DAX User-defined functions
Hatem Meddeb
Architect
In September 2025, a new feature was introduced in Power BI: User-Defined DAX Functions (UDFs) . This introduction represents a true revolution in the use of the DAX language. Developers are no longer limited to native DAX functions and no longer need to create measures with redundant and difficult-to-maintain logic. With UDFs, they can define their own functions, giving them greater flexibility in development.
1. The advantages for DAX development
- Centralization and reusability: A business rule is defined only once and can be used in multiple measures, calculated columns, visual calculations, or other user-defined functions.
- Simplified evolution and maintenance: Modifying a function automatically updates all dependent calculations.
- Portability: Functions can be grouped into libraries and shared between different semantic models.
- Readability: Measures are more readable by replacing complex blocks with function calls.
- Fewer errors: The use of type indicators and checking functions makes the function more error-resistant.
2. Definition and integration into the model
Before you can try out user-defined functions, you must enable them in the “features coming soon” section of the Power BI desktop options.
Functions can be defined either in the DAX query view (DQV) with the DEFINE command , or in the TDML (Tabular Model Definition Language) view with the createOrReplace command.
The syntax of a user-defined function is as follows:
FUNCTION <FunctionName> = ([ParameterName]: [Type] [SubType] [Mode], …) =>
<Function body>
The function name must be unique within the model; it may contain periods, but no spaces or special characters. It must not be the name of a built-in DAX function or a reserved word.
A user-defined function can accept zero or more parameters. A parameter name can contain alphanumeric characters or underscores “_”. It must not be a reserved word.
For each parameter defined in the function, the [Type] [SubType] [Mode] flags can be specified. If they are not, the default value “anyval val” will be used.
These indicators are summarized in the following table:
| Type | Subtype | Mode | Remarks |
|---|---|---|---|
| anyval | val | Accepts any type of scalar data or a table. This is the default type. | |
| scalar | val / expr | Accepts a scalar value. | |
| variant | val / expr | Accepts a scalar value. | |
| Int64 | val / expr | Accepts an integer. | |
| Decimal | val / expr | Accepts one decimal place with fixed precision. | |
| Double | val / expr | Accepts one decimal place with fixed precision. | |
| String | val / expr | Accepts text. | |
| Datetime | val / expr | Accepts a date/time. | |
| Boolean | val / expr | Accepts True/False. | |
| Numeric | val / expr | Accepts a numerical value. | |
| Table | val / expr | Accepts only one table. | |
| Anyref | expr | Accepts a reference to a table, column, measure, or calendar. |
The “mode” indicator defines how parameters are passed and influences the behavior of the function.
If the mode is “val”, the parameter is evaluated immediately before the function call and its value is fixed. Conversely, if it is “expr”, it is evaluated within the function body and changes according to the internal context.
A user-defined function can have built-in documentation. Using “///” directly above the function allows a description to be displayed in the tooltip when it is used. This differs from “//” or “/**/” which are used for comments.
Once a user-defined function is registered in the model, it is called in the same way as built-in DAX functions.
All user-defined functions can be displayed from the model explorer under the “Functions” node or via the DMV (Dynamic Management Views) query “EVALUATE INFO.FUNCTIONS(“ORIGIN”, “2”)”
3. Current limitations
- No recursion: A function cannot call itself.
- No optional parameters: All arguments of a function are required.
- OLS security: User-defined functions do not inherit object-level security.
- Refactoring: Renaming an object used in a UDF does not automatically update the function code.
4. Conclusion
With the arrival of user-defined functions in Power BI, DAX is moving from a formula expression language to a flexible and modular programming tool that allows business logic to be separated from the presentation layer.
Reference :
https://learn.microsoft.com/en-us/dax/best-practices/dax-user-defined-functions
