Why is it impossible to cast Func<T, bool> to Predicate<T>?

Question

I found myself in a situation where I had to do some stuff with elements of a list and then remove them from that list :

foreach (var element in someList.Where(e => e.IsMatching))
{
    // do some stuff
}

someList.RemoveAll(e => e.IsMatching);

This woks perfectly but I don't like the duplicate lambda, so I declared a Func<T, bool> x = e => e.IsMatching; and then tried to use x as a parameter for .Where and .RemoveAll but I wasn't able to use x for .RemoveAll because its overload only accepts Predicate<T> .

Why isn't an implicit nor explicit cast possible since both have the same return and parameter types?

(I am not looking for a way to make the conversion, everything about it is already in this quesion )

Answer 1

It is a little silly that the IEnumerable<T>.Where extension method and List<T> 's RemoveAll method take two different types. However, this is a simple work-around, since the local function can be used to initialize both a Func<int, bool> and a Predicate<int> :

var l = new List<int> {1, 2, 3, 5, 5, 6, 6};
bool Pred(int i) => i < 5;
var wh = l.Where(Pred);
var rem = l.RemoveAll(Pred);