• Specification checked in to csharplang.

Feature List

Features

• using declaration Champion "Implicitly scoped using statement" (16.3, Core 3) csharplang#1174
• pattern-based using for ref struct Proposal: Allow using statement to structurally match IDisposable on ref structs (16.3, Core 3) csharplang#1623

Code Examples

Using declarations

using var fileStream = new FileStream(@"temp\example.txt");

// vs. 

using (var fileStream = new FileStream(@"temp\example.txt")) {
  ...
}

// Works for IAsyncDisposable as well. 
await using var x = new AsyncDisposableType();

Pattern based using for ref struct

ref struct S { 
  public void Dispose() { ... }
}

using var local = new S();

Compiler Secnarios

• Ensure all features are tied to the C# 8.0 language version
• Goto and using declaration

  • Goto can't jump over a using declaration unless it also jumps over the closing brace

    { 
        { 
            goto target1; // error
            goto target2; // okay
        }
        using var fileStream = new FileStream(...);
    
        target1:
        ...
    }
    target2:
    ...

  • goto backwards over a using declaration must also cross the opening brace

  • goto can jump around inside a block when it doesn't cross using declaration

• pattern based using on ref struct
  • Follows the pattern based lookup specification
    • Consider extension methods
    • Consider generic methods where inference is possible
  • Applies to enumerators inside a foreach block (both classic and await foreach)
  • ensure a ref struct respects the lifetime of the initializer

    ref struct S {
        public Span<int> field;
        public void Dispose() { } 
    }
    
    S M() { 
        Span<int> span = stackalloc ... 
        using S local = new S() { field = span; }
        return local; // error 
    }

• using declaration restrictions
  • prohibit ref and ref readonly in a using declaration
  • test in conjunction with const: using const ... (should be binding error), const using ... (parsing error?)
  • test without initializer: using MyDisposable local;
  • test with non-disposable type
  • test with missing well-known IDisposable type (see uses of Compilation.MakeTypeMissing(WellKnownType) API)
  • test with missing well-known IAsyncDisposable type.
  • test await using declaration only legal in async method.
  • test can't use on a field or parameter
  • test disallowed in case label (unless inside an explicit block)
  • test in scripting
  • test that dynamic is allowed
• using declaration usage cases
  • test with null / default value
  • test with lambdas or local functions that capture using variables and non-using variables
  • variable is not assignable
  • test can't use as ref or out.
  • possible to use a struct variable as ref readonly or in

    struct S : IDisposable { 
        public void Dispose() { ... }
    }
    
    using S local = new S();
    ref readonly rr1 = ref local; // okay
    ref rr2 = ref local; // error

  • local functions and definite assignment

    public void M() {
        // legal because there isn't a definite assignment issue
        Local();
        
        using FileStream x;
        
        void Local()
        {
            x = null; // error is it's not assignable
            x.ToString();
        }
    }

  • order of dispose is reverse of declaration order
  • multi-variable declarator dispose order
  • multi-variable declarator dispose order in face of exception (all finally executed)
  • test inside an iterator
  • test inside an async method
• Semantic Model
  • Getting the awaiter info from a using declaration
  • Basic semantic information on the local from the API
• Consider impact on F#
• ref struct with Dispose() method should work if it returns Task or ValueTask (task-like)
  • same for DisposeAsync()
• test scripting (should disallow using-declaration, maybe some additional complications)
• check that using-declaration logic checks AwaitUsingAndForeachAddsPendingBranch flag (context)
• test ref struct with Task? DisposeAsync() method in await using (declaration or statement). Should produce a null warning
• test async using-declaration in async-iterator method
  • two async using-declarations with a yield return in between
• test obsolete Dispose method on a ref struct in using or in foreach

Productivity

• Find all references (moved all FAR issues into FindAllReferences should handle Dispose referenced in using and foreach statements #28228)
  • Finds Dispose method usage in using declaration
  • Finds Dispose / AsyncDispose method usage in foreach statements
• InlineVariable should not trigger for a using declaration variable (replaced by issue InlineVariable should not trigger for a using declaration #35645)
• Extract method should be disabled for blocks that have a using declaration (replaced by issue Extract Method should understand using declarations #35646)
• ENC
  • Verify it's functional for pattern based dispose in foreach
  • Using declarations going to be tricky. Mainly because adding “using” to a variable declaration implies
    wrapping the containing scope in a finally block. We need to be careful where to place the sequence point that is
    currently emitted for ‘}’ of such scope. EnC is especially concerned about all exception handling blocks and needs
    to be aware of them.)
• test MakeMethodAsync on using-declaration.
• verify sequence points and debugging experience
• Extras
  • fixer to use the right kind of declaration (await using or using) depending on the kind of disposable
  • refactoring to convert using-statement to using-declaration. (done)

Open LDM Issues

• Syntax for using var and IAsyncDisposable (await using var x = ...)?
• Should extension dispose be invoked for null values ( writing x.Dispose() would call it, GetPinnableReference() for fixed doesn't)
• Nullable-value types in using (should we fail if there isn't a lifted extension, or should we check null and call the underlying type if not)
• Handle the jump forward 'goto' case. Should we disallow labels in a block after a using var (you can't jump into a using statement today as the scope is different)

Proposal: dotnet/csharplang#1703
Championed issue (with LDM history): dotnet/csharplang#1174