Provide file contents as a string instead of a filename

Question

I'm very new to C#, but not to programming. I'm trying to use SSH.NET to make an SSH connection using a private key, using the example is here:

public ConnectionInfo CreateConnectionInfo()
{
    const string privateKeyFilePath = @"C:\some\private\key.pem";    
    ConnectionInfo connectionInfo;
    using (var stream = new FileStream(privateKeyFilePath, FileMode.Open, FileAccess.Read))
    {
        var privateKeyFile = new PrivateKeyFile(stream);
        AuthenticationMethod authenticationMethod =
            new PrivateKeyAuthenticationMethod("ubuntu", privateKeyFile);

        connectionInfo = new ConnectionInfo(
            "my.server.com",
            "ubuntu",
            authenticationMethod);
    }

    return connectionInfo;
}

I don't have the key stored in a file, it comes from another source and I have it in a string. I realize I could write it to a temporary file, then pass that filename, then trash the file, but I'd prefer not to have to write it out to disk if possible. I don't see anything in the documentation that lets me pass a string for this, only a filename. I've tried something like this to convert the string to a stream:

byte[] private_key     = Encoding.UTF8.GetBytes(GetConfig("SSHPrivateKey"));
var private_key_stream = new PrivateKeyFile(new MemoryStream(private_key));

And then pass private_key_stream as the second parameter of PrivateKeyAuthenticationMethod. There seem to be no complaints from the compiler, but the method doesn't appear to actually be getting the key (SSH doesn't authenticate, and external attempts like PuTTY using this key to the server do work), so it looks like I'm missing something.

Any thoughts on how to accomplish this without writing out a temp file?