.NET Aspire Gotchas: Why HttpClient Uri Isn’t Replaced in Blazor WebAssembly Standalone Apps in .NET 8

Have you ever stumbled upon an issue where .NET Aspire doesn’t replace the HTTP client Uri with the actual address in your Blazor WebAssembly standalone app in .NET 8? You’re not alone! In this article, we’ll dive into the reasons behind this issue and provide a step-by-step guide to troubleshoot and resolve it.

The Problem: HttpClient Uri Remains Unchanged

When working with Blazor WebAssembly standalone apps in .NET 8, you might encounter an issue where the HttpClient Uri doesn’t get replaced with the actual address. This can lead to issues with API calls, data retrieval, and overall app functionality.

For instance, let’s say you have the following code in your Blazor component:

@inject HttpClient _httpClient

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

In this example, you’d expect the `_httpClient` to make a GET request to https://example.com/api/data. However, when you run the app, the Uri remains unchanged, and the request is made to the original address instead of the actual one.

Why Does This Happen?

The reason behind this issue lies in the way .NET Aspire handles the HttpClient Uri in Blazor WebAssembly standalone apps. When you create an instance of HttpClient, Aspire doesn’t automatically replace the base address with the actual one.

This is because Blazor WebAssembly apps are executed in the browser, which has its own set of security constraints. By default, the browser restricts the HttpClient from making requests to external origins due to same-origin policy concerns.

To overcome this limitation, .NET Aspire provides a mechanism to configure the HttpClient to use a proxy or gateway to forward requests to external services. However, this requires additional setup and configuration.

Solving the Issue: Configure HttpClient with a Proxy

To resolve the issue, you’ll need to configure the HttpClient to use a proxy or gateway that can forward requests to external services. Here’s a step-by-step guide to do so:

Step 1: Create a Proxy Class

First, create a new class that will act as a proxy to forward requests to external services. This class will inherit from `DelegatingHandler` and override the `SendAsync` method:

public class ProxyHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // Forward the request to the actual address
        request.RequestUri = new Uri("https://example.com/api" + request.RequestUri.PathAndQuery);
        return await base.SendAsync(request, cancellationToken);
    }
}

Step 2: Configure HttpClient with the Proxy

Next, create an instance of HttpClient and configure it to use the proxy handler:

@inject IHttpClientFactory _httpClientFactory

@code {
    private async Task GetData()
    {
        var client = _httpClientFactory.CreateClient("exampleClient");
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "your_token");

        var proxyHandler = new ProxyHandler();
        client.Handler = proxyHandler;

        var response = await client.GetAsync("api/data");
        // ...
    }
}

Step 3: Register the HttpClient with the Proxy in DI

In your `Program.cs` file, register the HttpClient with the proxy in the Dependency Injection (DI) container:

builder.Services.AddHttpClient("exampleClient", client =>
{
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "your_token");
})
.AddHttpMessageHandler<ProxyHandler>();

Additional Considerations

When using a proxy to forward requests to external services, keep in mind the following considerations:

• Security: Ensure that your proxy handler is properly configured to handle security concerns, such as authentication and authorization.
• CORS: If your external service uses CORS, configure your proxy handler to handle CORS preflights and responses.
• Error Handling: Implement proper error handling mechanisms to handle cases where the proxy handler fails to forward requests.

Conclusion

In this article, we explored the reasons behind why .NET Aspire doesn’t replace the HttpClient Uri with the actual address in Blazor WebAssembly standalone apps in .NET 8. We also provided a step-by-step guide to configure the HttpClient with a proxy to forward requests to external services.

By following these instructions, you should be able to resolve the issue and ensure that your Blazor WebAssembly app makes requests to the correct address. Remember to consider security, CORS, and error handling when implementing the proxy handler.

Issue	Solution
.NET Aspire doesn’t replace HttpClient Uri with actual address	Configure HttpClient with a proxy to forward requests to external services

Happy coding!

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