Champion "Target-typed `new` expression" (VS 16.8, .NET 5)

[gafter]

Summary: Allow Point p = new (x, y);

Shipped in preview in 16.7p1.

---

• Proposal added
• Discussed in LDM
• Decision in LDM (let's go ahead with prototype)
• Finalized (done, rejected, inactive)
• Spec'ed

See also dotnet/roslyn#35

• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-21.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-06-25.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-08-22.md#target-typed-new
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-17.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-17.md#target-typed-new

Test plan: dotnet/roslyn#28489

[gulshan]

At least this can be done with object initializers without type-name, which currently generate anonymous types.

[Thaina]

I am more fond of var everywhere instead of empty new

[jnm2]

@Thaina I absolutely agree for locals, but for fields and properties this would actually be preferable to me and a_m_a_z_i_n_g, not gonna lie.

private readonly Dictionary<(Foo foo, Bar bar), List<Baz>> bazByFooAndBar = new();

Love it! Plus also:

private readonly Dictionary<Foo, Bar> barByFoo;

public ctor(IDictionary<Foo, Bar> initialValues)
{
   barByFoo = new(initialValues);
}

I do a lot of field initialization requiring access to this, so I end up saying new and repeating long types quite frequently in constructor bodies.

[Thaina]

@jnm2 Well, readonly in constructor seem great

But still that the only place I think we should do like that, I don't like mutable reference so field should not new anywhere. I wish we could have var field instead

[Richiban]

@Thaina

I wish we could have var field instead

I believe this is a limitation of the fact that C# has a two-pass compiler. The entire signature of a class needs to be known before any of the initialization expressions can be executed, so if you use var for a field the compiler doesn't know the type on the right-hand side of the =.