent="HandleCustomEvent">Handle</button>

@code
{
    private void HandleCustomEvent(CustomEventArgs eventArgs)
    {
        // eventArgs.CustomProperty1
        // eventArgs.CustomProperty2
    }
}

If the @oncustomevent attribute isn't recognized by IntelliSense, make sure that the component or the _Imports.razor file contains an @using statement for the namespace containing the EventHandler class.

Whenever the custom event is fired on the DOM, the event handler is called with the data passed from the JavaScript.

If you're attempting to fire a custom event, bubbles must be enabled by setting its value to true. Otherwise, the event doesn't reach the Blazor handler for processing into the C# custom xref:Microsoft.AspNetCore.Components.EventHandlerAttribute method. For more information, see MDN Web Docs: Event bubbling.

Custom clipboard paste event example

The following example receives a custom clipboard paste event that includes the time of the paste and the user's pasted text.

Declare a custom name (oncustompaste) for the event and a .NET class (CustomPasteEventArgs) to hold the event arguments for this event:

CustomEvents.cs:

[EventHandler("oncustompaste", typeof(CustomPasteEventArgs), 
    enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}

public class CustomPasteEventArgs : EventArgs
{
    public DateTime EventTimestamp { get; set; }
    public string? PastedData { get; set; }
}

Add JavaScript code to supply data for the xref:System.EventArgs subclass. In the wwwroot/index.html or Pages/_Host.cshtml file, add the following <script> tag and content immediately after the Blazor script. The following example only handles pasting text, but you could use arbitrary JavaScript APIs to deal with users pasting other types of data, such as images.

wwwroot/index.html (Blazor WebAssembly) or Pages/_Host.cshtml (Blazor Server) immediately after the Blazor script:

<script>
  Blazor.registerCustomEventType('custompaste', {
      browserEventName: 'paste',
      createEventArgs: event => {
        return {
          eventTimestamp: new Date(),
          pastedData: event.clipboardData.getData('text')
        };
      }
  });
</script>

The preceding code tells the browser that when a native paste event occurs:

Raise a custompaste event.
Supply the event arguments data using the custom logic stated:
- For the eventTimestamp, create a new date.
- For the pastedData, get the clipboard data as text. For more information, see MDN Web Docs: ClipboardEvent.clipboardData.

Event name conventions differ between .NET and JavaScript:

In .NET, event names are prefixed with "on".
In JavaScript, event names don't have a prefix.

In a Razor component, attach the custom handler to an element.

Pages/CustomPasteArguments.razor:

@page "/custom-paste-arguments"

<label>
    Try pasting into the following text box:
    <input @oncustompaste="HandleCustomPaste" />
</label>

<p>
    @message
</p>

@code {
    private string? message;

    private void HandleCustomPaste(CustomPasteEventArgs eventArgs)
    {
        message = $"At {eventArgs.EventTimestamp.ToShortTimeString()}, " +
            $"you pasted: {eventArgs.PastedData}";
    }
}

Lambda expressions

Lambda expressions are supported as the delegate event handler.

Pages/EventHandlerExample4.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample4.razor" highlight="6":::

It's often convenient to close over additional values using C# method parameters, such as when iterating over a set of elements. The following example creates three buttons, each of which calls UpdateHeading and passes the following data:

An event argument (xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs) in e.
The button number in buttonNumber.

Pages/EventHandlerExample5.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample5.razor" highlight="10,19":::

[!NOTE] Do not use a loop variable directly in a lambda expression, such as i in the preceding for loop example. Otherwise, the same variable is used by all lambda expressions, which results in use of the same value in all lambdas. Always capture the variable's value in a local variable and then use it. In the preceding example:

The loop variable i is assigned to buttonNumber.

buttonNumber is used in the lambda expression.

Creating a large number of event delegates in a loop may cause poor rendering performance. For more information, see xref:blazor/performance#avoid-recreating-delegates-for-many-repeated-elements-or-components.

EventCallback

A common scenario with nested components executes a parent component's method when a child component event occurs. An onclick event occurring in the child component is a common use case. To expose events across components, use an xref:Microsoft.AspNetCore.Components.EventCallback. A parent component can assign a callback method to a child component's xref:Microsoft.AspNetCore.Components.EventCallback.

The following Child component demonstrates how a button's onclick handler is set up to receive an xref:Microsoft.AspNetCore.Components.EventCallback delegate from the sample's ParentComponent. The xref:Microsoft.AspNetCore.Components.EventCallback is typed with MouseEventArgs, which is appropriate for an onclick event from a peripheral device.

