NET5.0 Blazor WASM CORS client exception

NET5.0 Blazor WASM CORS client exception

When making requests from a Blazor WebAssembly (WASM) application, you might encounter a CORS (Cross-Origin Resource Sharing) client exception if the API endpoint you're trying to access is on a different origin than the Blazor application itself.

To resolve this issue, you need to enable CORS on the server that hosts the API endpoint. This can be done by adding the appropriate CORS middleware to the API's Startup class, like this:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    // ...
    app.UseCors(builder => builder
        .AllowAnyOrigin()
        .AllowAnyMethod()
        .AllowAnyHeader());
    // ...
}

This code allows any origin to access the API, and allows any HTTP method and header to be used in requests. You can adjust the configuration to suit your needs.

Another common issue that can cause CORS client exceptions in Blazor WASM is the use of the HttpClient class to make requests. By default, the HttpClient class includes the origin of the Blazor application in the request's Referer header. Some APIs may reject requests that include a Referer header from a different origin. To work around this issue, you can configure the HttpClient to not include the Referer header by default:

services.AddHttpClient("MyApiClient", client =>
{
    client.DefaultRequestHeaders.ReferrerPolicy = new ReferrerPolicyHeaderValue("no-referrer");
});

This code configures an HttpClient instance named "MyApiClient" to not include the Referer header in requests by default. You can then use this HttpClient instance to make requests to your API:

@inject HttpClient MyApiClient

@code {
    private async Task MakeApiRequest()
    {
        var response = await MyApiClient.GetAsync("https://api.example.com");
        // ...
    }
}

Make sure to replace "https://api.example.com" with the actual URL of your API endpoint.

By enabling CORS on the server and configuring the HttpClient to not include the Referer header, you should be able to make requests from your Blazor WASM application to the API endpoint without encountering a CORS client exception.

Examples

  1. "NET 5.0 Blazor WASM CORS client exception handling"

    Description: Explore how to handle CORS (Cross-Origin Resource Sharing) exceptions in a .NET 5.0 Blazor WebAssembly application. CORS issues commonly arise when making requests to APIs hosted on different domains.

    Code:

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

public class MyDataService
{
    private readonly HttpClient _httpClient;

    public MyDataService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<string> GetDataFromServer()
    {
        try
        {
            var response = await _httpClient.GetAsync("https://example.com/api/data");
            response.EnsureSuccessStatusCode();
            return await response.Content.ReadAsStringAsync();
        }
        catch (HttpRequestException ex) when (ex.Message.Contains("CORS"))
        {
            return "CORS issue detected. Ensure CORS is configured correctly on the server.";
        }
    }
}

  2. "Blazor WebAssembly CORS client exception handling"

    Description: Learn how to implement client-side exception handling for CORS issues in a Blazor WebAssembly application targeting .NET 5.0. Proper error handling is essential when dealing with cross-origin requests.

    Code:

    @inject HttpClient HttpClient

@code {
    private string data;

    protected override async Task OnInitializedAsync()
    {
        try
        {
            data = await HttpClient.GetStringAsync("https://example.com/api/data");
        }
        catch (HttpRequestException ex) when (ex.Message.Contains("CORS"))
        {
            data = "CORS issue detected. Ensure CORS is configured correctly on the server.";
        }
    }
}

  3. "NET 5.0 Blazor WebAssembly CORS exception message"

    Description: Understand the error message associated with CORS exceptions in .NET 5.0 Blazor WebAssembly applications. Properly interpreting error messages helps in diagnosing and resolving CORS-related issues.

    Code:

    public class MyDataService
{
    private readonly HttpClient _httpClient;

    public MyDataService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<string> GetDataFromServer()
    {
        try
        {
            var response = await _httpClient.GetAsync("https://example.com/api/data");
            response.EnsureSuccessStatusCode();
            return await response.Content.ReadAsStringAsync();
        }
        catch (HttpRequestException ex) when (ex.Message.Contains("CORS"))
        {
            return ex.Message; // Return the exception message
        }
    }
}

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