csharplang/meetings/2019/LDM-2019-02-20.md

C# Language Design Notes for Feb 20th, 2019

Agenda

• Nullable Reference Types: Open LDM Issues https://github.com/dotnet/csharplang/issues/2201

Discussion

There are variants of this scenario with string! and string~. The question is whether we should learn in both branches, or whether we should treat one branch as unreachable.

'Deliberate' Tests for Nullability

This is a continuation of the discussion from the last meeting.

Proposal: divide into "pure" and "not-pure" null checks. The "pure" null checks will affect both branches, but the "not-pure" will only affect one branch. Precisely: when a "pure" check is used, the nullability state is split for both branches, while a non-pure check does not have a "maybe-null" state after split.

Proposed deliberate checks:

1. `x == null`

2. `x != null`

3. `(Type)x == null`

4. `(Type)x != null`

5. `x is null`

6. `s is string` (when the type of `s` is `string`)

7. `s is string s2` (when the type of `s` is `string`)

8. `s is string _` (when the type of `s` is `string`)

Other checks:

1. `x is string`

2. `x is string s`

3. `x is C { Property = 3 }`

4. `TryGetValue` (`[NotNullWhenTrue]`)

5. `string.IsNullOrEmpty(s)` (`[NotNullWhenFalse]`)

6. `x?.ToString() != null`

Follow up question: What about ?. and related operators (e.g., ??)?

An example would be

void M(string s)
{
    if (s?.ToString() == null)
    {
        // is `s` maybe null?
    }
}

A corresponding rewrite would be:

void M(string s)
{
    if (s == null || s.ToString() == null)
    {
        // s would be maybe-null after this point
    }
}

Conclusion

Pure:

1. `x == null`

2. `x != null`

3. `(Type)x == null`

4. `(Type)x != null`

5. `x is null`

6. `s is string` (when the type of `s` is `string`)

All conditional access (?., ??) included

Not-Pure

1. `x is string`

2. `x is string s`

3. `x is C { Property = 3 }`

4. `TryGetValue` (`[NotNullWhenTrue]`)

5. `string.IsNullOrEmpty(s)` (`[NotNullWhenFalse]`)

7. `s is string s2` (when the type of `s` is `string`)

8. `s is string _` (when the type of `s` is `string`)

Switch statement/expression: switch statement treated as essentially the equivalent of an if/else rewrite. No decisions for the switch expression yet.