Shared/Child.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/event-handling/Child.razor":::

The Parent component sets the child's xref:Microsoft.AspNetCore.Components.EventCallback%601 (OnClickCallback) to its ShowMessage method.

Pages/Parent.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/Parent.razor":::

When the button is selected in the ChildComponent:

The Parent component's ShowMessage method is called. message is updated and displayed in the Parent component.
A call to StateHasChanged isn't required in the callback's method (ShowMessage). xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A is called automatically to rerender the Parent component, just as child events trigger component rerendering in event handlers that execute within the child. For more information, see xref:blazor/components/rendering.

xref:Microsoft.AspNetCore.Components.EventCallback and xref:Microsoft.AspNetCore.Components.EventCallback%601 permit asynchronous delegates. xref:Microsoft.AspNetCore.Components.EventCallback is weakly typed and allows passing any type argument in InvokeAsync(Object). xref:Microsoft.AspNetCore.Components.EventCallback%601 is strongly typed and requires passing a T argument in InvokeAsync(T) that's assignable to TValue.

<ChildComponent 
    OnClickCallback="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

Invoke an xref:Microsoft.AspNetCore.Components.EventCallback or xref:Microsoft.AspNetCore.Components.EventCallback%601 with xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A and await the xref:System.Threading.Tasks.Task:

await OnClickCallback.InvokeAsync(arg);

Use xref:Microsoft.AspNetCore.Components.EventCallback and xref:Microsoft.AspNetCore.Components.EventCallback%601 for event handling and binding component parameters.

Prefer the strongly typed xref:Microsoft.AspNetCore.Components.EventCallback%601 over xref:Microsoft.AspNetCore.Components.EventCallback. xref:Microsoft.AspNetCore.Components.EventCallback%601 provides enhanced error feedback to users of the component. Similar to other UI event handlers, specifying the event parameter is optional. Use xref:Microsoft.AspNetCore.Components.EventCallback when there's no value passed to the callback.

Prevent default actions

Use the @on{DOM EVENT}:preventDefault directive attribute to prevent the default action for an event, where the {DOM EVENT} placeholder is a Document Object Model (DOM) event.

When a key is selected on an input device and the element focus is on a text box, a browser normally displays the key's character in the text box. In the following example, the default behavior is prevented by specifying the @onkeydown:preventDefault directive attribute. When the focus is on the <input> element, the counter increments with the key sequence Shift++. The + character isn't assigned to the <input> element's value. For more information on keydown, see MDN Web Docs: Document: keydown event.

Pages/EventHandlerExample6.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample6.razor" highlight="4":::

Specifying the @on{DOM EVENT}:preventDefault attribute without a value is equivalent to @on{DOM EVENT}:preventDefault="true".

An expression is also a permitted value of the attribute. In the following example, shouldPreventDefault is a bool field set to either true or false:

<input @onkeydown:preventDefault="shouldPreventDefault" />

...

@code {
    private bool shouldPreventDefault = true;
}

Stop event propagation

Use the @on{DOM EVENT}:stopPropagation directive attribute to stop event propagation within the Blazor scope. {DOM EVENT} is a placeholder for a Document Object Model (DOM) event.

The stopPropagation directive attribute's effect is limited to the Blazor scope and doesn't extend to the HTML DOM. Events must propagate to the HTML DOM root before Blazor can act upon them. For a mechanism to prevent HTML DOM event propagation, consider the following approach:

Obtain the event's path by calling Event.composedPath().
Filter events based on the composed event targets (EventTarget).

In the following example, selecting the checkbox prevents click events from the second child <div> from propagating to the parent <div>. Since propagated click events normally fire the OnSelectParentDiv method, selecting the second child <div> results in the parent <div> message appearing unless the checkbox is selected.

Pages/EventHandlerExample7.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample7.razor" highlight="4,15-16":::

Focus an element

Call xref:Microsoft.AspNetCore.Components.ElementReferenceExtensions.FocusAsync%2A on an element reference to focus an element in code. In the following example, select the button to focus the <input> element.

Pages/EventHandlerExample8.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample8.razor" highlight="16":::

:::moniker-end

:::moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

Specify delegate event handlers in Razor component markup with @on{DOM EVENT}="{DELEGATE}" Razor syntax:

The {DOM EVENT} placeholder is a Document Object Model (DOM) event (for example, click).
The {DELEGATE} placeholder is the C# delegate event handler.

