Delphi Basics : Null command

DelphiBasics

Null
Variable

A variable that has no value

Variants unit

var Null : Variant;

Description

The Null variable has an undefined value.

Null does not mean a zero number or empty string - it is undefined. Any expression using a null variable will yield a null result.

Null is particularly useful when handling SQL table data - providing a one for one correspondence with the SQL NULL value.

Notes

The Null variable is the Null Variant.

Calculations using nulls yield the EVariantError.

Related commands

Pointer		Defines a general use Pointer to any memory based data
Variant		A variable type that can hold changing data types

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Example code : Using Null to represent the result of a bad divide

var
  Answer : Variant;

begin
  Answer := Divide(4,2);

  // Show the result of this division
  if Answer = Null
  then ShowMessage('4 / 2 = Invalid')
  else ShowMessage('4 / 2 = '+IntToStr(Answer));

  Answer := Divide(4,0);

  // Show the result of this division
  if Answer = Null
  then ShowMessage('4 / 0 = Invalid')
  else ShowMessage('4 / 0 = '+IntToStr(Answer));

end;

function TForm1.Divide(Dividend, Divisor: Integer) : Variant;
begin
  // Try to divide the Dividend by the Divisor
  try
    Result := Dividend div Divisor;
  except
    Result := Null ;    // Assign Null if the division threw an error
  end;
end;

Show full unit code

4 / 2 = 2
4 / 0 = Invalid