How to leave the body of a lambda expression

问题:

I have some list and I do list.ForEach( l => { ... something ...}). Now, on certain condition I need to stop iterating over the list, but break doesn't work - I get "Control cannot leave the body of an anonymous method or lambda expression" compilation error.

Any idea how to overcome that restriction?

回答1:

Using break alone won't work here because the lambda executes in a different method than the for loop. A break statement is only useful for breaking out of constructs local to the current function.

In order to support a break style leave you'd need to add an overload of ForEach where the delegate can specify via a return value that loop execution should break. For example

public static void ForEach<T>(this IEnumerable<T> enumerable, Func<T, bool> func) {
  foreach (var cur in enumerable) {
    if (!func(cur)) {
      break;
    }
  }
}

Now a consumer of this ForEach method can specify a break by returning false from the provided callback

myCollection.ForEach(current => {
  if (someCondition) {
    // Need to break
    return false;
  }
  // Keep going
  return true;
}

回答2:

A lambda expression works just like a method.
It can return whenever you want.

However, List.ForEach does not offer any way to prematurely stop the iteration.
If you need to break, you just use a normal foreach loop.

回答3:

You cannot stop iteration from within a ForEach lambda since you do not have control of the outer loop that is calling the lambda. At that point why don't you use a regular foreach loop and a break statement - that would be much more readable for this case.