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When sending a request with the HttpClient class, we always get the 'The request was aborted: Could not create SSL/TLS secure channel' error. The code is part of a library and has successfully accessed other services before. That's why I suspect that something about the specific service we are trying to access is different and the question is how to identify that difference. The error message is not really helpful in that regard.

What I have tried:

The following code was compiled under .NET famework 4.5.1. I have read that switching to a higher framework version might fix the problem. I have NOT tried this yet, because this library is only part of a much larger solution and a version change involves a little more than just changing a setting in the current project.

This is how the HttpClient is initialized:

protected override void Initialize()

    ServicePointManager.Expect100Continue = true;
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
    //ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | 
SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | 

    handler = new HttpClientHandler()
        ClientCertificateOptions = ClientCertificateOption.Automatic,
        UseDefaultCredentials = false,
        PreAuthenticate = false,
        Credentials = new NetworkCredential
                      (Parameter.Name, Parameter.Password)

    client = new HttpClient(handler)
        Timeout = new TimeSpan(0, 1, 0)

    (new MediaTypeWithQualityHeaderValue("application/json"));

     Result.Succeeded = true;
Updated 13-Sep-23 4:04am
Dave Kreskowiak 31-Aug-23 8:23am    
It may work on the newer .NET, but I would check to see what protocols the server supports before making any changes to the code. If the server doesn't support TLS1.2, changing the code isn't going to do you any good.

All versions of SSL, and TLS 1.0 and 1.1, are all considered broken today. The minimum you/servers should be using is TLS 1.2.
Richard Deeming 31-Aug-23 8:30am    
Transport Layer Security (TLS) best practices with the .NET Framework - .NET Framework | Microsoft Learn[^]

But as Dave said, you need to know what protocols the remote server supports. You can use something like Qualys SSL Server Test[^] to determine that.
CodeWraith 15-Sep-23 4:28am    
Just a little update: It's working now, but it was a bit of trial and error. We had no choice but to 'fix by update'. Anything higher than framework version 4.5.1 would work without any further code changes. Thanks for the help.

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