why C# Func lambda expression requires cast for result

Question

I need to understand about Func type expression deeper.

public class TheResult : IResultEntry {
    ...
}

With above class, why does below second method require cast?

I can read the error message of course, but hard to understand.

// Success
public Task<IResultEntry> ProcessAsync_1()
{
    return Task.Factory.StartNew(() => (IResultEntry) new TheResult());
}

// Fail: Compiler error. Cannot implicitly convert...
public Task<IResultEntry> ProcessAsync_2()
{
    return Task.Factory.StartNew(() => new TheResult());
}

If we change it to named method with help of ReSharper, we can do without a cast.

public Task<IResultEntry> ProcessAsync_2_Changed()
{
    return Task.Factory.StartNew(function);
}

private IResultEntry function()
{
    return new TheResult();
}

you are expecting `IResultEntry` as a return value, but `TheResult` may implement other interfaces as well. — Amit Kumar Ghosh, Apr 19 '16 at 06:54
Just because two types, `A` and `B`, exhibit a certain inheritance/implementation relationship, that does *not* mean that `G` and `G` have the *same* relationship. — Damien_The_Unbeliever, Apr 19 '16 at 06:58
And also note that with `function()` you **do have a cast**, it's implicit but there it is. — Adriano Repetti, Apr 19 '16 at 06:58
@AmitKumarGhosh It's perfectly fine for `TheResult` to implement other interfaces. It's simply because making `Task` covarient would involve implementing `ITask`, which it doesn't. The dupe covers why it doesn't. — Rob, Apr 19 '16 at 07:01

why C# Func lambda expression requires cast for result

0 Answers0