Support null checking with property in Nullable Reference Type analysis

[MihaZupan]

class Foo
{
    public TaskCompletionSource<bool>? _completionSource;

    public bool IsStopped => !(_completionSource is null);
    // or
    public bool IsStopped => _completionSource != null;

    public void Bar()
    {
        if (IsStopped)
        {
            // Possible dereference
            _completionSource.SetResult(true);
        }

        if (!(_completionSource is null))
        {
            // This is fine
            _completionSource.SetResult(true);
        }
    }
}

I know I can stick a dammit operator in there, was just wondering if there are plans to support such null checks in analysis.

[jcouv]

There is no plan to support such an analysis at this point.
This issue is similar to #34800

[MihaZupan]

Okay. Thanks for the response.

[jcouv]

This will be supported in C# 9 after all, but requires an attribute to be placed saying that the return value of IsStopped affects our knowledge of the state of _completionSource.

class Foo
{
    public TaskCompletionSource<bool>? _completionSource;

    [MemberNotNullWhen(true, "_completionSource")]
    public bool IsStopped => !(_completionSource is null);
    
    public void Bar()
    {
        if (IsStopped)
        {
            // Possible dereference
            _completionSource.SetResult(true);
        }

        if (!(_completionSource is null))
        {
            // This is fine
            _completionSource.SetResult(true);
        }
    }
}

namespace System.Diagnostics.CodeAnalysis
{
    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]
    public sealed class MemberNotNullAttribute : Attribute
    {
        public MemberNotNullAttribute(params string[] members) { }
        public MemberNotNullAttribute(string member) { }
    }
}

namespace System.Diagnostics.CodeAnalysis
{
    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, AllowMultiple = true)]
    public sealed class MemberNotNullWhenAttribute : Attribute
    {
        public MemberNotNullWhenAttribute(bool when, params string[] members) { }
        public MemberNotNullWhenAttribute(bool when, string member) { }
    }
}

sharplab

[MihaZupan]

Thanks! I didn't realize the attribute will be supported on properties as well.

Thanks for reaching out even on such an old and closed issue