6.1 Microsoft visual basic delegates

Introduction

When developing Visual Basic applications in a multithreaded invironment it may be necessary for a thread to access a GUI object in the form thread. However it is not possible for the thread to access the object directly so it must us a delegate that runs in the thread that handles the GUI object. This module describes the very basics of how to use delegate methods so that more than one thread may access a GUI object.

Reading

• Delegate Class: (External Link)
• Control Class: (External Link)
• Control Class Invoke method: (External Link)

Background

When a simple Visual Basic (VB) GUI application is made, there is a form class that handles all the GUI controls. This form class is a thread that handles all the interaction with the controls. It is possible to have other threads in our application. However, it is not possible for the other threads to have direct access to the GUI controls. The reason for this is it could be possible for the form thread and the other thread to try and change the same control at the same time.

To fix the problem of accessing the same control from two threads, a delegate function is made in the thread that handles the control object. The delegate function is invoked by other threads and the delegate function operates in the thread that handles the control object.

From the MSDN: "Delegates are similar to function pointers in C or C++ languages. Delegates encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code that calls the referenced method, and the method to be invoked can be unknown at compile time. Unlike function pointers in C or C++, delegates are object-oriented, type-safe, and more secure."

"The Delegate Class represents a delegate, which is a data structure that refers to a static method or to a class instance and an instance method of that class."

Visual basic delegates

Suppose you have a TextBox control object in your Form called TextBox1 . You can change the text in the TextBox by issuing a command like this:

This will change the Text property of the TextBox control object. When this is done within the Form thread everything works as expected. For instance, suppose you have a button in your Form called Button1 . Then in the click method for that button you could have something like this:

This would all take place within the Form thread. The Form thread is the thread that created the TextBox control and the Button control so this is all within the same thread.

Suppose now you have a thread that you started that runs separate from the Form thread. If the thread tries to change the TextBox object you will get an error. If the thread looked like the following code, there would be a problem.