C# – Web Client Exception: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel

The following snippets will fix the case where there is something wrong with the SSL certificate on the server you are calling. For example, it may be self-signed or the host name between the certificate and the server may not match.

This is dangerous if you are calling a server outside of your direct control, since you can no longer be as sure that you are talking to the server you think you're connected to. However, if you are dealing with internal servers and getting a "correct" certificate is not practical, use the following to tell the web service to ignore the certificate problems and bravely soldier on.

The first two use lambda expressions, the third uses regular code. The first accepts any certificate. The last two at least check that the host name in the certificate is the one you expect.
... hope you find it helpful

//Trust all certificates
System.Net.ServicePointManager.ServerCertificateValidationCallback =
    ((sender, certificate, chain, sslPolicyErrors) => true);

// trust sender
System.Net.ServicePointManager.ServerCertificateValidationCallback
                = ((sender, cert, chain, errors) => cert.Subject.Contains("YourServerName"));

// validate cert by calling a function
ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateRemoteCertificate);

// callback used to validate the certificate in an SSL conversation
private static bool ValidateRemoteCertificate(object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors policyErrors)
{
    bool result = cert.Subject.Contains("YourServerName");
    return result;
}

As a workaround you could add a handler to the ServicePointManager's ServerCertificateValidationCallback on the client side:

System.Net.ServicePointManager.ServerCertificateValidationCallback +=
    (se, cert, chain, sslerror) =>
        {
            return true;
        };

but be aware that this is not a good practice as it completely ignores the server certificate and tells the service point manager that whatever certificate is fine which can seriously compromise client security. You could refine this and do some custom checking (for certificate name, hash etc). at least you can circumvent problems during development when using test certificates.