How to Fix 'The underlying connection was closed' Error with HttpWebRequest Basic Authentication?

Solving HttpWebRequest Basic Authentication Issues

This article addresses a common problem encountered when using HttpWebRequest with basic authentication: "The underlying connection was closed: An unexpected error occurred on a send." This error often arises from conflicts with the default authentication handling.

The solution is to bypass the default mechanism and directly manage the Authorization header. This header requires the authentication method ("Basic") and base64-encoded credentials.

Here's how to implement this:

<code class="language-csharp">var username = "abc";
var password = "123";
string encoded = Convert.ToBase64String(Encoding.GetEncoding("ISO-8859-1")
                                  .GetBytes(username + ":" + password));
httpWebRequest.Headers.Add("Authorization", "Basic " + encoded);</code>

This code snippet takes the username and password, encodes them using base64 with ISO-8859-1 encoding (to ensure compatibility with the server), and then adds the resulting string to the Authorization header of the HttpWebRequest. This allows the request to properly authenticate, preventing connection closure and enabling successful communication. The use of ISO-8859-1 encoding is crucial for resolving many authentication failures. By manually setting the header, the application avoids potential issues with the default authentication process.

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