How to get started using UDFs in Tabular Editor 3

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Picture of Morten Lønskov
Program Manager

Morten Lønskov is one of Tabular Editor’s Program Managers. In addition to producing content, recording release videos and engaging with our community, he plays an important strategic role in ensuring that our users get the best experience with Tabular Editor.

Tabular Editor 3 has full support for DAX User-Defined Functions (UDFs) built in right now. With the Tabular Editor version 3.23.1 or higher, we have worked hard to ensure that you have the best experience possible when writing and using UDFs. To get you started, here is a small guide on the first five steps of writing a UDF in Tabular Editor 3. 
NOTE
Tabular Editor 3 includes day 0 support for both new semantic model objects introduced at FabCon Europe 2025: DAX User-Defined Functions and Custom Calendars.
It is important to underline that this blog is not an attempt to teach you how UDFs work, but we will soon release a course on our learning platform that will give you all those details. So stay tuned.  
Step 1: New Compatibility Level – 1702
The first thing to do is to create a new model or upgrade your current model to compatibility level 1702. If UDFs do not show up as an option in the TOM Explorer you do not have the correct compatibility level set. 
NOTE

Compatibility Level is the TOM object term for enabling release-specific behaviors in the Analysis Service engine. For more information refer to Microsoft documentation on compatibility levels.  

UDFs are currently only available in Power BI Desktop and the Power BI/Fabric Service. Compatibility level 1702 is not available on SSAS and Azure AS. 

Additionally, compatibility level 1701 and higher also includes support for custom calendars. Check back soon for an article on these.

You update the compatibility level on an existing model by going to the TOM Explorer, selecting the top model object and under its properties locating the Compatibility level.  

Here is a small example of how that would look:
Figure_1_Compatability_Level
Figure 1: Setting the compatibility level to enable UDFs 
Step 2: Create a Function

As we saw in the previous section, UDFs introduce a new folder inside the TOM Explorer called Functions. This is where you create and store all the model’s functions. To create a new function, you can either: 

  • Go to the Model menu and select “Add User-Defined Function” or  
  • Right-click in the TOM Explorer on functions and choose Create -> User-Defined Function. 
Figure_2_Creating_Function
Figure 2: Create your first function with Tabular Editor 3 
Step 3: Starting from an empty Expression Editor

It’s very simple to make the function, but just like measures, you need to fill it with DAX and business logic for it to be valuable.  

Now on to write some DAX into that empty Expression Editor or? Well, not quite first we need to learn a little new syntax that is required when creating functions:  
Figure_3_FunctionBody
Figure 3: The core syntax of a UDF 
This syntax is required for a function definition. Luckily, you do not have to remember this by yourself, as Tabular Editor contains a nifty code action that inserts the syntax for you so it is as simple as one click to get started writing your first function.
Figure_4_UDF_Code_Action
Figure 4: UDF Code Action for quick getting started syntax.  
Step 4: Writing DAX UDFs

The function is there, the code action has given us a quick start on the UDF syntax, where do we go from here? A good place to start is to look through your model and identify a DAX pattern that you reuse across several measures.  

Here is an example of a measure that calculates a comparison related to a specific target measure. Here, the Orders Target is used to calculate the delta with the [Net Orders], and then a percentage difference calculation is done.  
Figure_5_Measure_To_Create_Function
Figure 5: An example of a measure, [Orders Target vs. Net Orders (%)] that could be turned into a function 

This pattern is used several times in the model. It is thus a prime candidate to use a function instead in order to centralize your business logic, making it easy to reuse and maintain across your model. 

In the following demo, we create a function that can replace the code in the measure with our new function.  

One thing to do before writing DAX is to give your new function a meaningful name. Giving your function a good name is the first step to you and others knowing how to use it. In this case, we will call it Comparison.RelativeToTarget  
Figure_6_Writing_A_UDF
Figure 6: Writing our first function: Comparison.RelativeToTarget  
TIP 
Creating a naming standard for your functions will help with maintenance throughout your development process. One current best practice is to use a [Area].[SubArea].[FunctionName] naming standard.  
The syntax of the [Comparison.RelativeToTarget] is described in the figure below. Tabular Editor 3 uses the comments you make for your functions and parameters to populate the tooltips when you use the function elsewhere. We will see this in action in the next step. 
Figure_7_FunctionExample
Figure 7: The finished function Comparison.RelativeToTarget  
Step 5: Using UDF in a measure
Now, let's see how we can use the function we just wrote within a measure to replace the logic. We will locate the measure from the previous step and see how our function can simplify the measure’s logic and be reused across other measures as well. 
Figure_8_Using_UDF_in_Measure
Figure 8: Using the function in a measure.  
As you can see in Figure 8, the DAX expression of the original measure can be simplified with the new comparison function we created. And while you are writing your functions, Tabular Editor’s code assistance guides you along and uses the comments of the functions and parameters in the call tips that are available to the user, making it easy to understand what the function is for and what to use in the parameters.  

UDF Code Example

If you want to try out how to set up this function yourself we have included the code for it here:

‘’’DAX 
// Compare to a target and return the relative percent 

// Example usage: Comparing sales to budget or forecast to get relative performance 

// Examples: 

// - 9/10  is -10% or -1 

// - 11/10 is +10% or +1 

( 

    // The actual value that you are measuring 

    actualMeasure : SCALAR NUMERIC VAL, 

    // The target that you are comparing to 

    target : SCALAR NUMERIC VAL 

) 

=>  

    VAR _Actual       = actualMeasure 

    VAR _Target       = target 

    VAR _Delta        = _Actual - _Target 

    VAR _Percentage   = DIVIDE ( _Delta, _Target ) 

    RETURN 

        _Percentage 

‘‘‘

DAX Lib

The community is bringing together a library of DAX UDF that you can use as templates. Find them at daxlib.org. It's all free and available for you to get started right away with adding functions to your semantic model.  

Conclusion

Tabular Editor 3 is a one-stop IDE for building User Defined Functions into your semantic model, and provides assistance on how to get started and get the syntax right the first time around, as well as helping you document how to use your functions inside your measures without having to refer back to their definition. 

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