For event handling:

Asynchronous delegate event handlers that return a xref:System.Threading.Tasks.Task are supported.
Delegate event handlers automatically trigger a UI render, so there's no need to manually call StateHasChanged.
Exceptions are logged.

The following code:

Calls the UpdateHeading method when the button is selected in the UI.
Calls the CheckChanged method when the checkbox is changed in the UI.

Pages/EventHandlerExample1.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample1.razor" highlight="10,17,27-30,32-35":::

In the following example, UpdateHeading:

Is called asynchronously when the button is selected.
Waits two seconds before updating the heading.

Pages/EventHandlerExample2.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample2.razor" highlight="10,19-24":::

Event arguments

Built-in event arguments

Pages/EventHandlerExample3.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample3.razor" highlight="17-20":::

Supported xref:System.EventArgs are shown in the following table.

Event	Class	Document Object Model (DOM) events and notes
Clipboard	xref:Microsoft.AspNetCore.Components.Web.ClipboardEventArgs	`oncut`, `oncopy`, `onpaste`
Drag	xref:Microsoft.AspNetCore.Components.Web.DragEventArgs	`ondrag`, `ondragstart`, `ondragenter`, `ondragleave`, `ondragover`, `ondrop`, `ondragend` xref:Microsoft.AspNetCore.Components.Web.DataTransfer and xref:Microsoft.AspNetCore.Components.Web.DataTransferItem hold dragged item data. Implement drag and drop in Blazor apps using JS interop with HTML Drag and Drop API.
Error	xref:Microsoft.AspNetCore.Components.Web.ErrorEventArgs	`onerror`
Event	xref:System.EventArgs	General `onactivate`, `onbeforeactivate`, `onbeforedeactivate`, `ondeactivate`, `onfullscreenchange`, `onfullscreenerror`, `onloadeddata`, `onloadedmetadata`, `onpointerlockchange`, `onpointerlockerror`, `onreadystatechange`, `onscroll` Clipboard `onbeforecut`, `onbeforecopy`, `onbeforepaste` Input `oninvalid`, `onreset`, `onselect`, `onselectionchange`, `onselectstart`, `onsubmit` Media `oncanplay`, `oncanplaythrough`, `oncuechange`, `ondurationchange`, `onemptied`, `onended`, `onpause`, `onplay`, `onplaying`, `onratechange`, `onseeked`, `onseeking`, `onstalled`, `onstop`, `onsuspend`, `ontimeupdate`, `ontoggle`, `onvolumechange`, `onwaiting` xref:Microsoft.AspNetCore.Components.Web.EventHandlers holds attributes to configure the mappings between event names and event argument types.
Focus	xref:Microsoft.AspNetCore.Components.Web.FocusEventArgs	`onfocus`, `onblur`, `onfocusin`, `onfocusout` Doesn't include support for `relatedTarget`.
Input	xref:Microsoft.AspNetCore.Components.ChangeEventArgs	`onchange`, `oninput`
Keyboard	xref:Microsoft.AspNetCore.Components.Web.KeyboardEventArgs	`onkeydown`, `onkeypress`, `onkeyup`
Mouse	xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs	`onclick`, `oncontextmenu`, `ondblclick`, `onmousedown`, `onmouseup`, `onmouseover`, `onmousemove`, `onmouseout`
Mouse pointer	xref:Microsoft.AspNetCore.Components.Web.PointerEventArgs	`onpointerdown`, `onpointerup`, `onpointercancel`, `onpointermove`, `onpointerover`, `onpointerout`, `onpointerenter`, `onpointerleave`, `ongotpointercapture`, `onlostpointercapture`
Mouse wheel	xref:Microsoft.AspNetCore.Components.Web.WheelEventArgs	`onwheel`, `onmousewheel`
Progress	xref:Microsoft.AspNetCore.Components.Web.ProgressEventArgs	`onabort`, `onload`, `onloadend`, `onloadstart`, `onprogress`, `ontimeout`
Touch	xref:Microsoft.AspNetCore.Components.Web.TouchEventArgs	`ontouchstart`, `ontouchend`, `ontouchmove`, `ontouchenter`, `ontouchleave`, `ontouchcancel` xref:Microsoft.AspNetCore.Components.Web.TouchPoint represents a single contact point on a touch-sensitive device.

