AspNetCore.Docs/aspnetcore/blazor/call-web-api.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	no-loc	uid	zone_pivot_groups
Call a web API in an ASP.NET Core Blazor app	guardrex	Learn how to call a web API in Blazor apps.	>= aspnetcore-3.1	riande	mvc	06/03/2021	Home Privacy Kestrel appsettings.json ASP.NET Core Identity cookie Cookie Blazor Blazor Server Blazor WebAssembly Identity Let's Encrypt Razor SignalR	blazor/call-web-api	blazor-hosting-models

Call a web API from ASP.NET Core Blazor

::: zone pivot="webassembly"

Blazor WebAssembly apps call web APIs using a preconfigured xref:System.Net.Http.HttpClient service, which is focused on making requests back to the server of origin. Additional xref:System.Net.Http.HttpClient service configurations for other web APIs can be created in developer code. Requests are composed using Blazor JSON helpers or with xref:System.Net.Http.HttpRequestMessage. Requests can include Fetch API option configuration.

Examples in this article

In this article's component examples, a hypothetical todo list web API is used to create, read, update, and delete (CRUD) todo items on a server. The examples are based on a TodoItem class that stores the following todo item data:

• ID (Id, long): Unique ID of the item.
• Name (Name, string): Name of the item.
• Status (IsComplete, bool): Indication if the todo item is finished.

Use the following TodoItem class with this article's examples if you build the examples into a test app:

public class TodoItem
{
    public long Id { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
}

For guidance on how to create a server-side web API, see xref:tutorials/first-web-api. For information on Cross-origin resource sharing (CORS), see the CORS guidance later in this article.

Packages

Reference the System.Net.Http.Json NuGet package in the project file.

Add the HttpClient service

In Program.Main, add an xref:System.Net.Http.HttpClient service if it isn't already present from a Blazor project template used to create the app:

builder.Services.AddScoped(sp => 
    new HttpClient
    {
        BaseAddress = new Uri(builder.HostEnvironment.BaseAddress)
    });

HttpClient and JSON helpers

xref:System.Net.Http.HttpClient is available as a preconfigured service for making requests back to the origin server.

xref:System.Net.Http.HttpClient and JSON helpers (xref:System.Net.Http.Json.HttpClientJsonExtensions?displayProperty=nameWithType) are also used to call third-party web API endpoints. xref:System.Net.Http.HttpClient is implemented using the browser's Fetch API and is subject to its limitations, including enforcement of the same-origin policy (discussed later in this article).

The client's base address is set to the originating server's address. Inject an xref:System.Net.Http.HttpClient instance into a component using the @inject directive:

@using System.Net.Http
@inject HttpClient Http

Use the xref:System.Net.Http.Json?displayProperty=fullName namespace for access to xref:System.Net.Http.Json.HttpClientJsonExtensions, including xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A, xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A, and xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A:

@using System.Net.Http.Json

GET from JSON (GetFromJsonAsync)

xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A sends an HTTP GET request and parses the JSON response body to create an object.

In the following component code, the todoItems are displayed by the component. xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A is called when the component is finished initializing (OnInitializedAsync).

@using System.Net.Http
@using System.Net.Http.Json
@using System.Threading.Tasks
@inject HttpClient Http

@if (todoItems == null)
{
    <p>No Todo Items found.</p>
}
else
{
    <ul>
        @foreach (var item in todoItems)
        {
            <li>@item.Name</li>
        }
    </ul>
}

@code {
    private TodoItem[] todoItems;

    protected override async Task OnInitializedAsync() => 
        todoItems = await Http.GetFromJsonAsync<TodoItem[]>("api/TodoItems");
}

POST as JSON (PostAsJsonAsync)

xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A sends a POST request to the specified URI containing the value serialized as JSON in the request body.

In the following component code, newItemName is provided by a bound element of the component. The AddItem method is triggered by selecting a <button> element.

@using System.Net.Http
@using System.Net.Http.Json
@using System.Threading.Tasks
@inject HttpClient Http

<input @bind="newItemName" placeholder="New Todo Item" />
<button @onclick="AddItem">Add</button>

@code {
    private string newItemName;

    private async Task AddItem()
    {
        var addItem = new TodoItem { Name = newItemName, IsComplete = false };
        await Http.PostAsJsonAsync("api/TodoItems", addItem);
    }
}

Calls to xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A return an xref:System.Net.Http.HttpResponseMessage. To deserialize the JSON content from the response message, use the xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A extension method. The following example reads JSON weather data:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast>();

PUT as JSON (PutAsJsonAsync)

xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A sends an HTTP PUT request with JSON-encoded content.

