[Proposal] WhereNotNull & SelectNotNull

[bram209]

With C# 8 nullability feature it would be nice to be able to easily filter out null values.
I always use

        <Nullable>enable</Nullable>
        <WarningsAsErrors>nullable</WarningsAsErrors>

For strict nullability warnings. Would be nice if we can have linq extensions that can automatically remove all null values and return a sequence of a non-nullable type.

     public static IEnumerable<T> WhereNotNull<T>(this IEnumerable<T?> enumerable) =>
            enumerable.Where(e => e != null)!;

    IEnumerable<Person?> nullableList = new List<Person?> { new("Foo"), null, new("Bar")};
    IEnumerable<Person> values = nullableList.WhereNotNull();
    Debug.Assert(values.Count() == 2);

[atifaziz]

I believe that this is a duplicate of #609, where I have presented arguments why Choose is what you want, better, more powerful and more generic. In short, by breaking down the problem another way, you can use Choose directly:

IEnumerable<Person?> nullableList = new List<Person?> { new("Foo"), null, new("Bar")};
IEnumerable<Person> values = nullableList.Choose(Nullable.Deconstruct);
Debug.Assert(values.Count() == 2);

record Person(string Name);
    
static partial class Nullable
{
    public static (bool HasValue, T Value) Deconstruct<T>(T? nullable) =>
        nullable is not null ? (true, nullable) : default;
}

Notice how the signature of Choose also adapts correctly instead of masquerading through the null-forgiving operator (!). Choose has two type parameters: T and TResult. In the example above, T is Person? and TResult is Person and you don't need any ! to change some T? into T.

I'll close this for now as a duplicate, but open to reconsidering if I've overlooked something.

[skarllot]

@atifaziz What you think of providing a WhereNotNull extension as a shortcut of using Choose and implementing the Desconstruct method?

[atifaziz]

What you think of providing a WhereNotNull extension as a shortcut of using Choose and

@skarllot Why have another way to do the same thing?

implementing the Desconstruct method?

It doesn't belong in MoreLINQ since it generally extends any type T whereas this library should only be extending sequential family of types like IEnumerable<> & IAsyncEnumerable<>.

[skarllot]

@atifaziz because it is a very common use case and avoid repeating the same code over and over.

And about the null forgiving, the proposed Desconstruct is masquerading the null too as it returns default that returns null too, but it is not detected by compiler.

[atifaziz]

@atifaziz because it is a very common use case and avoid repeating the same code over and over.

Right, but MoreLINQ is not a convenience library of small helpers for common code. See the discussion in PR #1001 for some background.

And about the null forgiving, the proposed Desconstruct is masquerading the null too as it returns default that returns null too, but it is not detected by compiler.

True, but if you get a (false, _) then using the second half of the couple is undefined and at the risk of the user.