Lambda event handler scope

Question

Can someone give me a an eloquent, in depth explaination of why is this ok:

EventHandler e;

private void foobar(){
     e = new EventHandler((o, s) => {
           somectl.LayoutUpdated -= e;
     }
     somectl.LayoutUpdated += e;
}

But this is not:

private void foobar(){
     EventHandler e = new EventHandler((o, s) => {
           somectl.LayoutUpdated -= e;
     }
     somectl.LayoutUpdated += e;
}

Nor is this:

private void foobar(){
    EventHandler e;
    e = new EventHandler((o, s) => {
          somectl.LayoutUpdated -= e;
    }
    somectl.LayoutUpdated += e;
}

See my article on this subject: blogs.msdn.com/b/ericlippert/archive/2006/08/18/… — Eric Lippert
– Eric Lippert, Commented Apr 12, 2012 at 14:36

StriplingWarrior · Accepted Answer · 2012-04-12 14:24:05Z

This has less to do with the lambdas than you might think. For example, this would fail:

int i = i + 1;

As would this:

int i;
if (condition) {i = 0;}
i = i + 1;

Or:

int i;
if (condition) {Console.WriteLine(i);}
i = 1;

The lambda expressions fail for the same reasons: the middle one because you can't refer to any variable within its declaration, and the last because you can't guarantee that e will have been initialized before you attempt to use it within the lambda.

The first example works fine because fields are always initialized. They will either be given a value in the class's constructor, or they will be set to their default automatically. If you wanted to make this work within the scope of the method, you just need to assign your variable when you declare it.

private void foobar(){
    EventHandler e = null;
    e = new EventHandler((o, s) => {
          somectl.LayoutUpdated -= e;
    }
    somectl.LayoutUpdated += e;
}

SLaks · Accepted Answer · 2012-04-12 14:20:39Z

2

The last two won't compile because you can't reference e before you finish assigning it.

The first one works since this restriction does not apply to fields.

You can make the last one work by assigning it to null first so that it will be definitely assigned.

EventHandler e = null;
e = (o, s) => {
      somectl.LayoutUpdated -= e;
};
somectl.LayoutUpdated += e;

edited Apr 12, 2012 at 14:20

answered Apr 12, 2012 at 14:15

SLaks

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Comments

huysentruitw · Accepted Answer · 2012-04-12 14:19:06Z

0

In the last 2 cases, the compiler can check that you're working with an unassigned local variable inside the lambda. In the first case, because of the global variable, the compiler can't check this.

answered Apr 12, 2012 at 14:19

huysentruitw

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