Common Flash Compiler Errors: #1042

This error shows the following in the Compiler Errors window:


1042: The this keyword can not be used in static methods. It can only be used in instance methods, function closures, and global code.


Quick Answer and Solution

Most likely you are you using the keyword “this” inside a class file, but it is not inside the curly braces of a function. By double-clicking on the error it will take you to the offending line of code. Check carefully to make sure where you have placed the keyword “this” is actually between the opening “{” and closing “}” braces of a function declaration. If it looks correct, but is accompanied by error #1126 (described here), then fix that error first. Also, if the “this” keyword appears in a function that has the “static” keyword in its declaration, error #1042 will also be thrown.

The following are three scenarios where a class file will throw this error:

package
{
	import flash.display.MovieClip;
 
	public class Main extends MovieClip
	{
		public function Main()
		{
		}
		// "this" keyword used outside a function
		this.visible = false;
	}
}

package
{
	import flash.display.MovieClip;
 
	public class Main extends MovieClip
	{
		public function Main()
		{
		}
 
		// function does not have a body because of misplaced semicolon,
		// so error #1126 is thrown
		public function hide():void;
		{
			// the "this" keyword is actually not in the body of a function
			// because of the above #1126 error
			this.visible = false;
		}
	}
}

package
{
	import flash.display.MovieClip;
 
	public class Main extends MovieClip
	{
		public function Main()
		{
		}
 
		// function is a "static method," meaning it runs inside the class,
		// not the object created from the class
		static public function hide():void
		{
			// the "this" keyword is inside a static method
			this.visible = false;
		}
	}
}

The corrected form of the above examples would be:

package
{
	import flash.display.MovieClip;
 
	public class Main extends MovieClip
	{
		public function Main()
		{
		}
 
		public function hide():void
		{
			// the reference to "this" is inside a non-static function
			// (called an instance method)
			this.visible = false;
		}
	}
}

Technical Overview

There are two types of methods (functions) that can appear in a class file, static methods and instance methods. Static methods refer to methods that are run via the class itself, while instance methods refer to methods run via an instance of the class. Consider the following class file:

package
{
	import flash.display.MovieClip;
 
	public class Main extends MovieClip
	{
		public function Main()
		{
		}
 
		static public function staticMethod():void
		{
			trace( "static method called!" );
		}
 
		public function instanceMethod():void
		{
			trace( "instance method called!" );
		}
	}
}

To call the first method, a new “Main” object does not need to be created, as the static method can be called on the class itself, like so:

Main.staticMethod();

Conversely, to access the other method, an (object) instance of the Main class needs to be created and then the method can be called through that object, like so:

var main:Main = new Main();
main.instanceMethod();

Use of the “this” keyword refers to the object instance of the class that the code is currently dealing with (in the above case if this appeared in the body of the instanceMethod() method it would be referring to the object placed in the main variable). Since no object instance has to be created to use static methods, the “this” keyword can not be used within those methods because there is no object in existence to refer to. Additionally, all code outside of a function declaration runs before an object instance is created (before the constructor function runs), so any references to “this” do not have an object to refer to in that context either.