[Proposal]: Nullable analysis of LINQ queries

[jcouv]

Nullable analysis of LINQ queries

Champion issue: #8999

Summary

Nullable analysis of LINQ queries (LDM expressed interested to handle Where, needs design proposal)

Motivation

Range variables in LINQ queries are currently treated as oblivious to minimize annoyances.
Most of the annoyance results from our inability to recognize method calls that filter out null elements, such as .Where(i => i is not null).

using System.Linq;
using System.Collections.Generic;

#nullable enable
public class C 
{
    public void M(IEnumerable<string?> list) 
    {
        _ = from item in list
            select item.ToString(); // should warn

        _ = list
            .Where(i => i is not null)
            .Select(i => i.ToString()); // should not warn
    }
}

Detailed design

TBD

Unresolved questions

• How to recognize Where methods?

Design meetings

Split issue from #3868
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-28.md#nullability-improvements
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-06.md#nullable-analysis-of-linq-queries

[gafter]

Would these warnings only be suppressed for the "standard" Linq implementations that have the expected semantics, or for all query expressions (even those that do not have the expected semantics)?

[JTeeuwissen]

Note that OfType currently can be used to filter out nulls whilst getting the correct return type. But perhaps there are more cases where this would not apply.

[tobias-tengler]

What's the status of this proposal?

[KillyMXI]

Last meeting note link is wrong. Should be this, it seems: https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-04.md#nullable-analysis-of-linq-queries

[JudahGabriel]

Slightly related: TypeScript added support for this last year:

var birds = nations
   .map(n => n.getNationalBird()) // getNationalBird returns Bird or null
   .filter(n => n != null);

birds.forEach(bird => bird.sing()); // Typescript understands bird will be non-null here

It's found great use within the TS community. I suspect the same would be true of C#.