In the following component code, editItem values for Name and IsCompleted are provided by bound elements of the component. The item's Id is set when the item is selected in another part of the UI (not shown) and EditItem is called. The SaveItem method is triggered by selecting the <button> element.

@using System.Net.Http
@using System.Net.Http.Json
@using System.Threading.Tasks
@inject HttpClient Http

<input type="checkbox" @bind="editItem.IsComplete" />
<input @bind="editItem.Name" />
<button @onclick="SaveItem">Save</button>

@code {
    private string id;
    private TodoItem editItem = new TodoItem();

    private void EditItem(long id)
    {
        editItem = todoItems.Single(i => i.Id == id);
    }

    private async Task SaveItem() =>
        await Http.PutAsJsonAsync($"api/TodoItems/{editItem.Id}", editItem);
}

Calls to xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A return an xref:System.Net.Http.HttpResponseMessage. To deserialize the JSON content from the response message, use the xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A extension method. The following example reads JSON weather data:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast>();

Additional extension methods

xref:System.Net.Http includes additional extension methods for sending HTTP requests and receiving HTTP responses. xref:System.Net.Http.HttpClient.DeleteAsync%2A?displayProperty=nameWithType is used to send an HTTP DELETE request to a web API.

In the following component code, the <button> element calls the DeleteItem method. The bound <input> element supplies the id of the item to delete.

@using System.Net.Http
@using System.Threading.Tasks
@inject HttpClient Http

<input @bind="id" />
<button @onclick="DeleteItem">Delete</button>

@code {
    private long id;

    private async Task DeleteItem() =>
        await Http.DeleteAsync($"api/TodoItems/{id}");
}

Named HttpClient with IHttpClientFactory

xref:System.Net.Http.IHttpClientFactory services and the configuration of a named xref:System.Net.Http.HttpClient are supported.

Reference the Microsoft.Extensions.Http NuGet package in the project file:

<PackageReference Include="Microsoft.Extensions.Http" Version="{VERSION}" />

In the preceding example, the {VERSION} placeholder is the version of the package.

In Program.Main of the Program.cs file:

builder.Services.AddHttpClient("WebAPI", client => 
    client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));

In the following component code:

• An instance of xref:System.Net.Http.IHttpClientFactory creates a named xref:System.Net.Http.HttpClient.
• The named xref:System.Net.Http.HttpClient is used to issue a GET request for JSON weather forecast data from the web API.

Pages/FetchDataViaFactory.razor:

@page "/fetch-data-via-factory"
@using System.Net.Http
@using System.Net.Http.Json
@using System.Threading.Tasks
@inject IHttpClientFactory ClientFactory

<h1>Fetch data via <code>IHttpClientFactory</code></h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

@code {
    private WeatherForecast[] forecasts;

    protected override async Task OnInitializedAsync()
    {
        var client = ClientFactory.CreateClient("WebAPI");

        forecasts = await client.GetFromJsonAsync<WeatherForecast[]>(
            "WeatherForecast");
    }
}

Typed HttpClient

Typed xref:System.Net.Http.HttpClient uses one or more of the app's xref:System.Net.Http.HttpClient instances, default or named, to return data from one or more web API endpoints.

WeatherForecastClient.cs:

using System.Net.Http;
using System.Net.Http.Json;
using System.Threading.Tasks;

public class WeatherForecastHttpClient
{
    private readonly HttpClient http;

    public WeatherForecastHttpClient(HttpClient http)
    {
        this.http = http;
    }

    public async Task<WeatherForecast[]> GetForecastAsync()
    {
        try
        {
            return await http.GetFromJsonAsync<WeatherForecast[]>(
                "WeatherForecast");
        }
        catch
        {
            ...

            return new WeatherForecast[0];
        }
    }
}

In Program.Main of the Program.cs file:

builder.Services.AddHttpClient<WeatherForecastHttpClient>(client => 
    client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));

Components inject the typed xref:System.Net.Http.HttpClient to call the web API.

In the following component code:

• An instance of the preceding WeatherForecastHttpClient is injected, which creates a typed xref:System.Net.Http.HttpClient.
• The typed xref:System.Net.Http.HttpClient is used to issue a GET request for JSON weather forecast data from the web API.

Pages/FetchDataViaTypedHttpClient.razor:

@page "/fetch-data-via-typed-httpclient"
@using System.Threading.Tasks
@inject WeatherForecastHttpClient Http

<h1>Fetch data via typed <code>HttpClient</code></h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

@code {
    private WeatherForecast[] forecasts;

    protected override async Task OnInitializedAsync()
    {
        forecasts = await Http.GetForecastAsync();
    }
}

HttpClient and HttpRequestMessage with Fetch API request options

HttpClient (API documentation) and xref:System.Net.Http.HttpRequestMessage can be used to customize requests. For example, you can specify the HTTP method and request headers. The following component makes a POST request to a web API endpoint and shows the response body.

