Runtime creation of generic Func

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?

Marc Gravell · Accepted Answer · 2009-03-18T14:02:37.340

30

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.

edited Mar 18 '09 at 14:02

answered Mar 18 '09 at 13:53

Marc Gravell

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Mark, I don't actually know what T is. If I knew what T is, this would be easy. – Craig Wilson Mar 18 '09 at 13:56
I modified the question to better state my problem. – Craig Wilson Mar 18 '09 at 14:01
Not exactly what I needed , but this pushed me down the right path. Thanks for your help. – Craig Wilson Mar 18 '09 at 14:16
Oooh, was playing with reflection the other day and wondered if there was a way to describe a generic type definition in code, and now I know: List<>, which presumably can only appear inside typeof? – Daniel Earwicker Mar 18 '09 at 15:37
@Earwicker: correct; you simply omit all the type-args - for example, KeyValuePair<,> instead of KeyValuePair – Marc Gravell Mar 18 '09 at 15:42

score 1 · Answer 2 · edited Nov 08 '20 at 10:33

1

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);

edited Nov 08 '20 at 10:33

Striped

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answered Nov 08 '20 at 03:36

John Ernest

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I use this with generics in EF Core including making Expression> for let's say a Where clause, you can chain MakeGenericType calls with nested generics. – John Ernest Nov 08 '20 at 03:41

score 0 · Answer 3 · answered Mar 18 '09 at 13:49

0

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.

answered Mar 18 '09 at 13:49

Daniel Earwicker

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So, this is in the bowels of an IoC container, and I have to return the object as a strongly typed dependency, not necessarily knowing that what was requested was a Func (it could be a Uri or string). – Craig Wilson Mar 18 '09 at 13:51

score 0 · Answer 4 · answered Mar 18 '09 at 14:06

0

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

answered Mar 18 '09 at 14:06

Joachim Kerschbaumer

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Runtime creation of generic Func

4 Answers4

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