Saturday, 15 February 2014

With Keyword in Delphi

With Keyword in Delphi

The With keyword is a convenience provided by Delphi for referencing elements of a complex variable, such as a record or object. 

It simplifies the code by removing the need to prefix each referenced element with the complex variable name. 

For example: 

myObject.colour := clRed;
myObject.size   := 23.5;
myObject.name   := 'Fred'; 

can be rewritten : 

With myObject do
begin
  colour := clRed;
  size   := 23.5;
  name   := 'Fred';
end; 

However be warned that it can surprisingly make your code more difficult to read, especially when nesting With clauses. More disturbingly, it can create maintenance problems, where a code change can mean that the wrong target for the 'child' field referenced.

Example code: Using the with keyword with a record structure

type
  // Declare a customer record
  TCustomer = Record
    firstName : string[20];
    lastName  : string[20];
    address1  : string[100];
    address2  : string[100];
    address3  : string[100];
    city      : string[20];
    postCode  : string[8];
  end;

var
  John, Sarah : TCustomer;

begin
  // Set up the John's customer details
  With John do
  begin
    firstName := 'John';
    lastName  := 'Smith';
    address1  := '7 Park Drive';
    address2  := 'Branston';
    address3  := 'Grimworth';
    city      := 'Banmore';
    postCode  := 'BNM 1AB';
  end;

  // Set up John's sister similarly - simply copying the whole record
  Sarah := John;

  // And then changing the first name to suit
  Sarah.firstName := 'Sarah';

  // Now show the details of both customers
  With John do ShowCustomer([firstName, lastName,
                             address1, address2, address3, city,
                             postCode]);
  With Sarah do ShowCustomer([firstName, lastName,
                             address1, address2, address3, city,
                             postCode]);
end;

// A procedure that displays a variable number of strings
procedure TForm1.ShowCustomer(const fields: array of string);
var
  i : Integer;

begin
  // Display all fields passed - note : arrays start at 0
  for i := 0 to Length(fields)-1 do
    ShowMessage(fields[i]);
end;

Output

John
Smith
7 Park Drive
Branston
Grimworth
Banmore
BNM 1AB

Sarah
Smith
7 Park Drive
Branston
Grimworth
Banmore
BNM 1AB

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