Proposal: Object expressions

[Porges]

The purpose of this is to allow things like:

var sw = Stopwatch.StartNew();
using (new IDisposable { void Dispose() => Console.WriteLine(sw.Elapsed) })
{
    // ... 
}

Currently one can write:

new { x = 1 }

The proposal is that one can also supply behaviour:

new { x = 1 } with { int XSquared => x * 2 }

The with is needed to disambiguate with collection/object initializers. Inside the with block, public visibility is the default.

This is not limited to anonymous types. Existing types can be augmented:

new List<int> { 1, 2, 3 } with { int Htgnel => this.Reverse().Count() }

Under the covers, this will generate a new type. Obviously the type specified needs to be unsealed/not a value type.

Additional interfaces can be implemented:

new List<int> { 1, 2, 3 } with IEquatable<List<int>>
    {
        bool Equals(List<int> other) => this.SequenceEqual(other)
    }

Anonymous types can also implement interfaces:

using (new with IDisposable { void Dispose() => Console.WriteLine("ended") })
{
    // ...
}

The with keyword is optional if the type specified is an interface, as there is no ambiguity with collection/object initializers, which gives the syntax used in the original example.

The last variation is a type with only behaviour, no interfaces or state.

new with { void Do() => Console.WriteLine("Done") }

Here with is required.

[Porges]

Already covered in #13, but I used the wrong search terms.