How to convert Func<T, bool> to Predicate<T>?

Question

Yes I've seen this but I couldn't find the answer to my specific question.

Given a lambda testLambda that takes T and returns a boolean (I can make it either Predicate or Func that's up to me)

I need to be able to use both List.FindIndex(testLambda) (takes a Predicate) and List.Where(testLambda) (takes a Func).

Any ideas how to do both?

Answer 1

I got this:

Func<object, bool> testLambda = x=>true;
int idx = myList.FindIndex(x => testLambda(x));

Works, but ick.

Answer 2

Easy:

Func<string,bool> func = x => x.Length > 5;
Predicate<string> predicate = new Predicate<string>(func);

Basically you can create a new delegate instance with any compatible existing instance. This also supports variance (co- and contra-):

Action<object> actOnObject = x => Console.WriteLine(x);
Action<string> actOnString = new Action<string>(actOnObject);

Func<string> returnsString = () => "hi";
Func<object> returnsObject = new Func<object>(returnsString);

If you want to make it generic:

static Predicate<T> ConvertToPredicate<T>(Func<T, bool> func)
{
    return new Predicate<T>(func);
}

Answer 3

Sound like a case for

static class ListExtensions
{
  public static int FindIndex<T>(this List<T> list, Func<T, bool> f) {
    return list.FindIndex(x => f(x));
  }
}

// ...
Func<string, bool> f = x=>Something(x);
MyList.FindIndex(f);
// ...

I love C#3 ...