--- title: Build your first Blazor app author: guardrex description: Build a Blazor app step-by-step. monikerRange: '>= aspnetcore-3.0' ms.author: riande ms.custom: mvc ms.date: 05/19/2019 uid: tutorials/first-blazor-app --- # Build your first Blazor app By [Daniel Roth](https://github.com/danroth27) and [Luke Latham](https://github.com/guardrex) This tutorial shows you how to build and modify a Blazor app. Follow the guidance in the article to create a Blazor project for this tutorial. ## Build components 1. Browse to each of the app's three pages in the *Pages* folder: Home, Counter, and Fetch data. These pages are implemented by the Razor component files *Index.razor*, *Counter.razor*, and *FetchData.razor*. 1. On the Counter page, select the **Click me** button to increment the counter without a page refresh. Incrementing a counter in a webpage normally requires writing JavaScript, but Blazor provides a better approach using C#. 1. Examine the implementation of the Counter component in the *Counter.razor* file. *Pages/Counter.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Counter1.razor)] The UI of the Counter component is defined using HTML. Dynamic rendering logic (for example, loops, conditionals, expressions) is added using an embedded C# syntax called [Razor](xref:mvc/views/razor). The HTML markup and C# rendering logic are converted into a component class at build time. The name of the generated .NET class matches the file name. Members of the component class are defined in an `@functions` block. In the `@functions` block, component state (properties, fields) and methods are specified for event handling or for defining other component logic. These members are then used as part of the component's rendering logic and for handling events. When the **Click me** button is selected: * The Counter component's registered `onclick` handler is called (the `IncrementCount` method). * The Counter component regenerates its render tree. * The new render tree is compared to the previous one. * Only modifications to the Document Object Model (DOM) are applied. The displayed count is updated. 1. Modify the C# logic of the Counter component to make the count increment by two instead of one. [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Counter2.razor?highlight=14)] 1. Rebuild and run the app to see the changes. Select the **Click me** button. The counter increments by two. ## Use components Include a component in another component using an HTML syntax. 1. Add the Counter component to the app's Index component by adding a `` element to the Index component (*Index.razor*). If you're using Blazor client-side for this experience, a Survey Prompt component (`` element) is in the Index component. Replace the `` element with the `` element. If you're using a Blazor server-side app for this experience, add the `` element to the Index component: *Pages/Index.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Index1.razor?highlight=7)] 1. Rebuild and run the app. The Index component has its own counter. ## Component parameters Components can also have parameters. Component parameters are defined using non-public properties on the component class decorated with `[Parameter]`. Use attributes to specify arguments for a component in markup. 1. Update the component's `@functions` C# code: * Add a `IncrementAmount` property decorated with the `[Parameter]` attribute. * Change the `IncrementCount` method to use the `IncrementAmount` when increasing the value of `currentCount`. *Pages/Counter.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Counter.razor?highlight=13,17)] 1. Specify an `IncrementAmount` parameter in the Index component's `` element using an attribute. Set the value to increment the counter by ten. *Pages/Index.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Index2.razor?highlight=7)] 1. Reload the Index component. The counter increments by ten each time the **Click me** button is selected. The counter in the Counter component continues to increment by one. ## Route to components The `@page` directive at the top of the *Counter.razor* file specifies that the Counter component is a routing endpoint. The Counter component handles requests sent to `/counter`. Without the `@page` directive, a component doesn't handle routed requests, but the component can still be used by other components. ## Dependency injection Services registered in the app's service container are available to components via [dependency injection (DI)](xref:fundamentals/dependency-injection). Inject services into a component using the `@inject` directive. Examine the directives of the FetchData component. If working with a Blazor server-side app, the `WeatherForecastService` service is registered as a [singleton](xref:fundamentals/dependency-injection#service-lifetimes), so one instance of the service is available throughout the app. The `@inject` directive is used to inject the instance of the `WeatherForecastService` service into the component. *Pages/FetchData.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/FetchData1.razor?highlight=3)] The FetchData component uses the injected service, as `ForecastService`, to retrieve an array of `WeatherForecast` objects: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/FetchData2.razor?highlight=6)] If working with a Blazor client-side app, `HttpClient` is injected to obtain weather forecast data from the *weather.json* file in the *wwwroot/sample-data* folder: *Pages/FetchData.razor*: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/FetchData1_client.razor?highlight=7-8)] A [\@foreach](/dotnet/csharp/language-reference/keywords/foreach-in) loop is used to render each forecast instance as a row in the table of weather data: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/FetchData3.razor?highlight=11-19)] ## Build a todo list Add a new component to the app that implements a simple todo list. 1. Add an empty file named *Todo.razor* to the app in the *Pages* folder: 1. Provide the initial markup for the component: ```cshtml @page "/todo"

Todo

``` 1. Add the Todo component to the navigation bar. The NavMenu component (*Shared/NavMenu.razor*) is used in the app's layout. Layouts are components that allow you to avoid duplication of content in the app. For more information, see . Add a `` for the Todo component by adding the following list item markup below the existing list items in the *Shared/NavMenu.razor* file: ```cshtml
  • Todo
    • ``` 1. Rebuild and run the app. Visit the new Todo page to confirm that the link to the Todo component works. 1. If building a Blazor server-side app, add the app's namespace to the *\_Imports.razor* file. The following `@using` statement assumes that the app's namespace is `WebApplication`: ```cshtml @using WebApplication ``` Blazor client-side apps include the app's namespace by default in the *\_Imports.razor* file. 1. Add a *TodoItem.cs* file to the root of the project to hold a class that represents a todo item. Use the following C# code for the `TodoItem` class: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/TodoItem.cs)] 1. Return to the Todo component (*Pages/Todo.razor*): * Add a field for the todo items in an `@functions` block. The Todo component uses this field to maintain the state of the todo list. * Add unordered list markup and a `foreach` loop to render each todo item as a list item. [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo4.razor?highlight=5-10,12-14)] 1. The app requires UI elements for adding todo items to the list. Add a text input and a button below the list: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo5.razor?highlight=12-13)] 1. Rebuild and run the app. When the **Add todo** button is selected, nothing happens because an event handler isn't wired up to the button. 1. Add an `AddTodo` method to the Todo component and register it for button clicks using the `onclick` attribute: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo6.razor?highlight=2,7-10)] The `AddTodo` C# method is called when the button is selected. 1. To get the title of the new todo item, add a `newTodo` string field and bind it to the value of the text input using the `bind` attribute: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo7.razor?highlight=2)] ```cshtml ``` 1. Update the `AddTodo` method to add the `TodoItem` with the specified title to the list. Clear the value of the text input by setting `newTodo` to an empty string: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo8.razor?highlight=19-26)] 1. Rebuild and run the app. Add some todo items to the todo list to test the new code. 1. The title text for each todo item can be made editable and a check box can help the user keep track of completed items. Add a check box input for each todo item and bind its value to the `IsDone` property. Change `@todo.Title` to an `` element bound to `@todo.Title`: [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/ToDo9.razor?highlight=5-6)] 1. To verify that these values are bound, update the `

    ` header to show a count of the number of todo items that aren't complete (`IsDone` is `false`). ```cshtml

    Todo (@todos.Count(todo => !todo.IsDone))

    ``` 1. The completed Todo component (*Pages/Todo.razor*): [!code-cshtml[](build-your-first-blazor-app/samples_snapshot/3.x/Todo.razor)] 1. Rebuild and run the app. Add todo items to test the new code. ## Publish and deploy the app To publish the app, see .