Cannot Convert Type 'Task<Derived>' to 'Task<Interface>'

Cannot convert type 'TaskDerived' to 'TaskInterface'

C# does not allow variance on classes, only interfaces and delegates that are parameterized with reference types. Task<T> is a class.

This is somewhat unfortunate, as Task<T> is one of those rare classes that could be made safely covariant.

However it is easy enough to convert a Task<Derived> to a Task<Base>. Just make a helper method / lambda that takes a Task<Derived> and returns Task<Base>, await the passed-in task, and return the value cast to Base. The C# compiler will take care of the rest. Of course you lose referential identity, but you weren't ever going to get that with a class.

Cannot convert TaskListTEntity to TaskIListTEntity

The easiest way in my opinion is to await the task. So it will work with minimum changes:

public async Task<IList<TEntity>> GetAsync(Func<IQueryable<TEntity>, IQueryable<TEntity>> 
getFunction)
{
    if (getFunction == null)
    {
        return new List<TEntity>();
    }
    return await getFunction(_dbSet).AsNoTracking().ToListAsync();
}

How to convert from TaskListDerived to TaskListBase

As the title states, I'm curious if it's possible to convert from Task<List<Derived>> to Task<List<Base>>?

No. Suppose it were legal, and see what goes wrong:

Task<List<Giraffe>> t1 = GetGiraffesAsync();
Task<List<Animal>> t2 = t1; // Suppose this were legal
(await t2).Add(new Fish());

And now we have a fish in a list of giraffes.

One of those three lines has to be illegal. Plainly it cannot be the first, since GetGiraffesAsync returns Task<List<Giraffe>>. Plainly it cannot be the last, since await t2 produces a List<Animal>, and a fish is an animal. Therefore it must be the middle line that is illegal.

Now, you could do this:

async Task<List<Animal>> ConvertAsync(Task<List<Giraffe>> t) => 
  (await t).Select(g => (Animal)g).ToList();

Or

async Task<List<Animal>> ConvertASync(Task<List<Giraffe>> t) => 
  (await t).Cast<Animal>().ToList();

or

async Task<List<Animal>> ConvertAsync(Task<List<Giraffe>> t) => 
  (await t).OfType<Animal>().ToList();

If you wanted to make it generic you could do

async Task<<List>Animal> ConvertAsync<T>(Task<List<T>> t) where T : Animal =>
  (await t).OfType<Animal>().ToList();

and now it works with giraffes and fish and tigers and so on.

That is, you can asynchronously wait for the original task to finish, and when it is done, you can create a new list of animals from your list of giraffes. That's perfectly legal. You now have two lists, one of giraffes and one of animals.

Or, you could do this:

async Task<IEnumerable<Animal>> ConvertAsync(Task<List<Giraffe>> t) => 
  await t;

since List<Giraffe> is convertible to IEnumerable<Animal>.

I would be inclined to write it as an extension method.

static class Extensions {
  public static Task<List<Animal>> ConvertAsync<T>(
    this Task<List<T>> t) where T : Animal {
      return (await t).OfType<Animal>().ToList();
  } 
}

And now your program fragment is:

var tasks = new List<Task<List<Animal>>>
{
  GetGiraffesAsync().ConvertAsync(),
  GetTigersAsync().ConvertAsync()
};

Cannot implicitly convert type 'string' to 'System.Threading.Tasks.Taskstring'

The listed return type of the method is Task<string>. You're trying to return a string. They are not the same, nor is there an implicit conversion from string to Task<string>, hence the error.

You're likely confusing this with an async method in which the return value is automatically wrapped in a Task by the compiler. Currently that method is not an async method. You almost certainly meant to do this:

private async Task<string> methodAsync() 
{
    await Task.Delay(10000);
    return "Hello";
}

There are two key changes. First, the method is marked as async, which means the return type is wrapped in a Task, making the method compile. Next, we don't want to do a blocking wait. As a general rule, when using the await model always avoid blocking waits when you can. Task.Delay is a task that will be completed after the specified number of milliseconds. By await-ing that task we are effectively performing a non-blocking wait for that time (in actuality the remainder of the method is a continuation of that task).

