ToDictionary TKey nullable check

[IgorMenshikov]

I have a linq query

var periodHash = context.Query<Entity>()
                .Where(i => i.Name != null)
                .ToDictionary(i => i.Name, i => i);

So, I can be sure key cannot be null.

But I get a warning:

Warning	CS8714	The type 'string?' cannot be used as type parameter 'TKey' in the generic type or method 'Enumerable.ToDictionary<TSource, TKey, TElement>(IEnumerable<TSource>, Func<TSource, TKey>, Func<TSource, TElement>)'. Nullability of type argument 'string?' doesn't match 'notnull' constraint.

Is there a way to avoid the warning?

[dstarkowski]

Either declare Name in Entity class as non nullable, or use ! to tell compiler i.Name is not null.

var periodHash = context.Query<Entity>()
    .Where(i => i.Name != null)
    .ToDictionary(i => i.Name!, i => i);

Also see #3951

[IgorMenshikov]

1. Name can be nullable. That is why I filter null values out.
2. Null-forgiving operator helps to hide the warning. Thanks

[FaustVX]

@IgorMenshikov Just FYI, the Where method don't change the type.
So, Name type don't change from string? to string.
And so, in .ToDictionary(i => i.Name, i => i), TKey is still string?.

[IgorMenshikov]

This is more about "null warning logic". If you write something like:

string? something = GetSomething();
if (something != null) {
    something.ToString(); // no warning here because of data flow.
}

Same logic would be good for Where. It is like "if" for collection.

But, we have a workaround using Null-forgiving operator.

[Joe4evr]

@IgorMenshikov That's literally what #3951 is about.