How do I exit a List<string>.ForEach loop when using an anonymous delegate?

There is no loop that one has access to, from which to break. And each call to the (anonymous) delegate is a new function call so local variables will not help. But since C# gives you a closure, you can set a flag and then do nothing in further calls:

bool stop = false;
myList.ForEach((a) => {
  if (stop) {
    return;
  } else if (a.SomeCondition()) {
    stop = true;
  }
});

(This needs to be tested to check if correct reference semantics for closure is generated.)

A more advanced approach would be to create your own extension method that allowed the delegate to return false to stop the loop:

static class MyExtensions {
  static void ForEachStoppable<T>(this IEnumerable<T> input, Func<T, bool> action) {
    foreach (T t in input) {
      if (!action(t)) {
        break;
      }
    }
  }
}

Do you have LINQ available to you? Your logic seems similar to Any:

bool any = blackList.Any(s=>inputString.Contains(s));

which is the same as:

bool any = blackList.Any(inputString.Contains);

If you don't have LINQ, then this is still the same as:

bool any = blackList.Find(inputString.Contains) != null;

If you want to run additional logic, there are things you can do (with LINQ) with TakeWhile etc

I don't think there's an elegant way to do it when using the ForEach method. A hacky solution is to throw an exception.

What's preventing you from doing an old fashioned foreach?

foreach (string item in blackList)
{
    if (!inputString.Contains(item)) continue;

    result = true;
    break;
}

If you want a loop, use a loop.

Action allows for no return value, so there's no way the ForEach function could possibly know that you want to break, short of throwing an exception. Using an exception here is overkill.

The only way to "exit" the loop is to throw an exception. There is no "break" style way of exiting the .ForEach method like you would a normal foreach loop.

The ForEach method is not mean to do this. If you want to know if a collection contains an item you should use the Contains method. And if you want to perform a check on all items in a collection you should try the Any extention method.

bool @break = false;

blackList.ForEach(item =>
 {  
    if(!@break && inputString.Contains(item))
     { @break = true;
       result = true;
     }

    if (@break) return;
    /* ... */
 });

Note that the above will still iterate through each item but return immediately. Of course, this way is probably not as good as a normal foreach.

    class Program
{
    static void Main(string[] args)
    {
        List<string> blackList = new List<string>(new[] { "jaime", "jhon", "febres", "velez" });
        string inputString = "febres";
        bool result = false;
        blackList.ForEach((item) =>
                              {
                                  Console.WriteLine("Executing");
                                  if (inputString.Contains(item))
                                  {
                                      result = true;
                                      Console.WriteLine("Founded!");
                                  }
                              },
                          () => result);
        Console.WriteLine(result);
        Console.ReadLine();
    }


}
public static class MyExtensions
{
    public static void ForEach<T>(this IEnumerable<T> enumerable, Action<T> action, Func<bool> breakOn)
    {
        foreach (var item in enumerable)
        {
            action(item);
            if (breakOn())
            {
                break;
            }
        }
    }
}

Would this work for you:

bool result = null != blackList.Find( item => inputString.Contains(item)) );

blackList.ForEach(new Action<string>(
    delegate(string item)
    {
        if(inputString.Contains(item)==true)
        {
            result = true;
            // I want to break here
            return; //add a return here to exit
        }
    }
));

if you realy want to exist a loop foreach in a list you could use the exception like this code:

public class ExitMyForEachListException : Exception
{
    public ExitMyForEachListException(string message)
        : base(message)
    {
    }
}
class Program
{
    static void Main(string[] args)
    {
        List<string> str = new List<string>() { "Name1", "name2", "name3", "name4", "name5", "name6", "name7" };
        try
        {
            str.ForEach(z =>
            {
                if (z.EndsWith("6"))
                    throw new ExitMyForEachListException("I get Out because I found name number 6!");
                System.Console.WriteLine(z);
            });
        }
        catch (ExitMyForEachListException ex)
        {
            System.Console.WriteLine(ex.Message);
        }

        System.Console.Read();
    }
}

hope this help to get other point of view.