For more information, see EventArgs classes in the ASP.NET Core reference source (dotnet/aspnetcore main branch).

[!INCLUDE]

Custom event arguments

Blazor supports custom event arguments, which enable you to pass arbitrary data to .NET event handlers with custom events.

General configuration

Custom events with custom event arguments are generally enabled with the following steps.

In JavaScript, define a function for building the custom event argument object from the source event:

function eventArgsCreator(event) { 
  return {
    customProperty1: 'any value for property 1',
    customProperty2: event.srcElement.value
  };
}

Register the custom event with the preceding handler in wwwroot/index.html (Blazor WebAssembly) or Pages/_Layout.cshtml (Blazor Server) immediately after the Blazor <script>:
```
<script>
  Blazor.registerCustomEventType('customevent', {
    createEventArgs: eventArgsCreator
  });
</script>
```
[!NOTE] The call to registerCustomEventType is performed in a script only once per event.

Define a class for the event arguments:

public class CustomEventArgs : EventArgs
{
    public string? CustomProperty1 {get; set;}
    public string? CustomProperty2 {get; set;}
}

Wire up the custom event with the event arguments by adding an xref:Microsoft.AspNetCore.Components.EventHandlerAttribute attribute annotation for the custom event. The class doesn't require members. Note that the class must be called EventHandlers in order to be found by the Razor compiler, but you should put it in a namespace specific to your app:
```
[EventHandler("oncustomevent", typeof(CustomEventArgs), enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}
```

Register the event handler on one or more HTML elements. Access the data that was passed in from JavaScript in the delegate handler method:

<button @oncustomevent="HandleCustomEvent">Handle</button>

@code
{
    private void HandleCustomEvent(CustomEventArgs eventArgs)
    {
        // eventArgs.CustomProperty1
        // eventArgs.CustomProperty2
    }
}

Whenever the custom event is fired on the DOM, the event handler is called with the data passed from the JavaScript.

Custom clipboard paste event example

The following example receives a custom clipboard paste event that includes the time of the paste and the user's pasted text.

Declare a custom name (oncustompaste) for the event and a .NET class (CustomPasteEventArgs) to hold the event arguments for this event:

CustomEvents.cs:

[EventHandler("oncustompaste", typeof(CustomPasteEventArgs), 
    enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}

public class CustomPasteEventArgs : EventArgs
{
    public DateTime EventTimestamp { get; set; }
    public string? PastedData { get; set; }
}

Add JavaScript code to supply data for the xref:System.EventArgs subclass. In the wwwroot/index.html or Pages/_Layout.cshtml file, add the following <script> tag and content immediately after the Blazor script. The following example only handles pasting text, but you could use arbitrary JavaScript APIs to deal with users pasting other types of data, such as images.

wwwroot/index.html (Blazor WebAssembly) or Pages/_Layout.cshtml (Blazor Server) immediately after the Blazor script:

<script>
  Blazor.registerCustomEventType('custompaste', {
      browserEventName: 'paste',
      createEventArgs: event => {
        return {
          eventTimestamp: new Date(),
          pastedData: event.clipboardData.getData('text')
        };
      }
  });
</script>

The preceding code tells the browser that when a native paste event occurs:

Raise a custompaste event.
Supply the event arguments data using the custom logic stated:
- For the eventTimestamp, create a new date.
- For the pastedData, get the clipboard data as text. For more information, see MDN Web Docs: ClipboardEvent.clipboardData.

Event name conventions differ between .NET and JavaScript:

In .NET, event names are prefixed with "on".
In JavaScript, event names don't have a prefix.

In a Razor component, attach the custom handler to an element.

Pages/CustomPasteArguments.razor:

@page "/custom-paste-arguments"

<label>
    Try pasting into the following text box:
    <input @oncustompaste="HandleCustomPaste" />
</label>

<p>
    @message
</p>

@code {
    private string? message;

    private void HandleCustomPaste(CustomPasteEventArgs eventArgs)
    {
        message = $"At {eventArgs.EventTimestamp.ToShortTimeString()}, " +
            $"you pasted: {eventArgs.PastedData}";
    }
}

Lambda expressions

Lambda expressions are supported as the delegate event handler.

Pages/EventHandlerExample4.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample4.razor" highlight="6":::

An event argument (xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs) in e.
The button number in buttonNumber.

