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What is the syntax for an interface method when using MarshalAs
public interface IMine
{
  [DispId(1)]
  object stringSize(string txt, string fontName, float fontSize)
  {
    [return: MarshalAs(UnmanagedType.Struct, SafeArraySubType = VarEnum.VT_ARRAY)]
  }
}

The name 'MarshalAs' does not exist in the current context

public object stringSize(string txt, string fontName, float fontSize)
    {
        System.Drawing.SizeF result = _textSize(txt, fontName, fontSize);
        return new object[] { result.Width, result.Height };
    }

public interface IMine
{
  [DispId(1)]
  [return: MarshalAs(UnmanagedType.Struct, SafeArraySubType = VarEnum.VT_ARRAY)]
  object stringSize(string txt, string fontName, float fontSize)
  {
    // stuff...
  }
}