CS8852: Init-only property cannot be assigned to in an 'init' accessor if accessing it requires a cast (e.g. an explicitly implemented interface type)

[ericwj]

Version Used:
Using "Csc" task from assembly "C:\Program Files\dotnet\sdk\5.0.200-preview.20601.7\Roslyn\Microsoft.Build.Tasks.CodeAnalysis.dll".

Steps to Reproduce:

Try to compile this.

public interface ISomething { int Property { get; init; } }
public record BaseType : ISomething { int ISomething.Property { get; init; } }
public record DerivedType : BaseType {
    public int SomethingElse {
        get => ((ISomething)this).Property;
        init => ((ISomething)this).Property = value;
    }
}

Expected Behavior:
Compiles.

Actual Behavior:
error CS8852: Init-only property or indexer 'ISomething.Property' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

[DataRostiashvili]

Same problem bugs me out. Is it intentional? if so why?

[jaredpar]

In the cases the cast is an identity preserving conversion on this the cast should be allowed here.

[jcouv]

LDM confirmed that this is by-design. If a base type needs to expose something to initialize, it should expose APIs to do so.

[ericwj]

What API? Can't do this through an interface since that introduces public API to circumvent the init-only-ness. Awkward.

[jcouv]

Simply using a public interface implementation would do it:

public interface ISomething { int Property { get; init; } }
public record BaseType : ISomething { int Property { get; init; } }

Note there is a similar problem that's already existing with a private implementation with set or init property:

public record BaseType : ISomething { int ISomething.Property { get; set; } }

new BaseType() { ... can't set property ... }

[ViIvanov]

What API? Can't do this through an interface since that introduces public API to circumvent the init-only-ness. Awkward.

I think it could be something like this:

public interface ISomething {
  int Property { get; init; }
}

public record BaseType : ISomething
{
  protected int SomethingProperty { get; init; }
  
  int ISomething.Property {
    get => SomethingProperty;
    init => SomethingProperty = value;
  }
}

public record DerivedType : BaseType
{
  public int SomethingElse {
    get => SomethingProperty;
    init => SomethingProperty = value;
  }
}

[ericwj]

I find at the very least the error message very confusing. It is also very weird to me that this works, but not the original code in the issue, with the only difference being that I don't want Property to be visible without the cast to ISomething:

public interface ISomething { int Property { get; init; } }
public record BaseType : ISomething { public int Property { get; init; } }
public record DerivedType : BaseType {
    public int SomethingElse {
        get => Property;
        init => Property = value;
    }
}