Invoke a method from another assembly

I have Tools.dll file that contains MyMethod() like this :

    public void MyMethod()
    {
        global::System.Windows.Forms.MessageBox.Show("Sth");
    }

Now , I am trying to run this assembly method from another file :

        System.Reflection.Assembly myDllAssembly = System.Reflection.Assembly.LoadFile(@"PATH\Tools.dll");
 myDllAssembly.GetType().GetMethod("MyMethod").Invoke(myDllAssembly, null); //here we invoke MyMethod.

After running 'System.NullReferenceException' happens . It says "Object reference not set to an instance of an object."

So How can I fix it ?!

Im sure this .dll building truth without problem .

Note : the assembly code come from : http://ift.tt/1PP9l7M