Anonymous Methods in Delphi

Introduction

Under the scope of Delphi, an anonymous method is either a procedure or function that’s unattached to an identifier. In other words, anonymous methods don’t have names, which is why they are called “anonymous”.

Basically, you can assign a block of code (in the form of a procedure or function) directly to a variable.

I am going to give you a simplistic example. I am going to Keep it simple, Stupid! to avoid distracting you with any complexity.

This is the wording of the task: create a console application which prints “Hello world” and “Good bye” to the standard output. Constraint: Use the Writeln function just once in the code.

To accomplish such reckless task in the old days (before the introduction of anonymous methods), you could do this:

program Project1;

{$APPTYPE CONSOLE}

procedure PrintString(aText: string);
begin
  Writeln(aText);
end;

begin
  PrintString('Hello world');
  PrintString('Good bye');
  Readln;
end.

How to do the same with anonymous methods? Take a look at the following code:

{$APPTYPE CONSOLE}

type
  TMyAnonymousProcedureType = reference to procedure(aText: string);

var
  A: TMyAnonymousProcedureType;

begin
  A:= procedure(aText: string) //No semicolon here
  begin
    Writeln(aText);
  end;
  A('Hello world');
  A('Good bye');
  Readln;
end.

As you can see in the example above, we have assigned code directly to the variable A. Then, we called A with parameters, and voilà!: we have accomplished our reckless task with anonymous methods as well.

Pay attention to this: if you are tented to declare variable A like this:

var
  A: reference to procedure(aText: string);

Don’t! That shortcut doesn’t work. You’ll get a compilation error… like this:

Undeclared identifier: ‘reference’

type 
  TMyAnonymousProcedureType = reference to procedure(aText: string);

Only later, you can define the type of variable A.

You might be asking by now: why to bother with all this? What's the benefit? Well, in the previous example there’s little or no benefit present.

Generally speaking, anonymous methods are handy in the following cases:

• You have been trying to name a local method for hours. You cannot think of a name for it. Well, think no more: use anonymous methods. Don’t put a tasteless name like Foo(), XXX(), Aux(), etc.
• You create a function that is called just once (it’s just called from one spot).
• You can use anonymous methods to provide elegant and simpler implementations. This is the case when combining generics types with anonymous methods for example. I should write about this shortly. Subscribe to my feed and stay tuned

With this post, I wanted to introduce anonymous methods to you. It’s OK if you don’t see the benefits clearly right now. You’ll get there.