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Add an example of using UnscopedRefAttribute to fix CS8170 #47078

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Add an example of using UnscopedRefAttribute to fix CS8170 #47078

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5f5411d
Add an example of using UnscopedRefAttribute to fix CS8170
BartoszKlonowski Jul 2, 2025
c3fb5a3
Use ref when assigning result of M to d var
BartoszKlonowski Jul 3, 2025
5cb6f63
Warn for being sure about safety of leaving the scope by ref
BartoszKlonowski Jul 3, 2025
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27 changes: 27 additions & 0 deletions docs/csharp/language-reference/compiler-messages/cs8170.md
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Original file line number Diff line number Diff line change
Expand Up @@ -83,3 +83,30 @@ public class Other
}
}
```

Another approach is to use the <xref:System.Diagnostics.CodeAnalysis.UnscopedRefAttribute?displayProperty=nameWithType> attribute. It will mark the reference to be allowed to escape the scope.
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This is great, but I think it's worth pointing out the danger here: you really need to know that the ref can escape safely.

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Please check with the latest, as I added the "warning" for being sure about the safety of leaving the scope.
Let me know, please if there's some other wording you think would suit this better.

Below is the example of applying <xref:System.Diagnostics.CodeAnalysis.UnscopedRefAttribute?displayProperty=nameWithType> to `int M()` method, which fixes the CS8170:

```csharp
using System.Diagnostics.CodeAnalysis;

struct Program
{
public int d;

[UnscopedRef]
public ref int M()
{
return ref d; // No error - ref is valid to escape the scope in this line of that method
}
}

public class Other
{
public void Method()
{
var p = new Program();
var d = p.M();
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Should this use = ref?

Suggested change
var d = p.M();
ref int d = ref p.M();

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Indeed - However, because I generally used the exact same example as the one showing the CS8170, I also updated the original with this suggestion.

}
}
```
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