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.
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 isn't 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.
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.
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 isn't 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: Setting the compatibility level to enable UDFs
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:
Figure 2: Create your first function with Tabular Editor 3
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: 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 for quick getting started syntax. *
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: 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 our first function: **Comparison.RelativeToTarget ***
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.
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 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.
If you want to try out how to set up this function yourself we have included the code for it here:
// 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
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.
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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