Pages/EventHandlerExample5.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample5.razor" highlight="10,19":::

[!NOTE] Do not use a loop variable directly in a lambda expression, such as i in the preceding for loop example. Otherwise, the same variable is used by all lambda expressions, which results in use of the same value in all lambdas. Always capture the variable's value in a local variable and then use it. In the preceding example:

The loop variable i is assigned to buttonNumber.

buttonNumber is used in the lambda expression.

EventCallback

Shared/Child.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/event-handling/Child.razor":::

The Parent component sets the child's xref:Microsoft.AspNetCore.Components.EventCallback%601 (OnClickCallback) to its ShowMessage method.

Pages/Parent.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/Parent.razor":::

When the button is selected in the ChildComponent:

The Parent component's ShowMessage method is called. message is updated and displayed in the Parent component.
A call to StateHasChanged isn't required in the callback's method (ShowMessage). xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A is called automatically to rerender the Parent component, just as child events trigger component rerendering in event handlers that execute within the child. For more information, see xref:blazor/components/rendering.

<ChildComponent 
    OnClickCallback="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

await OnClickCallback.InvokeAsync(arg);

Use xref:Microsoft.AspNetCore.Components.EventCallback and xref:Microsoft.AspNetCore.Components.EventCallback%601 for event handling and binding component parameters.

Prevent default actions

Use the @on{DOM EVENT}:preventDefault directive attribute to prevent the default action for an event, where the {DOM EVENT} placeholder is a Document Object Model (DOM) event.

Pages/EventHandlerExample6.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample6.razor" highlight="4":::

Specifying the @on{DOM EVENT}:preventDefault attribute without a value is equivalent to @on{DOM EVENT}:preventDefault="true".

An expression is also a permitted value of the attribute. In the following example, shouldPreventDefault is a bool field set to either true or false:

<input @onkeydown:preventDefault="shouldPreventDefault" />

...

@code {
    private bool shouldPreventDefault = true;
}

Stop event propagation

Use the @on{DOM EVENT}:stopPropagation directive attribute to stop event propagation within the Blazor scope. {DOM EVENT} is a placeholder for a Document Object Model (DOM) event.

Obtain the event's path by calling Event.composedPath().
Filter events based on the composed event targets (EventTarget).

Pages/EventHandlerExample7.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample7.razor" highlight="4,15-16":::

Focus an element

Pages/EventHandlerExample8.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample8.razor" highlight="16":::

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

Specify delegate event handlers in Razor component markup with @on{DOM EVENT}="{DELEGATE}" Razor syntax:

The {DOM EVENT} placeholder is a Document Object Model (DOM) event (for example, click).
The {DELEGATE} placeholder is the C# delegate event handler.

For event handling:

Asynchronous delegate event handlers that return a xref:System.Threading.Tasks.Task are supported.
Delegate event handlers automatically trigger a UI render, so there's no need to manually call StateHasChanged.
Exceptions are logged.

The following code:

Calls the UpdateHeading method when the button is selected in the UI.
Calls the CheckChanged method when the checkbox is changed in the UI.

Pages/EventHandlerExample1.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample1.razor" highlight="10,17,27-30,32-35":::

In the following example, UpdateHeading:

Is called asynchronously when the button is selected.
Waits two seconds before updating the heading.

Pages/EventHandlerExample2.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample2.razor" highlight="10,19-24":::

Event arguments

Pages/EventHandlerExample3.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample3.razor" highlight="17-20":::

Supported xref:System.EventArgs are shown in the following table.

