Nullable members dependency

[NN---]

Make it possible to add dependency between members similar way as you can do it for parameters using NotNullIfNotNullAttribute.

For instance either x or y has value but not both:

[NotNullIfNull(nameof(y)]
string? x;

[NotNullIfNull(nameof(x)]
string? y;

Another sample, both either null or non-null:

[NotNullIfNotNull(nameof(y)]
string? x;

[NotNullIfNotNull(nameof(x)]
string? y;

Also with boolean indicating nullability:

bool hasValue;

[NotNullWhen(nameof(hasValue))]
string? value;

etc.

[RikkiGibson]

I think MemberNotNullWhenAttribute may address your scenarios adequately.

For instance either x or y has value but not both:

public class C
{
    [MemberNotNullWhen(true, nameof(X))]
    [MemberNotNullWhen(false, nameof(Y))]
    public bool HasX
    {
        get
        {
            if (X != null && Y == null)
            {
                return true;
            }
            else if (X == null && Y != null)
            {
                return false;
            }
            else
            {
                throw new InvalidOperationException();
            }
        }
    }
    
    public string? X;
    public string? Y;
    
    public static void M(C c)
    {
        if (c.HasX)
        {
            c.X.ToString();
            c.Y.ToString(); // warning
        }
        else
        {
            c.X.ToString(); // warning
            c.Y.ToString();
        }
    }
}

Another sample, both either null or non-null:

public class C
{
    [MemberNotNullWhen(true, nameof(X), nameof(Y))]
    public bool HasXAndY
    {
        get
        {
            if (X != null && Y != null)
            {
                return true;
            }
            else if (X == null && Y == null)
            {
                return false;
            }
            else
            {
                throw new InvalidOperationException();
            }
        }
    }
    
    public string? X;
    public string? Y;
    
    public static void M(C c)
    {
        if (c.HasXAndY)
        {
            c.X.ToString();
            c.Y.ToString();
        }
        else
        {
            c.X.ToString(); // warning
            c.Y.ToString(); // warning
        }
    }
}

Also with boolean indicating nullability:

public class C
{
    [MemberNotNullWhen(true, nameof(Value))]
    public bool HasValue => Value != null;
    
    public string? Value;
    
    public void M()
    {
        if (HasValue)
        {
            Value.ToString();
        }
        else
        {
            Value.ToString();
        }
    }
}

[NN---]

Very nice, however it doesn’t solve the first sample.