Runtime creation of generic Func<T>

Question

I need to implement the method:

object GetFactory(Type type);

This method needs to return a Func<T> where typeparam 'T' is the 'type'.

So, my problem is that I don't know how to create a Func<?> at runtime using reflection. Activator.CreateInstance doesn't work because there are no constructors on delegates.

Any Ideas?

Answer 1

You use Delegate.CreateDelegate, i.e. from a MethodInfo; below, I've hard-coded, but you would use some logic, or Expression, to get the actual creation method:

using System;
using System.Reflection;
class Foo {}

static class Program
{
    static Func<T> GetFactory<T>()
    {
        return (Func<T>)GetFactory(typeof(T));
    }
    static object GetFactory(Type type)
    {
        Type funcType = typeof(Func<>).MakeGenericType(type);
        MethodInfo method = typeof(Program).GetMethod("CreateFoo",
            BindingFlags.NonPublic | BindingFlags.Static);
        return Delegate.CreateDelegate(funcType, method);
    }
    static Foo CreateFoo() { return new Foo(); }
    static void Main()
    {
        Func<Foo> factory = GetFactory<Foo>();
        Foo foo = factory();
    }
}

For non-static methods, there is an overload of Delegate.CreateDelegate that accepts the target instance.

Answer 2

I use this with generics in EF Core including making Expression<Func<SomeModelType,bool>> for let's say a Where clause, you can chain MakeGenericType calls with nested generics.

var someType = SomeDbContext.SomeDataModelExample.GetType();
var funcType1 = typeof(Func<>).MakeGenericType(new Type[] { someType });
var result = Activator.CreateInstance(funcType1);
var result2 = Activator.CreateInstance(funcType1, someParams);