Xamarin Forms调用ASP.NET Web API出现HttpRequestException请求发送错误

Hey there, let's tackle this System.Net.Http.HttpRequestException issue you're hitting with your Xamarin Forms + ASP.NET Web API app! First, let's recap your setup to make sure we're on the same page:

• You're building a mobile app with Xamarin Forms paired with an ASP.NET Web API
• Both projects live in the same Visual Studio 2017 solution
• The Web API was created using the template with individual user accounts
• Your registration binding model looks like this:

public class RegisterBindingModel { 
    public string Email { get; set; } 
    public string Password { get; set; }
    // ... other properties
}

Below are the most common fixes and troubleshooting steps for this error:

1. Fix the Web API Access URL (Don't Use localhost) #

When running the Web API and Xamarin app side-by-side, localhost won't work for simulators or physical devices—they can't reach your computer's local loopback address directly:

• For Android emulators: Replace localhost with 10.0.2.2 (this maps to your host machine's loopback)
• For iOS simulators: Use your computer's local network IP (e.g., 192.168.1.100) instead of localhost
• For physical devices: Ensure your phone and computer are on the same WiFi network, then use your computer's local IP address
• Double-check your Web API's port number (found in the project's Properties > Debug > Launch URL) and include it in your request URL (e.g., http://192.168.1.100:5000/api/Account/Register)

2. Resolve HTTPS Certificate Trust Issues #

If your Web API uses HTTPS (default for the individual accounts template), Xamarin apps won't trust the self-signed development certificate automatically:

• Quick test: Temporarily disable HTTPS in your Web API (go to Properties > Debug and uncheck "Enable SSL") and use HTTP for your requests
• Permanent fix for HTTPS: Export the Web API's certificate from your browser, then install and trust it on your simulator/physical device

3. Verify Route and Request Method Match #

The individual accounts template's registration endpoint is typically api/Account/Register (for older ASP.NET Identity setups). Make sure:

• Your Xamarin app is sending a POST request to this exact route
• Your Web API's routing config in WebApiConfig.cs is correctly set up to handle controller-based routes:

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

4. Ensure Request Body Matches the Binding Model #

Double-check that your Xamarin request sends a valid JSON body that matches all required properties in RegisterBindingModel:

• Example valid request code in Xamarin:

var registerData = new { 
    Email = "user@example.com", 
    Password = "P@ssw0rd123",
    ConfirmPassword = "P@ssw0rd123" // Don't forget this if your model requires it!
};
var jsonContent = new StringContent(
    JsonConvert.SerializeObject(registerData),
    Encoding.UTF8,
    "application/json"
);

var response = await _httpClient.PostAsync("http://192.168.1.100:5000/api/Account/Register", jsonContent);

5. Get the Real Error from Inner Exception #

HttpRequestException is often a wrapper for a more specific issue. Capture the inner exception to see the root cause:

try
{
    // Your request code here
}
catch (HttpRequestException ex)
{
    if (ex.InnerException != null)
    {
        // Log or print this message—it will tell you if it's a timeout, DNS error, or certificate issue
        Console.WriteLine($"Root Cause: {ex.InnerException.Message}");
    }
}

内容的提问来源于stack exchange，提问作者Chandru Vijay