C#: Overriding ToString() method for custom exceptions

This is all overkill. Your exception should just override the Message Property.

public override String Message {
    get {  
        return base.Message + String.Format(", HttpStatusCode={0}, RequestId='{1}'", 
                    httpStatusCode, 
                    RequestId);
    }
}

The default ToString method for the Exception class is basically “ClassName: Message --> InnerException.ToString() StackTrace“. So overriding the Message puts your message text exactly where it should be.