Event	Class	Document Object Model (DOM) events and notes
Clipboard	xref:Microsoft.AspNetCore.Components.Web.ClipboardEventArgs	`oncut`, `oncopy`, `onpaste`
Drag	xref:Microsoft.AspNetCore.Components.Web.DragEventArgs	`ondrag`, `ondragstart`, `ondragenter`, `ondragleave`, `ondragover`, `ondrop`, `ondragend` xref:Microsoft.AspNetCore.Components.Web.DataTransfer and xref:Microsoft.AspNetCore.Components.Web.DataTransferItem hold dragged item data. Implement drag and drop in Blazor apps using JS interop with HTML Drag and Drop API.
Error	xref:Microsoft.AspNetCore.Components.Web.ErrorEventArgs	`onerror`
Event	xref:System.EventArgs	General `onactivate`, `onbeforeactivate`, `onbeforedeactivate`, `ondeactivate`, `onfullscreenchange`, `onfullscreenerror`, `onloadeddata`, `onloadedmetadata`, `onpointerlockchange`, `onpointerlockerror`, `onreadystatechange`, `onscroll` Clipboard `onbeforecut`, `onbeforecopy`, `onbeforepaste` Input `oninvalid`, `onreset`, `onselect`, `onselectionchange`, `onselectstart`, `onsubmit` Media `oncanplay`, `oncanplaythrough`, `oncuechange`, `ondurationchange`, `onemptied`, `onended`, `onpause`, `onplay`, `onplaying`, `onratechange`, `onseeked`, `onseeking`, `onstalled`, `onstop`, `onsuspend`, `ontimeupdate`, `ontoggle`, `onvolumechange`, `onwaiting` xref:Microsoft.AspNetCore.Components.Web.EventHandlers holds attributes to configure the mappings between event names and event argument types.
Focus	xref:Microsoft.AspNetCore.Components.Web.FocusEventArgs	`onfocus`, `onblur`, `onfocusin`, `onfocusout` Doesn't include support for `relatedTarget`.
Input	xref:Microsoft.AspNetCore.Components.ChangeEventArgs	`onchange`, `oninput`
Keyboard	xref:Microsoft.AspNetCore.Components.Web.KeyboardEventArgs	`onkeydown`, `onkeypress`, `onkeyup`
Mouse	xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs	`onclick`, `oncontextmenu`, `ondblclick`, `onmousedown`, `onmouseup`, `onmouseover`, `onmousemove`, `onmouseout`
Mouse pointer	xref:Microsoft.AspNetCore.Components.Web.PointerEventArgs	`onpointerdown`, `onpointerup`, `onpointercancel`, `onpointermove`, `onpointerover`, `onpointerout`, `onpointerenter`, `onpointerleave`, `ongotpointercapture`, `onlostpointercapture`
Mouse wheel	xref:Microsoft.AspNetCore.Components.Web.WheelEventArgs	`onwheel`, `onmousewheel`
Progress	xref:Microsoft.AspNetCore.Components.Web.ProgressEventArgs	`onabort`, `onload`, `onloadend`, `onloadstart`, `onprogress`, `ontimeout`
Touch	xref:Microsoft.AspNetCore.Components.Web.TouchEventArgs	`ontouchstart`, `ontouchend`, `ontouchmove`, `ontouchenter`, `ontouchleave`, `ontouchcancel` xref:Microsoft.AspNetCore.Components.Web.TouchPoint represents a single contact point on a touch-sensitive device.

For more information, see EventArgs classes in the ASP.NET Core reference source (dotnet/aspnetcore main branch)

[!INCLUDE]

Lambda expressions

Lambda expressions are supported as the delegate event handler.

Pages/EventHandlerExample4.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample4.razor" highlight="6":::

An event argument (xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs) in e.
The button number in buttonNumber.

Pages/EventHandlerExample5.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample5.razor" highlight="10,19":::

[!NOTE] Do not use a loop variable directly in a lambda expression, such as i in the preceding for loop example. Otherwise, the same variable is used by all lambda expressions, which results in use of the same value in all lambdas. Always capture the variable's value in a local variable and then use it. In the preceding example:

The loop variable i is assigned to buttonNumber.

buttonNumber is used in the lambda expression.

EventCallback

Shared/Child.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/event-handling/Child.razor":::

The Parent component sets the child's xref:Microsoft.AspNetCore.Components.EventCallback%601 (OnClickCallback) to its ShowMessage method.

Pages/Parent.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/Parent.razor":::

When the button is selected in the ChildComponent:

The Parent component's ShowMessage method is called. message is updated and displayed in the Parent component.
A call to StateHasChanged isn't required in the callback's method (ShowMessage). xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A is called automatically to rerender the Parent component, just as child events trigger component rerendering in event handlers that execute within the child. For more information, see xref:blazor/components/rendering.

<ChildComponent 
    OnClickCallback="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

await OnClickCallback.InvokeAsync(arg);

Use xref:Microsoft.AspNetCore.Components.EventCallback and xref:Microsoft.AspNetCore.Components.EventCallback%601 for event handling and binding component parameters.

Prevent default actions

Use the @on{DOM EVENT}:preventDefault directive attribute to prevent the default action for an event, where the {DOM EVENT} placeholder is a Document Object Model (DOM) event.

Pages/EventHandlerExample6.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample6.razor" highlight="4":::

Specifying the @on{DOM EVENT}:preventDefault attribute without a value is equivalent to @on{DOM EVENT}:preventDefault="true".

