API Proposal: MemberNotNullWhenNull

[chris-smith-zocdoc]

Today MemberNotNullWhen allows methods to change the nullity of a member provided the method returns a boolean. This allows for methods like int.TryParse where out parameters & booleans are used together but doesn't allow for handling of other response types which may change the nullity.

We have some common use cases where operations are either expected to succeed and return a result value or fail and return an error value. With the introduction of a new attribute like MemberNotNullWhenNull we can use the nullity check on one property to modify the nullity of another

public class Either<TError, TResult>
{
    [MemberNotNullWhenNull(nameof(Value))]
    public TError? Error { get; set; }
    
    [MemberNotNullWhenNull(nameof(Error))]
    public TResult? Value { get; set; }

    // The addition this IsSuccess method is a workaround to the lack of MemberNotNullWhenNull today
    [MemberNotNullWhen(true, nameof(Value))]
    [MemberNotNullWhen(false, nameof(Error))]
    public bool IsSuccess => Value != null;
}

API proposal

public MemberNotNullWhenNullAttribute(string member) {}

public MemberNotNullWhenNullAttribute(params string[] members) {}

Example usage

var t = new Either<A, B>();
if (t.Error != null)
{
    t.Error.FailureStuff();
} else {
    t.Value.SuccessfulStuff();
}

This can also be used as a switch expression. Without MemberNotNullWhenNull the switch is not exhaustive because the branches aren't known to be mutually exclusive

return res switch
{
    { Value : { } v } => v.SuccessfulStuff(),
    { Error : { } err } => err.FailureStuff(),
};