All the functions in RadExpressionEditor's runtime are actual CLR methods, which have just been exposed in a list to make them easily discoverable for end-users. However, as every value in the RadExpressionEditor's expression is an actual CLR object, you can also call .Net methods, not only the predefined functions.
In short, when you enter a function call expression in RadExpressionEditor, the engine tries to resolve the function from all the methods in the type of the function call operand. So when you type "myField.StartsWith("bla")", it tries to call the StartsWith method of the type of the "myField" field, which is System.String.
Tl;dr version: even though a method or property is not exposed in RadExpressionEditor's list of available items, you can still call it in the expression, provided that the CLR type you are calling it on actually contains it.
the Telerik team
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