An expression is also a permitted value of the attribute. In the following example, shouldPreventDefault is a bool field set to either true or false:

<input @onkeydown:preventDefault="shouldPreventDefault" />

...

@code {
    private bool shouldPreventDefault = true;
}

Stop event propagation

Use the @on{DOM EVENT}:stopPropagation directive attribute to stop event propagation within the Blazor scope. {DOM EVENT} is a placeholder for a Document Object Model (DOM) event.

Obtain the event's path by calling Event.composedPath().
Filter events based on the composed event targets (EventTarget).

Pages/EventHandlerExample7.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample7.razor" highlight="4,15-16":::

Focus an element

Pages/EventHandlerExample8.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample8.razor" highlight="16":::

:::moniker-end

:::moniker range="< aspnetcore-5.0"

Specify delegate event handlers in Razor component markup with @on{DOM EVENT}="{DELEGATE}" Razor syntax:

The {DOM EVENT} placeholder is a Document Object Model (DOM) event (for example, click).
The {DELEGATE} placeholder is the C# delegate event handler.

For event handling:

Asynchronous delegate event handlers that return a xref:System.Threading.Tasks.Task are supported.
Delegate event handlers automatically trigger a UI render, so there's no need to manually call StateHasChanged.
Exceptions are logged.

The following code:

Calls the UpdateHeading method when the button is selected in the UI.
Calls the CheckChanged method when the checkbox is changed in the UI.

Pages/EventHandlerExample1.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample1.razor" highlight="10,17,27-30,32-35":::

In the following example, UpdateHeading:

Is called asynchronously when the button is selected.
Waits two seconds before updating the heading.

Pages/EventHandlerExample2.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample2.razor" highlight="10,19-24":::

Event arguments

Pages/EventHandlerExample3.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample3.razor" highlight="17-20":::

Supported xref:System.EventArgs are shown in the following table.

Event	Class	Document Object Model (DOM) events and notes
Clipboard	xref:Microsoft.AspNetCore.Components.Web.ClipboardEventArgs	`oncut`, `oncopy`, `onpaste`
Drag	xref:Microsoft.AspNetCore.Components.Web.DragEventArgs	`ondrag`, `ondragstart`, `ondragenter`, `ondragleave`, `ondragover`, `ondrop`, `ondragend` xref:Microsoft.AspNetCore.Components.Web.DataTransfer and xref:Microsoft.AspNetCore.Components.Web.DataTransferItem hold dragged item data. Implement drag and drop in Blazor apps using JS interop with HTML Drag and Drop API.
Error	xref:Microsoft.AspNetCore.Components.Web.ErrorEventArgs	`onerror`
Event	xref:System.EventArgs	General `onactivate`, `onbeforeactivate`, `onbeforedeactivate`, `ondeactivate`, `onfullscreenchange`, `onfullscreenerror`, `onloadeddata`, `onloadedmetadata`, `onpointerlockchange`, `onpointerlockerror`, `onreadystatechange`, `onscroll` Clipboard `onbeforecut`, `onbeforecopy`, `onbeforepaste` Input `oninvalid`, `onreset`, `onselect`, `onselectionchange`, `onselectstart`, `onsubmit` Media `oncanplay`, `oncanplaythrough`, `oncuechange`, `ondurationchange`, `onemptied`, `onended`, `onpause`, `onplay`, `onplaying`, `onratechange`, `onseeked`, `onseeking`, `onstalled`, `onstop`, `onsuspend`, `ontimeupdate`, `onvolumechange`, `onwaiting` xref:Microsoft.AspNetCore.Components.Web.EventHandlers holds attributes to configure the mappings between event names and event argument types.
Focus	xref:Microsoft.AspNetCore.Components.Web.FocusEventArgs	`onfocus`, `onblur`, `onfocusin`, `onfocusout` Doesn't include support for `relatedTarget`.
Input	xref:Microsoft.AspNetCore.Components.ChangeEventArgs	`onchange`, `oninput`
Keyboard	xref:Microsoft.AspNetCore.Components.Web.KeyboardEventArgs	`onkeydown`, `onkeypress`, `onkeyup`
Mouse	xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs	`onclick`, `oncontextmenu`, `ondblclick`, `onmousedown`, `onmouseup`, `onmouseover`, `onmousemove`, `onmouseout`
Mouse pointer	xref:Microsoft.AspNetCore.Components.Web.PointerEventArgs	`onpointerdown`, `onpointerup`, `onpointercancel`, `onpointermove`, `onpointerover`, `onpointerout`, `onpointerenter`, `onpointerleave`, `ongotpointercapture`, `onlostpointercapture`
Mouse wheel	xref:Microsoft.AspNetCore.Components.Web.WheelEventArgs	`onwheel`, `onmousewheel`
Progress	xref:Microsoft.AspNetCore.Components.Web.ProgressEventArgs	`onabort`, `onload`, `onloadend`, `onloadstart`, `onprogress`, `ontimeout`
Touch	xref:Microsoft.AspNetCore.Components.Web.TouchEventArgs	`ontouchstart`, `ontouchend`, `ontouchmove`, `ontouchenter`, `ontouchleave`, `ontouchcancel` xref:Microsoft.AspNetCore.Components.Web.TouchPoint represents a single contact point on a touch-sensitive device.

