MemberMaybeNull

[soraphis]

I propose adding a new attribute called MemberMaybeNull that can be applied to methods and gives the compiler information about the nullability of a member variable after the method is executed.

[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
public class MemberMaybeNullAttribute : Attribute
{
    public string MemberName { get; }

    public MemberMaybeNullAttribute(string memberName)
    {
        MemberName = memberName;
    }
}

This attribute can be applied to a method like this:

public class ExampleClass
{
    public string Value { get; set; }

    [MemberMaybeNull("Value")]
    public void DoSomething(bool makeNull)
    {
        if (makeNull)
        {
            Value = null;
        }
    }
}

In this example, the DoSomething method has the MemberMaybeNull attribute applied to it, with the Value member specified as the argument. This tells the compiler that the Value member may be null after the DoSomething method is executed.

A use case would be in the following method:

public class ExampleClass
{
   // ...


   public void MyMethod(bool flag){

       if(Value == null) return;
       // use Value ...

       DoSomething(flag); 

       Console.WriteLine(Value.ToLower()); // potential null reference 
   }
}

[UniqeId6542]

An interesting idea.

Another similar example:

public class ExampleClass
{
    public void Test()
    {
        ExampleClass a = new();
        var v = a.Value; // null, intellisense says: may be null

        a.DoSomething();
        var v2 = a.Value; // not null, intellisense says: not null

        a.DoSomethingElse();
        var v3 = a.Value; // null, intellisense says: not null
    }

    public string? Value { get; set; }

    [MemberNotNull(nameof(Value))]
    public void DoSomething()
    {
        this.Value = "hi";
    }

    public void DoSomethingElse()
    {
        this.Value = null;
    }
}

[RikkiGibson]

Adding a new nullability attribute is a language suggestion so moving to csharplang.