The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

I was getting an error message "The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel." when trying to connect to an HTTPS site using an HttpWebRequest.

As the site I was connecting to was a test site from a 3rd party known to me, but not set up correctly at their end, to bypass this check I added the following line before calling GetRequestStream on my HttpWebRequest.

ServicePointManager.ServerCertificateValidationCallback 
+= new 
System.Net.Security.RemoteCertificateValidationCallback
(CustomValidation);

and added the CustomValidation function as follows:

private static bool CustomValidation(object sender, 
X509Certificate cert, X509Chain chain, 
System.Net.Security.SslPolicyErrors error)
{
    return true;
}