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Ignore overloads with ref struct parameters within Expression lambdas

meetings/working-groups/ref-improvements/ignore-overloads-in-expressions.md

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Ignore overloads with ref struct parameters within Expression lambdas

Ignore overloads with ref struct parameters when binding within Expression lambdas.

Motivation

C#13 adds support for params span, and C# preview adds first-class span types. Both of these features allow overload resolution to prefer overloads with span type parameters in more cases than with earlier language versions.

However, within an Expression, binding to a member that requires a ref struct instance may result in a compile-time or runtime error, and is therefore a breaking change.

Example 1: params span, see #110592

csharp
Expression<Func<string, string, string>> expr =
    (x, y) => string.Join("", x, y); // String.Join(string?, params ReadOnlySpan<string?>)

Example 2: First-class span types, see #109757

csharp
Expression<Func<string[], string, bool>> expr =
    (x, y) => x.Contains(y); // System.MemoryExtensions.Contains<T>(this ReadOnlySpan<T>, T)

var f = expr.Compile(preferInterpretation: true); // Exception

Design

12.6.4.2 Applicable function member could be updated as follows:

  • A static method is only applicable if the method group results from a simple_name or a member_access through a type.
  • ...
  • Within an Expression, if any parameters of the candidate method, other than the implicit instance method receiver, may have a ref struct type, the candidate is not applicable.

Note that the disqualifying parameter:

  • May be a params parameter
  • May be an optional parameter
  • May have a generic parameter type with allows ref struct

Drawbacks

Ignoring certain overloads may be a breaking change itself, in limited scenarios where ref struct instances are supported within an Expression currently. Examples?

Alternatives

No compiler change; add IDE fixer

Make no additional compiler changes. Instead, require callers to rewrite Expression instances to work around the overload resolution changes. An IDE fixer could be provided to rewrite calls for these cases.

Support ref struct arguments within Expression lambdas

The Expression interpreter relies on reflection, so supporting these cases would require supporting ref struct arguments in reflection or rewriting parts of the interpreter.