D365F&O - X++ - The first parameter of AifCollectionTypeAttribute must match either a parameter of the method it is defined on or "return".

 The value in your attribute calls the parameter in method definition which is _TestParm in our case.

[DataMemberAttribute ("TestClass"),

AifCollectionTypeAttribute('_TestParm', Types::Class, classStr(CustClass)),

AifCollectionTypeAttribute('return', Types::Class, classStr(CustClass))]

public List parmTestMethod(List _TestParm = TestParm)

{

    TestParm = _TestParm;

    return TestParm;

}