Pages/TodoRequest.razor:

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

[!code-razor]

::: moniker-end

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

[!code-razor]

::: moniker-end

Blazor WebAssembly's implementation of xref:System.Net.Http.HttpClient uses Fetch API. Fetch API allows the configuration of several request-specific options. Options can be configured with xref:System.Net.Http.HttpRequestMessage extension methods shown in the following table.

Extension method	Fetch API request property
xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestCache%2A	cache
xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestCredentials%2A	credentials
xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestIntegrity%2A	integrity
xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestMode%2A	mode

Set additional options using the generic xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestOption%2A extension method.

The HTTP response is typically buffered to enable support for synchronous reads on the response content. To enable support for response streaming, use the xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserResponseStreamingEnabled%2A extension method on the request.

To include credentials in a cross-origin request, use the xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestCredentials%2A extension method:

requestMessage.SetBrowserRequestCredentials(BrowserRequestCredentials.Include);

For more information on Fetch API options, see MDN web docs: WindowOrWorkerGlobalScope.fetch(): Parameters.

Call web API example

The following example calls a web API. The example requires a running web API based on the sample app described by the xref:tutorials/first-web-api article. This example makes requests to the web API at https://localhost:10000/api/TodoItems. If a different web API address is used, update the ServiceEndpoint constant value in the component's @code block.

The following example makes a cross-origin resource sharing (CORS) request from http://localhost:5000 or https://localhost:5001 to the web API. Add the following CORS middleware configuration to the web API's service's Startup.Configure method:

app.UseCors(policy => 
    policy.WithOrigins("http://localhost:5000", "https://localhost:5001")
    .AllowAnyMethod()
    .WithHeaders(HeaderNames.ContentType));

Adjust the domains and ports of WithOrigins as needed for the Blazor app. For more information, see xref:security/cors.

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

By default, ASP.NET Core apps use ports 5000 (HTTP) and 5001 (HTTPS). To run both apps on the same machine at the same time for testing, use a different port for the web API app (for example, port 10000). For more information on setting the port, see xref:fundamentals/servers/kestrel/endpoints.

::: moniker-end

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

By default, ASP.NET Core apps use ports 5000 (HTTP) and 5001 (HTTPS). To run both apps on the same machine at the same time for testing, use a different port for the web API app (for example, port 10000). For more information on setting the port, see xref:fundamentals/servers/kestrel#endpoint-configuration.

::: moniker-end

Pages/CallWebAPI.razor:

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

[!code-razor]

::: moniker-end

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

[!code-razor]

::: moniker-end

Handle errors

Handle web API response errors in developer code when they occur. For example, xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A expects a JSON response from the web API with a Content-Type of application/json. If the response isn't in JSON format, content validation throws a xref:System.NotSupportedException.

In the following example, the URI endpoint for the weather forecast data request is misspelled. The URI should be to WeatherForecast but appears in the call as WeatherForcast, which is missing the letter e in Forecast.

The xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A call expects JSON to be returned, but the web API returns HTML for an unhandled exception with a Content-Type of text/html. The unhandled exception occurs because the path to /WeatherForcast isn't found and middleware can't serve a page or view for the request.

In xref:Microsoft.AspNetCore.Components.ComponentBase.OnInitializedAsync%2A on the client, xref:System.NotSupportedException is thrown when the response content is validated as non-JSON. The exception is caught in the catch block, where custom logic could log the error or present a friendly error message to the user.

Pages/FetchDataReturnsHTMLOnException.razor:

@page "/fetch-data-returns-html-on-exception"
@using System.Net.Http
@using System.Net.Http.Json
@using System.Threading.Tasks
@inject HttpClient Http

<h1>Fetch data but receive HTML on unhandled exception</h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

<p>
    @exceptionMessage
</p>

@code {
    private WeatherForecast[] forecasts;
    private string exceptionMessage;

    protected override async Task OnInitializedAsync()
    {
        try
        {
            // The URI endpoint "WeatherForecast" is misspelled on purpose on the 
            // next line. See the preceding text for more information.
            forecasts = await Http.GetFromJsonAsync<WeatherForecast[]>("WeatherForcast");
        }
        catch (NotSupportedException exception)
        {
            exceptionMessage = exception.Message;
        }
    }
}

[!NOTE] The preceding example is for demonstration purposes. A web API can be configured to return JSON even when an endpoint doesn't exist or an unhandled exception occurs on the server.

For more information, see xref:blazor/fundamentals/handle-errors.

