PCount() and a single NULL param

[Karl1155]

PCount() returns 0 when I pass a single NULL param. There is no problem if more than one NULL param is passed.

The Fox- and VO-dialect results are the same.

FUNCTION Start() AS VOID 
	
? TestNullParams()	  // ok, 0
? TestNullParams(NULL)  // 0 <--- instead of 1
? TestNullParams(NULL,NULL) // ok, 2 
? TestNullParams(NULL,NULL,NULL) // ok, 3 
? TestNullParams(NULL,NULL,NULL,NULL) // ok, 4 	 

RETURN

FUNCTION TestNullParams( one , two , three ) AS INT CLIPPER
	
	RETURN PCount() 

Karl-Heinz

[RobertvanderHulst]

Karl,
Clipper calling convention translates the variables into a params array of usuals.
I think the Roslyn backend assumes that the call with 0 tries to send in a NULL for the array instead of an array with a single value null in it.
Fortunately the code that is generated for clipper calling covention checks for null, otherwise this would have created a runtime error.

[Karl1155]

Robert,

If I call the function with a fox .NULL. , PCount() returns 1.

? TestNullParams(NULL)  // 0 <--- instead of 1
? TestNullParams(.NULL.)  // ok, 1

Looking at the generated code shows the differences:

XSharp.RT.Functions.QOut(TestNullParams(null))
XSharp.RT.Functions.QOut(TestNullParams(DBNull.Value))

Are you aware of that in VFP .NULL. and NULL are the same thing? At least, this test seems to prove it.

VFP code:

private x , y 

x = NULL 
y = .NULL.

? x   && .NULL.
? y   && .NULL.

Karl-Heinz

[RobertvanderHulst]

Karl,
The FoxPro .NULL. value is translated to DBNull.Vaue by the compiler. This is not a valid value for the __Usual[] parameter of the TestNullParams() function, so the compiler creates an array of usuals and stores the value in the first element.
Null however is seen as a valid argument, so the compiler does not create an array.
I will fix this in a future build.
We cannot make NULL and .NULL. the same in X#. That would cause all kind of side effects.

[jpmoschi]

Robert, Karl
I want to contribute from my point of view, which is the need to grow the compiler to encompass FoxPro.
The parameter of a function, method o free procedure is an array of cero or more values, any objects, not only an array of usual values. For example a null or .null. value may be an argument , similar to NIL Usual value

[RobertvanderHulst]

Juan,
I am sorry but you apparently did not get how we have implemented parameter passing in what we call "CLIPPER" calling convention. It should probably have been called XBase calling convention.
Since functions and procedures in Xbase can declare untyped parameters and can receive more or less parameters than declared we have created a special method to handle these parameters in what's called a param array. This is a standard dotnet features that is also used by methods such as String.Format (https://docs.microsoft.com/en-us/dotnet/api/system.string.format?view=net-6.0#system-string-format(system-string-system-object())) where second argument receives zero or more values.
In our implementation we have given the array a name that cannot be used by normal variables (Xs$Args) and we have added an attribute to the function/method that contains the parameter names, so the parameter tips in the editor can show the correct names. We also use this attribute in the compiler to check if you are not overriding a method with clipper calling convention with a strict method (non clipper method).
Inside the function/method we assign the values from the Xs$Args array to local variables with the names of the parameters that you have declared, and the PCOUNT() function returns the length of the array.
The problem that Karl describes is that the compiler does not see null as the first value that should be stored in the array, but it sees null as the array itself.

[Karl1155]

Juan,

in addition to what Robert said: The problems is that TestNullParams(NULL) doesn´t create an array of type usual. There are some pseudo functions that makes it easier to understand how such so called "CLIPPER" functions work. The pseudo function _Args() that i´m using in the modified TestNullParams() is described here:

https://www.xsharp.eu/help/pseudo-functions.html

?? TestNullParams()	  // ok, 0 
?? TestNullParams(NULL) // 0 <---  instead of 1
?? TestNullParams(NULL,NULL) // ok, 2  
?? TestNullParams(NULL,NULL,NULL) // ok, 3 
?? TestNullParams(NULL,NULL,NULL,NULL) // ok, 4 	

FUNCTION TestNullParams( one , two , three ) AS INT CLIPPER 

	// the only reason i added the check is to avoid a runtime error
	// due to the posted issue.

	IF _ARGS() != NULL 
		? _ARGS() ,  _ARGS():Length == PCount() , "" 
	ELSE	
		? "This must never happen, but something went wrong - no array !" , ""
	ENDIF

	RETURN PCount() 

Karl-Heinz

[cpyrgas]

This also looks to be fully fixed now.