Records (C# reference)|Inheritance|with expressions in derived records – Clone does not use *covariant return type*

[bravequickcleverfibreyarn]

From Records (C# reference)/Inheritance/with expressions in derived records

Because the synthesized clone method uses a covariant return type, the result of a with expression has the same run-time type as the expression's operand.

This seems rendered as a lie.

Decompilation

private void Test()
{
  DerivedRec dr = (DerivedRec)new DerivedRec().<Clone>$();
}

public override BaseRec <Clone>$()
{
  return new DerivedRec(this);
}

Definition

class CovariantReturnType
{
    record BaseRec () { }
    record DerivedRec : BaseRec { }

    void Test ()
    {       
      var dr = new DerivedRec () with { };
    }
}

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Document Details

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• ID: 20818c64-4ad8-fb15-5d79-e7f98614e9c4
• Version Independent ID: de1f383b-ef20-4776-57a8-04edae892166
• Content: Records - C# reference
• Content Source: docs/csharp/language-reference/builtin-types/record.md
• Product: dotnet-csharp
• Technology: csharp-language-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[Youssef1313]

This is being worked on currently in Roslyn. See dotnet/roslyn#53404.

[bravequickcleverfibreyarn]

This is being worked on currently in Roslyn. See dotnet/roslyn#53404.

Ok, I see. Documentation antedates implementation. Not big problem.

[Youssef1313]

@BillWagner Do you think it's worth specifying the compiler version which have this update when it gets merged?

[BillWagner]

@Youssef1313 @Uzivatel919

As a fix, I think I'd prefer removing that implementation detail from the description of records. The important concept is that with expressions don't accidentally slice a record if the source argument has a compile time type that is a base class of the runtime type.