Microsoft Office

Part VI
Programming Excel with VBA
What a Function Can’t Do
Almost everyone who starts creating custom worksheet functions using VBA makes a fatal mistake: They try
to get the function to do more than is possible.
A worksheet function returns a value, and the function must be completely “passive.” In other words, the
function can’t change anything on the worksheet. For example, you can’t develop a worksheet function that
changes the formatting of a cell. (Every VBA programmer has tried, and not one of them has been successful!)
If your function attempts to perform an action that isn’t allowed, the function simply returns an error.
VBA functions that aren’t used in worksheet formulas can do anything that a regular Sub procedure can do —
including changing cell formatting.
To create a custom function, follow these steps:
1. Activate the VB Editor (press Alt+F11).
2. Select the workbook in the Project window.
3. Choose Insert ➪ Module to insert a VBA module. (Or you can use an existing module.)
4. Enter the keyword Function followed by the function’s name and a list of the arguments (if
any) in parentheses.
5. Insert the VBA code that performs the work — and make sure that the variable corresponding
to the function’s name has the appropriate value when the function ends. (This is the
value that the function returns.)
6. End the function with an End Function statement.
Function names that are used in worksheet formulas must adhere to the same rules as variable names.
Executing Function Procedures
You can execute a Sub procedure in many ways, but you can execute a Function procedure in just two ways:
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Call it from another VBA procedure.
Use it in a worksheet formula.
Calling custom functions from a procedure
You can call custom functions from a procedure just as you call built-in VBA functions. For example, after
you define a function called CalcTax, you can enter a statement such as the following:
Tax = CalcTax(Amount, Rate)
This statement executes the CalcTax custom function with Amount and Rate as its arguments. The function’s
result is assigned to the Tax variable.
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