CODEWITHSUNDEEP
c#.netreflection
Craig Wilson
Craig Wilson

Reputation: 12624

Runtime creation of generic Func<T>

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?

Upvotes: 17

Views: 8768

Answers (4)

John Ernest
John Ernest

Reputation: 827

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);

Upvotes: 1

Joachim Kerschbaumer
Joachim Kerschbaumer

Reputation: 9891

you could create Expression objects instead of a func, and compile() the expression to get a Func delegate.

Upvotes: 0

Marc Gravell
Marc Gravell

Reputation: 1064094

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.

Upvotes: 30

Daniel Earwicker
Daniel Earwicker

Reputation: 116744

I think the usual approach would be to make the "dumb" version be the thing that you spoof at runtme, and then provide a helper extension method to provide the type-safe version on top of it.

Upvotes: 0

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