::: zone-end

::: zone pivot="server"

Blazor Server apps call web APIs using xref:System.Net.Http.HttpClient instances, typically created using xref:System.Net.Http.IHttpClientFactory. For guidance that applies to Blazor Server, see xref:fundamentals/http-requests.

A Blazor Server app doesn't include an xref:System.Net.Http.HttpClient service by default. Provide an xref:System.Net.Http.HttpClient to the app using the HttpClient factory infrastructure.

In Startup.ConfigureServicesofStartup.cs`:

services.AddHttpClient();

The following Blazor Server Razor component makes a request to a web API for GitHub branches similar to the Basic Usage example in the xref:fundamentals/http-requests article.

Pages/CallWebAPI.razor:

@page "/call-web-api"
@using System.Text.Json
@using System.Text.Json.Serialization;
@inject IHttpClientFactory ClientFactory

<h1>Call web API from a Blazor Server Razor component</h1>

@if (getBranchesError)
{
    <p>Unable to get branches from GitHub. Please try again later.</p>
}
else
{
    <ul>
        @foreach (var branch in branches)
        {
            <li>@branch.Name</li>
        }
    </ul>
}

@code {
    private IEnumerable<GitHubBranch> branches = Array.Empty<GitHubBranch>();
    private bool getBranchesError;
    private bool shouldRender;

    protected override bool ShouldRender() => shouldRender;

    protected override async Task OnInitializedAsync()
    {
        var request = new HttpRequestMessage(HttpMethod.Get,
            "https://api.github.com/repos/dotnet/AspNetCore.Docs/branches");
        request.Headers.Add("Accept", "application/vnd.github.v3+json");
        request.Headers.Add("User-Agent", "HttpClientFactory-Sample");

        var client = ClientFactory.CreateClient();

        var response = await client.SendAsync(request);

        if (response.IsSuccessStatusCode)
        {
            using var responseStream = await response.Content.ReadAsStreamAsync();
            branches = await JsonSerializer.DeserializeAsync
                <IEnumerable<GitHubBranch>>(responseStream);
        }
        else
        {
            getBranchesError = true;
        }

        shouldRender = true;
    }

    public class GitHubBranch
    {
        [JsonPropertyName("name")]
        public string Name { get; set; }
    }
}

For an additional working example, see the Blazor Server file upload example that uploads files to a web API controller in the xref:blazor/file-uploads#upload-files-to-a-server article.

::: zone-end

Cross-origin resource sharing (CORS)

Browser security restricts a webpage from making requests to a different domain than the one that served the webpage. This restriction is called the same-origin policy. The same-origin policy restricts (but doesn't prevent) a malicious site from reading sensitive data from another site. To make requests from the browser to an endpoint with a different origin, the endpoint must enable cross-origin resource sharing (CORS).

::: zone pivot="webassembly"

For information on CORS requests in Blazor WebAssembly apps, see xref:blazor/security/webassembly/additional-scenarios#cross-origin-resource-sharing-cors.

For information on CORS, see xref:security/cors. The article's examples don't pertain directly to Blazor WebAssembly apps, but the article is useful for learning general CORS concepts.

::: zone-end

::: zone pivot="server"

For more information, see xref:security/cors.

::: zone-end

Blazor framework component examples for testing web API access

Various network tools are publicly available for testing web API backend apps directly, such as Fiddler, Firefox Browser Developer, and Postman. Blazor framework's reference source includes xref:System.Net.Http.HttpClient test assets that are useful for testing:

HttpClientTest assets in the dotnet/aspnetcore GitHub repository

[!INCLUDE]

Additional resources

::: zone pivot="webassembly"

• xref:blazor/security/webassembly/additional-scenarios: Includes coverage on using xref:System.Net.Http.HttpClient to make secure web API requests.
• xref:security/cors: Although the content applies to ASP.NET Core apps, not Blazor WebAssembly apps, the article covers general CORS concepts.
• Cross Origin Resource Sharing (CORS) at W3C
• Fetch API

::: zone-end

::: zone pivot="server"

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

• xref:blazor/security/server/additional-scenarios: Includes coverage on using xref:System.Net.Http.HttpClient to make secure web API requests.
• xref:fundamentals/http-requests
• xref:security/enforcing-ssl
• xref:security/cors
• Kestrel HTTPS endpoint configuration
• Cross Origin Resource Sharing (CORS) at W3C

::: moniker-end

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

• xref:blazor/security/server/additional-scenarios: Includes coverage on using xref:System.Net.Http.HttpClient to make secure web API requests.
• xref:fundamentals/http-requests
• xref:security/enforcing-ssl
• xref:security/cors
• Kestrel HTTPS endpoint configuration
• Cross Origin Resource Sharing (CORS) at W3C

::: moniker-end

::: zone-end