pbiplintRules / Error Prevention
User-defined function with a one-word name
What it checks
User-defined functions whose name holds neither a dot nor an underscore, such as AddTax. Tabular Editor 3 has a built-in rule with the same test.
Each finding names the function as the model does and points at its function line in definition/functions.tmdl. Functions installed from a DAX Lib package are checked too; their names start with the package's name, so they pass.
Example
table Sales
column Amount
dataType: decimal
sourceColumn: Amount
measure 'Sales With Tax' = AddTax(SUM('Sales'[Amount]))
formatString: #,0
/// Adds 10 percent sales tax to an amount.
function AddTax = (amount: NUMERIC) => amount * 1.1
table Sales
column Amount
dataType: decimal
sourceColumn: Amount
measure 'Sales With Tax' = Local.AddTax(SUM('Sales'[Amount]))
formatString: #,0
/// Adds 10 percent sales tax to an amount.
function 'Local.AddTax' = (amount: NUMERIC) => amount * 1.1
Why it matters
Most of DAX's built-in functions have one-word names, and new ones arrive with Power BI releases. Microsoft's naming rules say a function's name "Must not conflict with built-in DAX functions" (Define and manage user-defined functions), but a one-word name that is free today can be taken by a built-in tomorrow. Microsoft does not say what happens to the model's function then. Tabular Editor's guidance says that "the built-in function takes precedence and your UDF will stop working" (Use compound names for user-defined functions), so every call to it would reach the built-in instead.
A dot or an underscore marks the name as the model's own. SQLBI's naming conventions recommend a Local. prefix for a model's own functions "to avoid conflicts with future DAX function names" (Function names), and a library's functions start with the library's name.
How to fix it
Rename the function to a compound name, such as Local.AddTax for a function of the model's own, or a prefix for your organization or library.
In Power BI Desktop's Model view, select Model at the top of the Data pane to open Model explorer, right-click the function under Functions, choose Rename, and enter the new name; Desktop updates the measures and functions that call it.
In TMDL, change the name on the function's line in definition/functions.tmdl, in single quotes when it holds a dot (function 'Local.AddTax' =), and at every call in the files under definition/, where it is written without quotes (Local.AddTax(). Searching those files for the old name followed by an opening parenthesis finds the calls.
When to ignore it
When the name is fixed by something outside the model: a live-connected report's own measures, or a DAX query kept elsewhere, calls the function by that name, and renaming it would break them where nothing in the model shows it. Otherwise the finding is not noise, though the risk it guards against lies in future releases rather than in the model today.
To ignore this rule on one object, add annotation pbiplint.ignore = UDF_USE_COMPOUND_NAMES under the object in its TMDL file. Power BI Desktop keeps the annotation. To turn the rule off for a whole project, set "UDF_USE_COMPOUND_NAMES": "off" under rules in pbiplint.config.json.
Quirks
- The test is Tabular Editor's: a dot or an underscore anywhere in the name passes, so
_toggleButtonpasses, and so doesadd_tax. - A dot is no guarantee against a clash. Microsoft's own built-ins include dotted names, such as
INFO.USERDEFINEDFUNCTIONS. - A function from a DAX Lib package is checked as any other, since its name is what callers write.
Related rules
UDF_NOT_CALLEDreports a function nothing calls, which is cheaper to delete than to rename.UDF_WITHOUT_DESCRIPTIONreports a function with no description, a good moment to add one while the function is open.