Casting TResult in Task to System.Object

Question

I am loading an assembly and calling a static method that will create a new object of type “MyClass1” (this type is specified at runtime) through reflection using MethodInfo.Invoke(). This works fine when the method is a normal sync method. However, the method being called is an async method which returns Task<MyClass1>, which will be used to retrieve the result using task.Result.

Ideally I should be using MyClass1 as TResult in the task, but the type is determined only at runtime so I can't do that. I am looking for a way to get the task and the result. I am trying to cast the TResult to System.Object and get the class as a generic Object. The following is the code I am using for this purpose.

public static void LoadAssembly()
{
    // Calling static async method directly works fine
    Task<MyClass1> task1 = MyClass1.MakeMyClass1();
    MyClass1 myClass1 = task1.Result;

    // Calling static async method through reflection through exception.
    Assembly assembly = Assembly.LoadFrom(dllName);
    Type type = assembly.GetType("AsyncDll.MyClass1");
    var types = assembly.GetTypes(); 
    MethodInfo[] methodInfos = types[0].GetMethods(BindingFlags.Public | BindingFlags.Static);
    Type myClassType = types[0];
    MethodInfo mi = myClassType.GetMethod("MakeMyClass1");
    Object obj = Activator.CreateInstance(mi.ReflectedType);
    Task<Object> task = (Task<Object>)mi.Invoke(obj, null); // Exception occurs here.
    Object result = task.Result;
}

Following is the method (test code) being called through reflection. This

public class MyClass1
{
    public static async Task<MyClass1> MakeMyClass1()
    {
        MyClass1 newObject = null;
        await Task.Run(() =>
        {
            newObject = new MyClass1();
        });
        return newObject;
    }
    ...
}

Unfortunately, the casting of TResult is causing System.InvalidCastException.

An unhandled exception of type 'System.InvalidCastException' occurred in Test.exe

Additional information: Unable to cast object of type 'System.Threading.Tasks.Task`1[MyClass1]' to type 'System.Threading.Tasks.Task`1[System.Object]'.

How can I cast the TResult in Task<> to a generic object and get the result using task.Result? I would appreciate any help resolving this issue.

Answer 1

You cannot cast Task<T> to Task<object>, because Task<T> is not covariant (it's not contravariant, either). The simplest solution would be to use some more reflection:

var task   = (Task) mi.Invoke (obj, null) ;
var result = task.GetType ().GetProperty ("Result").GetValue (task) ;

This is slow and inefficient, but usable if this code is not executed often. As an aside, what is the use of having an asynchronous MakeMyClass1 method if you are going to block waiting for its result?

Answer 2

Another possibility is to write an extension method to this purpose:

    public static Task<object> Convert<T>(this Task<T> task)
    {
        TaskCompletionSource<object> res = new TaskCompletionSource<object>();

        return task.ContinueWith(t =>
        {
            if (t.IsCanceled)
            {
                res.TrySetCanceled();
            }
            else if (t.IsFaulted)
            {
                res.TrySetException(t.Exception);
            }
            else
            {
                res.TrySetResult(t.Result);
            }
            return res.Task;
        }
        , TaskContinuationOptions.ExecuteSynchronously).Unwrap();
    }

It is none-blocking solution and will preserve original state/exception of the Task.

Answer 3

As an enhancement to the accepted answer, you can avoid the blocking by awaiting the Task in between:

var task = (Task)mi.Invoke(obj, null);
await task;
var result = task.GetType().GetProperty("Result").GetValue(task);

Of course this will be properly asynchronous only if all methods up the call stack are marked async and you're using await instead of .Result everywhere.