.NET: Why aren't Enum's Range/Value Checked?

Question

This has always bugged me. Perhaps someone with some hardcore knowledge of .NET internals can explain it to me.

Suppose I define an enum as follows:

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Answer 1

The issue was performance. It is quite simple to have a checked enum for normal enums such as Color

enum Color {
  Red,
  Blue
}

The problem though is for enum's which are used as bit flags.

enum Property {
  IsFirst = 0x1,
  IsDefault = 0x2,
  IsLastAccessed = 0x4
}

Having to do a bitwise check for every single integer which is converted to an Enum value was deemed to be too expensive. Hence the relaxed conversion to enum values.

Answer 2

Range-checking has a potentially unnecessary cost. It's therefore reasonable to not perform it implicitly. As already mentioned, [Flags] require that no such checking takes place. If the runtime would check for the presence of [Flags], this would still incur a runtime penalty every time you perform a conversion.

The only way around this would be for the compiler to be aware of the [Flags] attribute. I guess this wasn't done to reduce the amount of runtime knowledge hardcoded into the compiler.