When to use closure? [closed]

Closures are simply great tools. When to use them? Any time you like… As has already been said, the alternative is to write a class; for example, pre C# 2.0, creating a parameterised thread was a real struggle. With C# 2.0 you don’t even need the `ParameterizedThreadStart’ you just do:

string name = // blah
int value = // blah
new Thread((ThreadStart)delegate { DoWork(name, value);}); // or inline if short

Compare that to creating a class with a name and value

Or likewise with searching for a list (using a lambda this time):

Person person = list.Find(x=>x.Age > minAge && x.Region == region);

Again – the alternative would be to write a class with two properties and a method:

internal sealed class PersonFinder
{
    public PersonFinder(int minAge, string region)
    {
        this.minAge = minAge;
        this.region = region;
    }
    private readonly int minAge;
    private readonly string region;
    public bool IsMatch(Person person)
    {
        return person.Age > minAge && person.Region == region;
    }
}
...
Person person = list.Find(new PersonFinder(minAge,region).IsMatch);

This is fairly comparable to how the compiler does it under the bonnet (actually, it uses public read/write fields, not private readonly).

The biggest caveat with C# captures is to watch the scope; for example:

        for(int i = 0 ; i < 10 ; i++) {
            ThreadPool.QueueUserWorkItem(delegate
            {
                Console.WriteLine(i);
            });
        }

This might not print what you expect, since the variable i is used for each. You could see any combination of repeats – even 10 10’s. You need to carefully scope captured variables in C#:

        for(int i = 0 ; i < 10 ; i++) {
            int j = i;
            ThreadPool.QueueUserWorkItem(delegate
            {
                Console.WriteLine(j);
            });
        }

Here each j gets captured separately (i.e. a different compiler-generated class instance).

Jon Skeet has a good blog entry covering C# and java closures here; or for more detail, see his book C# in Depth, which has an entire chapter on them.