[Proposal]: nested members in with and object creation

[jcouv]

nested members in with and object creation

Champion issue: #9003

Summary

If we go ahead and add support for extended names in property patterns (if (e is MethodCallExpression { Method.Name: "MethodName" })), it makes sense for symmetry to allow this in with and object creation expressions:

methodCallExpression with { Method.Name = "MethodName" }
new MethodCallExpression(...) { Method.Name = "MethodName" }

Motivation

This is particularly useful in the with expression, so you don't have to write:
methodCallExpression with { Method = methodCallExpression.Method with { Name = "MethodName" } }

Detailed design

Drawbacks

Alternatives

Unresolved questions

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-09-22.md#nested-members-in-with-and-object-creation
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-01-24.md#nested-members-in-with-and-object-creation

[jnm2]

Existing discussion on nested with expressions: #77

Could there be names in scope to avoid doing a deep traversal twice? A common scenario (usually deeper):

// Names in scope after '=': NestedThing, Collection
thing = thing with { NestedThing.Collection = Collection.Remove(value) };

As opposed to:

thing = thing with { NestedThing.Collection = thing.NestedThing.Collection.Remove(value) };

[DustinCampbell]

Here's another potential alternate syntax:

methodCallExpression with
{
    Method with { Name = "MethodName" }
}

[XDracam]

I hacked together my own Lenses with source generators today just to avoid annoying nested with statements. If anyone needs a quick workaround, here it is (MIT license, use at your own risk). But I'd prefer a convenient builtin syntax.