c# exit generic ForEach that use lambda

问题

Does anyone know if it is possible to exit a generic ForEach that uses lambda? e.g.

someList.ForEach(sl =>
  {
    if (sl.ToString() == "foo")
        break;

    // continue processing sl here
    // some processing code
  }
);

This code itself won't compile. I know I could use a regular foreach but for consistency I want to use lambda.

Many thanks.

回答1:

Sure. But first, note that I recommend against this; I say that a sequence operator should not have a side effect, and a statement should have a side effect. If you're doing something in that ForEach lambda, then make it a statement in the body of a foreach loop rather than making it look like a sequence operator.

That said, here's what you do. First, you write yourself a ForEach that works on arbitrary sequences, not just lists:

public static void ForEach<T>(this IEnumerable<T> sequence, Action<T> action)
{
    foreach(var item in sequence) action(item);
}

And now you write your break like this:

someList
    .TakeWhile(x=>x.ToString() != "foo")
    .ForEach(sl=>
    {/*your action here*/});

回答2:

From MSDN

The following rules apply to variable scope in lambda expressions:

snip

A lambda expression cannot contain a goto statement, break statement, or continue statement whose target is outside the body or in the body of a contained anonymous function.

Don't know if that helps given the code you posted. The relevant quote is from the end of the MSDN article.

回答3:

Warning: the code below is not to be taken seriously and is provided for entertainment purposes only!

You can 'simulate' a continue with an early return like this:

Enumerable.Range(1, 20)
          .ForEach(n =>
                {
                    if (n == 10) return;
                    Console.WriteLine("This is not 10: {0}", n);
                });

That said, I think that side effects within lambda's are a sign that you're doing it wrong. Use a proper foreach instead. Or something like TakeWhile, as Eric kindly demonstrated already.

回答4:

How about this?

        Enumerable.Range(1, 10)
        .Where(x => x % 2 != 0)
        .ToList()
        .ForEach(x => Console.WriteLine(x));

来源：https://stackoverflow.com/questions/2249420/c-sharp-exit-generic-foreach-that-use-lambda

标签

generics

lambda

foreach

break