csharplang/meetings/2020/LDM-2020-02-05.md

C# LDM for Feb. 5, 2020

Agenda

1. Dependent nullability attribute
2. Null checking in unconstrained generics

Discussion

Nullability

Dependent calls:

We'd like to be able to support patterns like:

if (x.HasValue)
{
    x.Value // should not warn when HasValue is true
}

We would add attributes to support these use cases: EnsuresNotNull and EnsuresNotNullWhen (to parallel the existing NotNull attributes). The proposal as stands is to name the fields or properties and that would be the inputs and outputs for the flow analysis. We propose that the lookup rules for resolving the names in these instances would be similar to the rules for the DefaultMemberAttribute.

This could also be used for helpers in constructor initialization, where today constructors which only call base are required to initialize all members, even if a helper initializes some of the members.

There's a follow-up question: should you be able to specify that members of the parameter are not-null after the call? For example,

class Point
{
    public object? X;
}
static void M([EnsuresNotNull(nameof(Point.X))]Point p) { ... }

We could also allow it for nested annotation

class Point
{
    public object? X;
}
class Line
{
    public Point P1;
    public Point P2;
}
static void M([EnsuresNotNull("P1.X", "P1.Y")]Line l) { ... }
static bool M([EnsuresNotNullWhen(true, "P1.X", "P1.Y")]Line l) { ... }

The nested names could also be used for return annotations.

However, we're not sure this is worth it. We see the usefulness in theory, but we're not sure how often it would actually be used. If we want to leave the space for later, we could produce an error when writing an attribute with an unsupported string form.

Similarly, if we don't want to support referring to members of types through the parameters, as in the first example, we can also provide an error for these scenarios. Or, we could say that the initial proposal is qualitatively different from all these scenarios:

[MemberNotNull(nameof(X))]
void M() { }

For the situations in parameters and return types we are referring to the target the attribute is being applied to, while the original proposal is returning to the containing type, somewhat unrelated to the location of the attribute.

We're also not sure exactly what the name or shape of these attributes would look like. We think this could be valuable, but we'd like to decide with the full context of other attributes we're considering.

Conclusion

We see the usefulness of the original scenario, but the broadening we're not sure on the return on investment. Let's support the original scenario through a new attribute, MemberNotNull, that only has members as valid attribute targets to start.

If we find users still hit significant limitations without the parameter and return type support, we can consider broadening in a future release.

Pure non-null check in generic

public T Id<T>(T t)
{
    if (t is null) Console.WriteLine();
    return t; // warn?
}

The question here is whether we should consider T to move to MaybeDefault after checking for null. We never do this today.

The scenario where a "pure" null check would come into play is:

public T Id<T>(T t) where T : notnull
{
    if (t is null) Console.WriteLine();
    return t;
}

It appears that this does not warn today, which looks like a bug. The analogous scenario for T?? is

public T ID<T>(T t)
{
    if (t is default) Console.WriteLine();
    return t;
}

However, this is illegal as there is no way to check if T is default(T).

Conclusion

The original scenario should not warn.