如何将对象类型转换为任务<T>类型？

Question

How can I cast an object instance of type object to type Task<T> ?

I store a Task<T> reference in a variable of type object and need to cast it back to Task<T> because the method needs to return type Task<T> .

Answer 1

Try using the Task.FromResult<TResult> method . From MSDN:

T obj = new T();
Task<T> a = Task.FromResult(obj);

Answer 2

If T is known at the time of casting, there should be no problem doing:

Task<T> myTask = (Task<T>)myObject;

Example: https://dotnetfiddle.net/oOg4E8

Answer 3

You can use the as operator. If the object is of type Task<T> you get a correct references. Otherwise you get null .

var task = obj as Task<T>;

Answer 4

If the object you have stored is actually the Task<T> you can just use a direct cast.

var task = (Task<X>)obj;

If you have the object result, unwrapped, and you need a task result from the instance, you can use Task.FromResult(X) which return a completed Task<T> of type X .

Answer 5

Given:

• The variable is defined as object
• The value is type Task<T1>
• T1 is not available at compile time.
• You need to cast the object to Task<T2> where T1 : T2 , but not T1 == T2 (likely T2 is an interface implemented by T1 ).

The other answers will not work when T1 != T2 .

This extension method works well for me to preform the cast.

public static class TaskExtensions
{
    public static Task<T> CastTask<T>(this object taskObj)
    {
        var taskType = taskObj.GetType();
        if (!taskType.IsSubClassOfGeneric(typeof(Task<>)))
            throw new ArgumentException($"{taskType.FullName} is not of type Task<>");
        var resultType = taskType.GenericTypeArguments.First();
        var castTaskMethodGeneric = typeof(TaskExtensions)
            .GetMethod("CastTaskInner", BindingFlags.Static | BindingFlags.Public);
        var castTaskMethod = castTaskMethodGeneric.MakeGenericMethod(
            new Type[] { resultType, typeof(T) });
        var objCastTask = castTaskMethod.Invoke(null, new object[] { taskObj });
        return objCastTask as Task<T>;
    }

    public static async Task<TResult> CastTaskInner<T, TResult>(Task<T> task)
    {
        var t = await task;
        var tObj = (object)t;
        return (TResult)tObj;
    }
}