Event unsubscription via anonymous delegate [duplicate]

Question

This question already has an answer here:

• Unsubscribe anonymous method in C# 11 answers

I am using Resharper 5.1 code analysis many a times i get a comment from resharper as

"Event unsubscription via anonymous delegate"

#Part of Code  

if (((bool)e.NewValue))
{
    listView.PreviewTextInput += (o,args) =>
        listView_PreviewTextInput(o,args,listView);
}
else
{
    listView.PreviewTextInput -= (o, args) => 
        listView_PreviewTextInput(o, args, listView);
}

How could i correct or optimze this thing

Answer 1

You can extract the lamdba to a variable:

EventHandler func = (sender, e) =>
    listView_PreviewTextInput(sender, e, listView);

if (((bool)e.NewValue))
{
    listView.PreviewTextInput += func;
}
else
{
    listView.PreviewTextInput -= func;
}

Answer 2

Warning! Accepted answer from Steven is wrong, all it does is just masking a problem that resharper is warning about.

Every time given code is executed

 EventHandler func = (sender, e) =>
     listView_PreviewTextInput(sender, e, listView);

you'll get a fresh (since you may capture different listView) instance of anonymous delegate saved to func, an instance that's not subscribed to any events yet, so in turn this code

listView.PreviewTextInput -= func;

will effectively do nothing, since you cant unsubscribe from an event you're not subscribed to. This will lead to mind-boggling bugs like event handlers 'called twice', memory leaks etc.

Actually, Jon Skeet says it may work in some cases:

The C# specification explicitly states (IIRC) that if you have two anonymous functions (anonymous methods or lambda expressions) it may or may not create equal delegates from that code.

e.g. when compiler doesn't generate new instance every time, you'll see nice behavior.

But that's not reliable and certainly wouldn't work in case described in starter question with captured variable listView.

So my suggestion is:

Use anonymous functions as event handlers ONLY if you will never have to unsubscribe.