For more information, see EventArgs classes in the ASP.NET Core reference source (dotnet/aspnetcore main branch)

[!INCLUDE]

Lambda expressions

Lambda expressions are supported as the delegate event handler.

Pages/EventHandlerExample4.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample4.razor" highlight="6":::

An event argument (xref:Microsoft.AspNetCore.Components.Web.MouseEventArgs) in e.
The button number in buttonNumber.

Pages/EventHandlerExample5.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample5.razor" highlight="10,19":::

[!NOTE] Do not use a loop variable directly in a lambda expression, such as i in the preceding for loop example. Otherwise, the same variable is used by all lambda expressions, which results in use of the same value in all lambdas. Always capture the variable's value in a local variable and then use it. In the preceding example:

The loop variable i is assigned to buttonNumber.

buttonNumber is used in the lambda expression.

EventCallback

Shared/Child.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/event-handling/Child.razor":::

The Parent component sets the child's xref:Microsoft.AspNetCore.Components.EventCallback%601 (OnClickCallback) to its ShowMessage method.

Pages/Parent.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/Parent.razor":::

When the button is selected in the ChildComponent:

The Parent component's ShowMessage method is called. message is updated and displayed in the Parent component.
A call to StateHasChanged isn't required in the callback's method (ShowMessage). xref:Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged%2A is called automatically to rerender the Parent component, just as child events trigger component rerendering in event handlers that execute within the child. For more information, see xref:blazor/components/rendering.

<ChildComponent 
    OnClickCallback="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

await OnClickCallback.InvokeAsync(arg);

Use xref:Microsoft.AspNetCore.Components.EventCallback and xref:Microsoft.AspNetCore.Components.EventCallback%601 for event handling and binding component parameters.

Prevent default actions

Use the @on{DOM EVENT}:preventDefault directive attribute to prevent the default action for an event, where the {DOM EVENT} placeholder is a Document Object Model (DOM) event.

Pages/EventHandlerExample6.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample6.razor" highlight="4":::

Specifying the @on{DOM EVENT}:preventDefault attribute without a value is equivalent to @on{DOM EVENT}:preventDefault="true".

An expression is also a permitted value of the attribute. In the following example, shouldPreventDefault is a bool field set to either true or false:

<input @onkeydown:preventDefault="shouldPreventDefault" />

...

@code {
    private bool shouldPreventDefault = true;
}

Stop event propagation

Use the @on{DOM EVENT}:stopPropagation directive attribute to stop event propagation within the Blazor scope. {DOM EVENT} is a placeholder for a Document Object Model (DOM) event.

Obtain the event's path by calling Event.composedPath().
Filter events based on the composed event targets (EventTarget).

Pages/EventHandlerExample7.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/event-handling/EventHandlerExample7.razor" highlight="4,15-16":::

:::moniker-end

64 KiB Raw Blame History

ASP.NET Core Blazor event handling

Event arguments

Built-in event arguments

Custom event arguments

General configuration

Custom clipboard paste event example

Lambda expressions

EventCallback

Prevent default actions

Stop event propagation

Focus an element

Event arguments

Built-in event arguments

Custom event arguments

General configuration

Custom clipboard paste event example

Lambda expressions

EventCallback

Prevent default actions

Stop event propagation

Focus an element

Event arguments

Lambda expressions

EventCallback

Prevent default actions

Stop event propagation

Focus an element

Event arguments

Lambda expressions

EventCallback

Prevent default actions

Stop event propagation

64 KiB

Raw Blame History