Excel UDF not working: problems and solutions

This article addresses common problems encountered when using custom functions (UDFs) in Excel. We'll explore troubleshooting solutions for recalculation issues, volatile vs. non-volatile functions, function unavailability, and missing help text.

Key Topics:

• Excel UDF Recalculation Problems
• Volatile vs. Non-Volatile UDFs
• UDF Unavailability
• Missing Excel UDF Help Text

Why Excel UDFs Don't Always Recalculate:

Unlike standard Excel functions, UDFs aren't automatically linked to cell changes for recalculation. Excel can't analyze VBA code to determine dependencies. To force recalculation, use Application.Volatile. (See below for details).

Volatile vs. Non-Volatile Custom Functions:

By default, UDFs are non-volatile; they recalculate only when their input cell values change. File or worksheet name changes won't trigger recalculation. For example, a function returning the workbook name (ThisWorkbook.Name) won't update if the workbook is renamed unless declared volatile.

To make a UDF volatile, add Application.Volatile at the function's beginning. This ensures recalculation whenever any worksheet change occurs. However, overuse can slow down Excel.

Why Custom Functions May Be Unavailable:

UDFs must reside in standard VBA modules (the "Modules" folder). Placing them in "Microsoft Excel Objects" or worksheet code prevents them from appearing in the function dropdown or working correctly.

Missing Excel Custom Function Help Text:

To add help text (descriptions and argument hints) to your UDFs, use Application.MacroOptions. This displays descriptions in the Function Arguments window (accessed via the fx button or Ctrl A).

Example using Application.MacroOptions for the GetMaxBetween function:

Sub RegisterUDF()
    Application.MacroOptions Macro:="GetMaxBetween", _
                            Description:="Maximum number in a specified range", _
                            Category:="My Custom Functions", _
                            ArgumentDescriptions:=Array("Numeric value range", "Lower bound", "Upper bound")
End Sub

This adds the function to the "My Custom Functions" category and provides descriptions for the function and its arguments. To remove the help text, use Application.MacroOptions with empty values. Alternatively, Ctrl Shift A provides argument hints, though without descriptions. For richer IntelliSense, consider the Excel-DNA IntelliSense extension.

This guide provides solutions to common UDF issues. If problems persist, please provide detailed information in the comments.

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