转换功能 <object, Task> 到功能 <T, Task> 在运行时

Question

I have a Func<object, Task> . I am trying to pass it as an argument into a function that takes Func<T, Task> . I am using reflection to create a MethodInfo for the function, and the T being filled in is not known until runtime.

How can I do this with reflection?

Answer 1

You don't need to do anything special.

Func<in T, out TResult> is contravariant on T - its input parameter, and every type inherits from object , which means you can just cast Func<object, Task> to Func<T, Task> for any reference type T - pass it as-is and it will just work.

Example

Now, this won't work for value types, since those require unboxing beforehand.

So if your T is a value type, you'll have to wrap it in another delegate which will perform a cast.

One simple way would be to define a wrapper method:

private static Func<T, TResult> CastFunc<T, TResult>(Func<object, TResult> fn)
    => param => fn(param);

Then create a delegate through reflection:

var result = (Func<int, Task>)typeof(WrapperClass)
    .GetMethod(nameof(CastFunc), BindingFlags.NonPublic | BindingFlags.Static)
    .MakeGenericMethod(typeof(int), typeof(Task)).Invoke(null, new object[] { fn });

Answer 2

I'd go for reflection using MakeGenericMethod

Here's a quick example:

    public class Example
    {
        public void Start()
        {
             Func<object, Task> func = o => null;
             object objFunc = func; // got it from a generic place as an object or something
             Type type = objFunc.GetType().GetGenericArguments()[0]; // get T in runtime
             var method = typeof(Example).GetMethod("DoSomething", BindingFlags.Public | BindingFlags.Instance).MakeGenericMethod(type);
             var result = method.Invoke(this, new object[1] { func });
        }

        public int DoSomething<T>(Func<T, Task> input)
        {
            return 1;
        }
    }