System.Variants.Null
Delphi
function Null: Variant;
C++
extern DELPHI_PACKAGE System::Variant __fastcall Null(void);
Properties
| Type | Visibility | Source | Unit | Parent |
|---|---|---|---|---|
| function | public | System.Variants.pas System.Variants.hpp |
System.Variants | System.Variants |
Description
Returns a Null variant.
Use Null to obtain a Null variant that can indicate unknown or missing data. Null Variants can be assigned to variant variables in an application that must contain a null value. Assigning Null to a variant variable does not cause an error, and Null can be returned from any function with a variant return value.
Assigning Null to a variable of any type other than Variant causes either a compile-time error or raises an EVariantTypeMismatch exception. For example, in the following code the assignment of v, the Null variant, to variant q is successful; whereas the conversion of variant v, which is now Null, to the integer return type of the Test function, raises an exception.
function Test(v: Variant): Integer; var q: Variant; msg: string; begin q := v; { this is ok, since q is a variant } if VarIsNull(q) then msg := 'q is a null variant' else msg := 'q is not a null variant'; ShowMessage(msg); Result := v; { this raises an exception!!!} end; procedure TForm1.Button1Click(Sender: TObject); begin Test(Null); end;
int Test(Variant v)
{
Variant q = v; // this is ok, since q is a variant
AnsiString msg;
if (VarIsNull(q))
msg = "q is a null variant";
else
msg = "q is not a null variant";
ShowMessage(msg);
return v; // this throws an exception!!!
}
void __fastcall TForm1::Button1Click(TObject *Sender)
{
Test(Null());
}
Except for comparisons, which always result in Boolean values, expressions involving Null variants always result in Null variants.
Note: The Null variant returned by the Null function is different from the nil constant in Delphi code and the NULL macro in C++ code. Neither nil nor NULL is a variant. The nil constant is an object reference that does not refer to any actual object. The NULL macro is a pointer that does not refer to any actual object or value.