Assigning a lambda as a parameter to a generic method called through reflection

Question

Consider a generic method as follow:

class SomeClass
{
    public static void SomeMethod<T>(Func<T>);
}

I would like to call this method using reflection. This is how far I could make it:

_SomeMethod = typeof(SomeClass).GetMethod("SomeMethod",
    BindingFlags.Public | BindingFlags.Static);
Type type = typeof(SomeType); //Actually SomeType is extracted using reflection and it's not fixed
MethodInfo toBeCalled = _SomeMethod.MakeGenericMethod(type);
object obj = Activator.CreateInstance(type);
toBeCalled.Invoke(null, () => obj);

But it gives compile error:

Error CS1660: Cannot convert `lambda expression' to non-delegate type `object' (CS1660)

Which is absolutely acceptable but what's the work around?

Please keep in mind that the closure created using the lambda is something that I need, so don't eliminate it.

[UPDATE]

To clarify the situation so it makes sense; the functor created using the lambda is an instantiator used by SomeMethod to create an instance of the class SomeType. But I don't want to actually create the object in functor, instead return it from a collection of instantiated objects from before. The easiest way would be having the obj in a closure returned by the lambda, don't you think?

Well what's the type of obj? If you don't know the T involved, it's hard to see how you can create a Func<T> from a lambda expression. Are you always just returning a constant value, or would you sometimes have other lambda expressions? More information would be really helpful. — Jon Skeet
– Jon Skeet, Commented Jul 17, 2015 at 15:48
But you want to create a Func which always just returns obj? Nothing more complicated than that, ever? You say you want to keep the closure - does that mean you're going to change the value of obj afterwards? Again, more clarity would be really helpful. — Jon Skeet
– Jon Skeet, Commented Jul 17, 2015 at 16:00
@JamesCurran No, actually SomeMethod belongs to a library and I didn't write it! — Mehran
– Mehran, Commented Jul 17, 2015 at 16:09

Community · Accepted Answer · 2017-05-23 12:29:48Z

3

I think you a looking to pass known object ot generic method that takes Func<T> and that "func" is always identity. One more wrapper may help:

public void SomeMethodInvokerRuntime(Type typeofSomeClass, object obj)
{
    var _SomeMethod = this.GetType().GetMethod("SomeMethodInvoker", 
        BindingFlags.Public | BindingFlags.Static);
    MethodInfo toBeCalled = _SomeMethod.MakeGenericMethod(obj.GetType());
    toBeCalled.Invoke(null, new[]{typeofSomeClass, obj});
}

public static void SomeMethodInvoker<T>(Type typeofSomeClass, T obj)
{
    var _SomeMethod = typeofSomeClass.GetMethod("SomeMethod",
          BindingFlags.Public | BindingFlags.Static);
    MethodInfo toBeCalled = _SomeMethod.MakeGenericMethod(typeof(T));
    Func<T> that = () => obj; // Main part - strongly typed delegate
    toBeCalled.Invoke(null, new[]{that});
}

Sample usage:

static class Test
{
    public static void SomeMethod<T>(Func<T> f)
    {
        Console.WriteLine((T)(f()));
    }
}

SomeMethodInvoker(typeof(Test), 3);
object obj = "test";

SomeMethodInvokerRuntime(typeof(Test), obj);

Alternatively you can build expression and compile into a function as shown in Jon Skeet's answer.

edited May 23, 2017 at 12:29

CommunityBot

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answered Jul 17, 2015 at 17:19

Alexei Levenkov

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8 Comments

Alexei Levenkov Over a year ago

PS I may post sample for expression approacy if this is what OP is looking for.

James Curran Over a year ago

The problem is you have Type typeofSomeClass, but not T obj. T needs to be set at compile time, but we don't know it until runtime.

Alexei Levenkov Over a year ago

@JamesCurran but you already know how to call generic function with unknown type (MakeGenericMethod on SomeMethodInvoker and pass methodInfoOfSomeMethodInvoker.Invoke(null, obj)), so I assume that the only problem is how to build Func<some-run-time-type>... Would that provide complete answer?

Alexei Levenkov Over a year ago

Ignore this answer - Jon Skeet awaited long enough :) and posted awesome answer.

Mehran Over a year ago

@AlexeiLevenkov I like yours better. It's much simpler and easier to understand. Thanks.

|

Jon Skeet · Accepted Answer · 2015-07-17 17:22:57Z

3

It doesn't sound like you really need a lambda expression here. You can build a function to return a constant value very easily using expression trees:

var expression = Expression.Constant(obj, type);
var delegateType = typeof(Func<>).MakeGenericType(type);
var func = Expression.Lambda(delegateType, expression).Compile();
toBeCalled.Invoke(null, func);

answered Jul 17, 2015 at 17:22

Jon Skeet

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Comments

James Curran · Accepted Answer · 2015-07-17 16:20:43Z

0

Reduced to it's simplest form, we have this:

void Main()
{
    MethodInfoInvoke(()=> {} );
}

void MethodInfoInvoke(object func)
{
}

Which fails with the same error.

Now, if we pass the lambda in a delegate, it seems happier:

void Main()
{
    Object obj = new Object();
    Func<object> action = ()=> obj; ;
    MethodInfoInvoke(action);      // accepted!
}

answered Jul 17, 2015 at 16:20

James Curran

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1 Comment

Mehran Over a year ago

Your reduction is not proper. You can not reduce having SomeMethod<T>(Func<T>), introduce such a method and try calling it with a lambda as the parameter using reflection and then you'll see the problem. Your current solution does not work then.

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