[Proposal] MemberNotNullWhenMemberAttribute for Property and Field

[metaone01]

Usage like this:

public class Item: IDisposable {
    public bool IsDisposed;
    public bool IsLoaded;

    [MemberNotNullWhenMember(nameof(IsDisposed),false)]
    public int? Id;

    [MemberNotNullWhenMember([(nameof(IsDisposed),false),(nameof(IsLoaded),true)])]
    public List<string>? Resource;
    ...
}

If this is done, we can have better code analysis in the situation below:

public List<string>? UseResource(Item item) {
    if(item is {IsDisposed:false,IsLoaded:true}) {
        return item.Resource; //No null reference warning now.
    }
    ...
}

I'm not sure if it should have two overloads, one that receives a List<(string,bool)> for AND, and the other receives a string and a bool for OR.

[huoyaoyuan]

It's correct to ask at csharplang.

The current practice is to use MemberNotNullWhen on the condition property:

[MemberNotNullWhen(true, nameof(Id))]
public bool IsDisposed;
public string? Id;

Conjunction of multiple properties is unlikely to be supported because of the logical complexity.
Moreover, it's not the recommended practice to mark the nullability for invalid state. Usage of disposed/uninitialized object should be undefined or always throwing.

[metaone01]

Got it. Thank you!