using statement in C# 8 without a variable

Question

Is there a mechanism for the new c# 8 using statement to work without a local variable?

Given ScopeSomething() returns a IDisposable (or null)...

Previously:

using (ScopeSomething())
{
    // ...
}

However in C# 8 with the using statement, it requires a variable name:

using var _ = ScopeSomething();

The _ here is not treated as a discard.

I would have expected this to have worked:

using ScopeSomething();

@Saeid it’s a common pattern in logging and other areas to provide a sort of context or explicit scope — Daniel A. White
– Daniel A. White, Commented Apr 11, 2020 at 2:44
Thanks for explaining, if I'm not mistaken, the compiler decides the scope of the "using' statement in C# 8 based on the last usage of the disposable instance. I'm guessing with the pattern you explained, you want to do something (E.g., logging the execution) when the instance is disposed. I think with what you are proposing, the compiler supposed to dispose the instance when the method block ends. I don't think there is a way to do that in C# 8. It can be a good new feature for C# 9! — Saeid
– Saeid, Commented Apr 11, 2020 at 2:53
@Saeid - "The lifetime of a using local will extend to the end of the scope in which it is declared." - That's the current C# 8 implementation. No need to wait for C# 9. — Enigmativity
– Enigmativity, Commented Apr 11, 2020 at 3:16
Filed github.com/dotnet/roslyn/issues/43292 . Thanks for finding this issue! — Julien Couvreur
– Julien Couvreur, Commented Apr 12, 2020 at 3:28

Community · Accepted Answer · 2020-06-20 09:12:55Z

16

There are two kinds of resource_acquisition supported by using as per the specifications: local_variable_declaration and expression.

I believe the local using in C# 8 is a shortcut to local_variable_declaration form not expression form as per the language feature proposal, where you could see:

Restrictions around using declaration:

Must have an initializer for each declarator.

This also provides the ability to use more than one resources in a single using statement as per the language specs states:

When a resource_acquisition takes the form of a local_variable_declaration, it is possible to acquire multiple resources of a given type.

So you could do:

using IDisposable a = Foo1();
using IDisposable a = Foo1(), b = Foo2(), c = Foo3();

It may be possible that the language team could add expression form support in the future versions, but for now, expression is not yet supported.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 11, 2020 at 3:49

weichch

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4 Comments

41686d6564 Over a year ago

+1 This appears to be a valid answer. I just want to point out that with the classic using statement, it has always been possible to acquire multiple resources with/without declared variable declarations. For example, using (Foo1()) using ( Foo2()) { } was/is supported with the classic using statement (assuming Foo1() and Foo2() return an IDisposable).

weichch Over a year ago

@AhmedAbdelhameed Not within the same using though, like using (Foo1(), Foo2()) is invalid, but using(IDisposable a = Foo1(), b = Foo2()) is valid.

41686d6564 Over a year ago

They compile to the same IL, AFAICT. See this vs this.

weichch Over a year ago

Yes because they both valid using statements and translated as per the C# specs.

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