Enhanced var pattern

[KyouyamaKazusa0805]

Today C# allows us using var pattern to get result from an expression like:

if (expr is { Property: var p, _field: var f })
    // ...

Sometimes, a property or field may be a ref return rather than a value return. Here may exist a need for defining inlining variables with ref using var pattern like:

// properties
if (expr is
{
    RefProperty: ref var r,
    RefProperty2: ref readonly var s,
    RefReadOnlyProperty: ref readonly var t
})
    // ...

// indexers
if (span is [ref var a, ref readonly var b, ..])
    // ...

// methods
if (Method() is ref var r)
    // ...

Thank you.

[DavidArno]

Does C# support ref properties now? Think I missed that one 😊

[DavidArno]

When was that added?

[HaloFour]

I think the same time ref returns were?

[KyouyamaKazusa0805]

C# 7.x ... I've forgot the minor version of the feature born. I just know that C# 7 introduced 20+ features, and one of the features is ref return (properties/indexers/methods).

[DavidArno]

Well, you learn something every day. Apologies for distracting away from the request. Thanks, both.

[FaustVX]

You probably already used it, Span<> uses ref property.

[jnm2]

Since every line in your example should unconditionally succeed, I'd turn this around and say that having nominal deconstruction with var would be the ideal way to approach the first:

var {
    RefProperty: ref r,
    RefProperty2: ref readonly s,
    RefReadOnlyProperty: ref readonly t,
} = expr;

The next would be list destructuring which hasn't been discussed yet that I'm aware of, but which keeps coming to my mind as we work on collection literals (@CyrusNajmabadi):

var [ref a, ref readonly b, ..] = span;

The last is already possible in the ideal (unconditional) form today:

ref var r = ref Method();

[KyouyamaKazusa0805]

To be honest there may be some other examples to be used, but I have no idea what are they... Because ref variable definition is special definition than non-ref one; however sometimes I will use some expressions with ref return. Pattern matching cannot create ref var pattern.

Just thinking further, I may use this to check nullability such as

ref var r = ref obj.Property;
if (!Unsafe.IsNullRef(ref r))
    // ...

I cannot write pattern matching like obj.Property is not null because I want to check its reference of the property Property. If we can define ref null as equivalent to Unsafe.NullRef<T>(), can we define the pattern matching like

if (obj.Property is ref var and not ref null)
    // ...

?

I don't know whether this example is reasonable to be introduced here. I just want to describe a way to use ref var pattern. 😄

[jnm2]

I don't know either. Every time I see the topic come up of checking refs to see if they are ref null, the sentiment seems to be ref null is so surprising in the .NET ecosystem that making it mainstream is a risky move. Without having thought deeply, it seems like NRT-style analysis could make this less risky. (Would we need a full-fledged ref? keyword? New NullableContextAttribute encoding scheme? That sounds expensive to design and implement.)

If you started a separate discussion to suggest the pattern ref null which could be applied to refs, I'd be interested to see what the response would be. It makes sense to me. But I also see how it is currently considered niche.

[KyouyamaKazusa0805]

Thank you so much!