Why a `Predicate<T>` doesn't match a `Func<T,bool>`?

Question

I try to compile the following code in C#:

public static T FirstEffective(IEnumerable<T> list) 
{
    Predicate<T> pred = x => x != null;
    return Enumerable.FirstOrDefault(list, pred);
}

The compiler (Mono/.NET 4.0) gives the following error:

File.cs(139,47) The best overloaded method match for `System.Linq.Enumerable.FirstOrDefault<T>(this System.Collections.Generic.IEnumerable<T>,System.Func<T,bool>)' has some invalid arguments
/usr/lib/mono/4.0/System.Core.dll (Location of the symbol related to previous error)
File.cs(139,47): error CS1503: Argument `#2' cannot convert `System.Predicate<T>' expression to type `System.Func<T,bool>'

This is rather strange since a Predicate<T> is in fact a function that takes as input a parameter T and returns a bool (T is even "covariant" thus a specialization of T is allowed). Do delegates do not take the "Liskov Substitution principle" into account to derive that Predicate<T> is equivalent to Func<T,bool>? As far as I know this equivalence problem should be decidable.

Answer 1

C# specification is clear about that:

15.1 Delegate declarations

Delegate types in C# are name equivalent, not structurally equivalent. Specifically, two different delegate types that have the same parameter lists and return type are considered different delegate types.

That's why your code doesn't compile.

You can make it work by calling the delegate, instead of passing it:

public static T FirstEffective (IEnumerable<T> list) {
    Predicate<T> pred = x => x != null;
    return Enumerable.FirstOrDefault (list, x => pred(x));
}

Update

There is a great blog post by Eric Lippert: former member of C# Team as Microsoft, which answers your question in much details: Delegates and structural identity.