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Local variables with Delegates

Solutons:


currentValue is no longer a local variable: it is a captured variable. This compiles to something like:

class Foo {
  public string currentValue; // yes, it is a field

  public void SomeMethod(object sender, EventArgs e) {
    Response.Write(currentValue);
  }
}
...
public Page_Index() {
  Foo foo = new Foo();
  foo.currentValue = "This is the FIRST value";
  this.Load += foo.SomeMethod;

  foo.currentValue = "This is the MODIFIED value";
}

Jon Skeet has a really good write up of this in C# in Depth, and a separate (not as detailed) discussion here.

Note that the variable currentValue is now on the heap, not the stack – this has lots of implications, not least that it can now be used by various callers.

This is different to java: in java the value of a variable is captured. In C#, the variable itself is captured.

I suppose more the question I am asking is that how is it working with a local variable
[MG edit: “Ack – ignore this…” was added afterwards]

That is the point; it really isn’t a local variable any more – at least, not in terms of how we normally think of them (on the stack etc). It looks like one, but it isn’t.

And for info, re “not good practice” – anonymous methods and captured variables are actually an incredibly powerful tool, especially when working with events. Feel free to use them, but if you are going down this route, I would recommend picking up Jon’s book to make sure you understand what is actually happening.

You need to capture the value of the variable within the closure/delegate, else it can be modified, like you saw.

Assign currentValue to a variable local (inside) to the delegate.

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