C# 8.0: unmanaged constructed types

[pkulikov]

Fixes #14101
Supported by dotnet/samples#1411

[BillWagner]

This is a great start @pkulikov There's a couple scenarios that we should cover, but it may require a slightly different sample. For discussion, I'll use the Coords<T> type you've defined.

We should explain that if the type argument is not an unmanaged type, the constructed type is not unmanaged:

var objCoords = new Coords<object>();

That's a contrived example, but you see the point.

It is also interesting that adding the unmanaged constraint would mean that any constructed type would be an unmanaged type:

public struct Coords<T> where T : unmanaged
~
    public T X;
    public T Y;
}

Thoughts on how to update to make those two points?

[pkulikov]

@BillWagner

We should explain that if the type argument is not an unmanaged type...

I understand what you mean, but wording should be different. Type argument can be a reference type. Not practical example:

public struct Coords<T>
{
    public int X;
    public int Y;
}

With that definition, Coords<T> is unmanaged for any type argument. All fields are of an unmanaged type, regardless of the type argument.

I'll think how to update the article.

[pkulikov]

@BillWagner please take a look at the latest commit; I've also added one more snippet to the sample PR.

[pkulikov]

ok, I think the current edition is final. What do you think @BillWagner?

[BillWagner]

This is great work again @pkulikov

I had one small grammar nit to consider, and then I'll