If you prefer a 4.0 way of doing it, without using await , you can do this:

private Task<string> methodAsync() 
{
    return Task.Delay(10000)
        .ContinueWith(t => "Hello");
}

The first version will compile down to something that is more or less like this, but it will have some extra boilerplate code in their for supporting error handling and other functionality of await we aren't leveraging here.

If your Thread.Sleep(10000) is really meant to just be a placeholder for some long running method, as opposed to just a way of waiting for a while, then you'll need to ensure that the work is done in another thread, instead of the current context. The easiest way of doing that is through Task.Run:

private Task<string> methodAsync() 
{
    return Task.Run(()=>
        {
            SomeLongRunningMethod();
            return "Hello";
        });
}

Or more likely:

private Task<string> methodAsync() 
{
    return Task.Run(()=>
        {
            return SomeLongRunningMethodThatReturnsAString();
        });
}

Return class derived from generic constraint from a task

Since your implementation always returns an object of the same type, I would formulate the interface differently

public interface ICalcLoaderTask<T>
    where T : CalcLoaderTaskResult
{   
    Task<T> Execute(BaseTaskParameters taskParams, CancellationToken cancellationToken)
}

I.e. the type (interface) is generic instead of the method.

And then let your implementation implement the concrete type

public class MyImplementation : ICalcLoaderTask<CopyNMStoreResult>
{
    public async Task<CopyNMStoreResult> Execute(BaseTaskParameters taskParams,
                                                 CancellationToken cancellationToken)
    {
        ...
        var result = new CopyNMStoreResult
        {
            NMStoreResultsFilePath = GlobalNMStoreResultsFilePath
        };
        return result;
    }
}

Because the caller cannot choose the type of the return value anyway.

It is technically possible to let the the generic type parameter open

public class MyImplementation<T> : ICalcLoaderTask<T> where T ...

but since the return type of your method is hard-coded, there is no advantage in doing so.

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Note: It is possible to let a method return different types. Adding the generic type constraint new allows you to create an object with new T(). The constraint new means that the type T must have a default constructor.

T MyMethod<T>()
    where T : MyBase, new
{
    return new T();
}

CS1503 cannot convert compile error when registering generic type

You are getting a compile error for the same reason as this code would give you a compile error:

IAuthenticationService<IRequest, IResponse> service =
    new DummyAuthenticationService<AuthenticateRequest, AuthenticateResponse>();

The C# compiler is telling you that the created instance is not convertible to IAuthenticationService<IRequest, IResponse>.

This has to do with variance. Covariance and contravariance is a complicated topic, which is hard for me to explain in a few sentences. There's is actually great Microsoft documentation on this topic, which I encourage you to check out.

Although to support variance interfaces can be marked with in and out parameters, that not get your code to compile. Instead, to get this compiling, you must change the code to the following:

IAuthenticationService<AuthenticateRequest, AuthenticateResponse> service =
    new DummyAuthenticationService<AuthenticateRequest, AuthenticateResponse>();

And, therefore, your ConfigureServices method to the following:

services
  .AddSingleton<IAuthenticationService<AuthenticateRequest, AuthenticateResponse>>(
     new DummyAuthenticationService<AuthenticateRequest, AuthenticateResponse>());

Or, alternatively:

services.AddSingleton(
    typeof(IAuthenticationService<,>),
    typeof(DummyAuthenticationService<,>));

Await the result of TaskTDerived using reflection in a non-generic method

As per my comment:

Unlike interfaces, concrete types such as Task<TResult> cannot be covariant. See Why is Task not co-variant?. So Task<B> cannot be assigned to a Task<A>.

The best solution I can think of is to use the underlying type Task to perform the await like so:

var task = (Task)method.Invoke(this, new object[] { "humpf" });
await task;

Then you can use reflection to get the value of the Result:

var resultProperty = typeof(Task<>).MakeGenericType(type).GetProperty("Result");
A a = (A)resultProperty.GetValue(task);
return a.Id;

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