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

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