8. Debugging Code

8.1 Execution Run, Pause and Stop

Once you have reached a stage where your code is written and you want to test it, you can run the application within the Visual Basic design environment. To start the application, click the "Start" button in the top toolbar (this is a sideways triangle similar to the 'play' symbol on a tape recorder) or press the F5 key on the keyboard.

In some cases, making alterations to the code, even whilst paused, may require the code to re-start. If this happens you will be asked to confirm the change.

Finally, clicking the "End" button (this looks like the 'stop' symbol on a tape recorder) will stop all execution and return control back to the Visual Basic design environment.

8.2 Breakpoints

Breakpoints to not form part of the executable code; they are only there as a guide to Visual Basic. If you compile and run your code externally, the Breakpoints that you specify will have no effect.

8.2.1 Adding and Removing Breakpoints
A Breakpoint can be added to any executable line of code. This means that declaration lines (e.g. Function and Variable declarations) cannot be used as Breakpoints. An active Breakpoint is represented as a red dot in the margin beside the code and highlighted text, in the View Code window.

There are two ways to add a Breakpoint:

1. Move the text cursor to the line of code that you want to add the Breakpoint to and press the F9 key on the keyboard. Pressing F9 again whilst on a Breakpoint will remove the Breakpoint.

2. Click the margin adjacent to the line of code that you want to add the Breakpoint to. Clicking here again will remove the Breakpoint.

To remove all Breakpoints from the project, press Ctrl + Shift + F9 on the keyboard.

8.3 Execution Cursor

In the margin adjacent to the execution cursor, a small arrow is used to move the execution. If you want to skip a line of code, you could drag this arrow down past the line and resume execution (or step - see below). Alternatively, you could move the arrow back up to a previous line of code in order to re-run it.

When moving the execution cursor, be careful what is skipped and what is re-run as this can give you the impression that your code is functioning incorrectly, when in fact is runs fine - or vice-versa!

8.3.1 Continue, Step Into and Step Over
Once you have encountered a Breakpoint during the execution of your application, you may want to advance through the code step-by-step or simply resume execution until the next Breakpoint is reached.

To continue executing the application, click the "Run" button on the top toolbar (or press F5 on the keyboard). This will resume execution from the current line of code and continue as normal, unless another Breakpoint is encountered. If you have added a Breakpoint to a line of code within a loop (eg For..Next or While.Wend), the Breakpoint will pause the code on each iteration of the loop.

There are also 2 ways to execute the code on a step-by-step basis:

1. Step Into - press F8 on the keyboard (or select "Step Into" from the "Debug" menu)
The code is executed 1 line at a time. If the code invokes a procedure, such as a function or sub, the code within the procedure is also executed 1 line at a time.

2. Step Over - press Shift + F8 (or select "Step Over" from the "Debug" menu)
The code is still executed 1 line at a time, however, if the code invokes any procedure, they are considered external to the current code and run in full until execution returns back to the piece of code where the Breakpoint was encountered. This is very useful if you already know that your functions work and don't want to step through them line-by-line.

During stepwise execution of code, a highlight is placed over the next line of code to be executed. This shows you the current position of the code cursor.

8.3.2 Using the mouse to display variable values
When the code execution is paused as a result of a Breakpoint, the mouse pointer can be used to view the current values of the variables in the code window. Moving the mouse pointer over a variable will cause a pop-up display to appear next to it, indicating the current value.

This is very useful when testing parameters being passed to procedures or within loops.

8.4 Watches

In this way, several variables can be monitored together and compared to give you a better idea of what may be causing a fault or other undesired effect in your application.

8.4.1 Adding and Removing Watches
To add a Watch to the Watches window, select "Add Watch." from the "Debug" menu. In the dialogue box that appears, type in the name of the expression that you want to watch (the expression at the current text cursor is entered automatically by default).

It is important to note that the dialogue box prompts for an expression as opposed to a variable. This means that you may Watch for results of calculations or parameters returned from functions, as well as just variables.

For example, if you were interested in the first 4 characters of a String variable called "strA" you could add a Watch for the whole of "strA", but you could add a Watch for "Left(strA, 4)", which would show the first 4 characters. Similarly, you could add a watch for the total of two Integer type variables; A + B.

Set the "Context" for the Watch, depending upon where you want to be able to view it. This is important because you may have several localised variables with the same name, so adding a Watch in 'all procedure' may be confusing.

There are other options on the dialogue box that can be used to generate a break (like a Breakpoint) if the expression changes or becomes true. Click "OK" when done.

If you haven't added any Watches already or the Watches window is closed, it should appear automatically, listing all of the Watches in the project. The Watches window displays the name of the expression, it's current value and other useful information about each entry.

8.4.2 Interrogating Watches
During pauses in code execution, the Watches window will display the current values for the Watches on the project. If you have placed a Watch on an object (e.g. a ListBox), the Watch may be expanded to show properties at lower levels in the object:

8.5 The Debug Window

Debug.Print "Book number is:" & lngBookNumber
lngBookNumber = GetBookNumber()
Debug.Print "Book number changed to:" &  lngBookNumber

Book number is: 0
Book number changed to: 37

Debug.Assert  lngBookNumber <> 0	' Stop if lngBookNumber is not 0

8.6 Comments

You may use as many comments as you like in your code, as they are not compiled by Visual Basic, so have no effect on the final size or performance of the application.