>Hi,
>
>I need to check return values of many API function calls within class. In order to simplify this task I have the following idea: assign a return value to a property of a class and test a value in Assign method for that property.
>

>Define Class test as Custom
>	Protected RetVal
>	Protected Procedure RetVal_Assign(tnRetVal)
>		If tnRetVal != 0
>			Error "Error #" + Trans(tnRetVal)
>		EndIf
>	EndProc
>	Procedure SomeMethod()
>		This.RetVal = SomeAPIFunction()
>	EndProc
>EndDefine
>

>However this code will generate an error within line with Error statement, but I want to generate an error within line where API function is called. Any ideas?

The problem you face is that no VFP error is actually created by the vast majority of API call error returns, so there's nothing to catch the error spontaneously. Further, not all API functions return a consistent value for - in some cases, a 0 represents ERROR_SUCCESS, other times, it indicates a failure. You have to code each API call separately.