Sunday, 9 December 2012

Excel VBA Error Handling and Debugging Techniques

 What is Use of Locals Window , Watch Window ,Immediate Window,Debug.Print and Debug.Assert Techniques.
The most important step in programming is testing and debugging. Once you finished writing the VBA Procedures and compile , the next step you need to take is debugging the code to check the Macro is giving the correct result or not using the below techniques.

This post describes the various debugging resources available in the VBA Editor (VBE) and how to use them.

Step by Step Through Code 
Normally, your code runs unattended. It executes until its logical end. However, when you are testing code, it is often useful to step through the code line by line, watching each line of code take effect. This makes it easy to determine exactly what line is causing incorrect behavior. You can step through code line by line by pressing the F8 key to start the procedure in which the cursor is, or when VBA is paused at a break point. Pressing F8 causes VBA to execute each line one at a time, highlighting the next line of code in yellow. Note, the highlighted line is the line of code that will execute when you press F8. It has not yet been executed. 

If your procedure calls another procedure, pressing F8 will cause VBA to step inside that procedure and execute it line by line. You can use SHIFT+F8 to "Step Over" the procedure call. This means that the entire called procedure is executed as one line of code. This can make debugging simpler if you are confident that the problem does not lie within a called procedure. 

When you are in a called procedure, you can use CTRL+SHIFT+F8 to "Step Out" of the current procedure. This causes VBA to execute until the end of the procedure is reached (an End Sub or Exit Sub statement) and then stop at the line of code immediately following the line which called the procedure. 

Locals Window 
The Locals Window displays all the variables in a procedure (as well as global variables declared at the project or module level) and their values. This makes it easy to see exactly what the value of each variable is, and where it changes, as you step through the code. You can display the Locals Window by choosing it from the View menu. The Locals Window does not allow you to change the values of variables. It simply displays their names and values.

Watch Window 
The Watch Window allows you to "watch" a specific variable or expression and cause code execution to pause and enter break mode when the value of that variable or expression is True (non-zero) or whenever that variable is changed. (Note, this is not to be confused with the Watch object and the Watches collection). 

There are three types of watches, shown in the Watch Type group box. "Watch Expression" causes that watch to work much like the Locals Window display. It simply displays the value of a variable or expression as the code is executed. "Break When Value Is True" causes VBA to enter break mode when the watch variable or expression is True (not equal to zero). "Break When Value Changes" causes VBA to enter break mode when the value of the variable or expression changes value. 

You can have many watches active in your project, and all watches are displayed in the Watch Window. This makes is simple to determine when a variable changes value. 

Immediate Window 
The Immediate Window is a window in the VBE in which you can enter commands and view and change the contents of variables while you code is in Break mode or when no macro code is executing. (Break mode is the state of VBA when code execution is paused at a break point (see Breakpoints, below). To display the Immediate Window, press CTRL+G or choose it from the View menu. 

In the Immediate Window, you can display the value of a variable by using the ? command. Simply type ? followed by the variable name and press Enter. VBA will display the contents of the variable in the Immediate Window. For example, 


You can also execute VBA commands in the Immediate Window by omitting the question mark and entering the command followed by the Enter key:
Range("A1").Value = 1234

The Immediate Window won't let you enter VBA code snippets and execute them together because the Immediate Windows executes what you enter when you press the Enter key. However, you can combine several "logical" lines of code in to a single "physical" line of code using the ':' character, and execute this entire command. For example, to display each element of the array variable Arr use the following in the Immediate Window.

or N= LBound(Arr) To UBound(Arr): Debug.Print Arr(N) : Next N 

The Immediate Window always acts as if there were no Option Explicit statement in the active code module; that is, you don't have to declare variables you might use in Immediate Window commands. In fact, this is prohibited and you'll receive an error message if you attempt to use Dim in the Immediate Window. 

You can use the Debug.Print statement anywhere in your code to display messages or variable values in the Immediate Window. These statements don't require any confirmation or acknowledgement from the user so they won't affect the operation of your code. For example, you can send a message to the Immediate Window when a particular section of code is executed.
' some code

Debug.Print "Starting Code Section 1"

Unfortunately, there is no way to programmatically clear the Immediate Window. This is a shortcoming that has frustrated many programmers. 

In Excel 2000 and later, you can use Debug.Assert statements to cause the code to break if a condition is not met. The syntax for Debug.Assert is: 

Debug.Assert (condition) 

where condition is some VBA code or expression that returns True (any numeric non-zero value) or False (a zero value). If condition evaluates to False or 0, VBA breaks on that line (see Breakpoints, below). For example, the following code will break on the Debug.Assert line because the condition ( X < 100) is false. 

Dim X As Long
X = 123
Debug.Assert (X < 100)

Debug.Assert is a useful way to pause code execution when special or unexpected conditions occur. It may seem backwards that Debug.Assert breaks execution when condition is False rather than True, but this peculiarity traces its roots back to early C-language compilers. 

Remember, your end users don't want the code to enter break mode under any circumstances, so be sure to remove the statements before distributing your code, or use Conditional Compilation (see below) to create "release" and "debug" versions of your project. Note that Debug.Assert is not available in Excel97 or earlier versions. 

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