Why are operations between different enum types allowed in another enum declaration but not elsewhere?

Question

The C# compiler allows operations between different enum types in another enum type declaration, like this:

public enum VerticalAnchors
{
    Top=1,
    Mid=2,
          


        

Answer 1

Interesting. You can also ask why this is allowed:

enum MyType
{
  Member = DayOfWeek.Thursday | StringComparison.CurrentCultureIgnoreCase,
}

when this is not allowed:

var local = DayOfWeek.Thursday | StringComparison.CurrentCultureIgnoreCase;

The reason seems to be that inside the declaration of an enum, in the enum member initializers, any enum value, even a value of an unrelated enum, is considered to be cast to its underlying type. So the compiler sees the above example as:

enum MyType
{
  Member = (int)(DayOfWeek.Thursday) | (int)(StringComparison.CurrentCultureIgnoreCase),
}

I find this very strange. I knew that you could use values of the same enum directly (without stating the cast to the underlying type), as in the last line of:

enum SomeType
{
  Read = 1,
  Write = 2,
  ReadWrite = Read | Write,
}

but I find it very surprising that other enums' members are also cast to their underlying integer types.