It is very interesting to me why this code will not give an exception and 12356 will be output to the console? What is the meaning of this behavior enum?
enum Dummy { D1 = 123, D2 = 1234, D3 } class Program { static void Main(string[] args) { Dummy d = (Dummy)12356; Console.WriteLine((int)d); Console.ReadKey(); } } Is this a bug or a feature?
Is this a bug or a feature?
A feature, although it might bring you in curious circumstances when the code doesn't work as expected.
You could for example combine enum values, for example:
enum Dummy { D1 = 1, D2 = 2, D3 = 4 } Dummy v = Dummy.D1 | Dummy.D2; The backing value here is 3.
You can check for either of the values like this:
bool isD1 = (v & Dummy.D1) == Dummy.D1; You could check your enum for single value validness like this:
Dummy v = Dummy.D1; bool isValid = Enum.IsDefined(typeof(Dummy), v); 
This is by design but discouraged, as specified in the documentation:
It's possible to assign any arbitrary integer value to meetingDay. For example, this line of code does not produce an error: meetingDay = (Day) 42. However, you should not do this because the implicit expectation is that an enum variable will only hold one of the values defined by the enum. To assign an arbitrary value to a variable of an enumeration type is to introduce a high risk for errors.
You can always check if a given value is defined for an enum type by using IsDefined:
Console.WriteLine(Enum.IsDefined(typeof(Dummy), 123)); //True Console.WriteLine(Enum.IsDefined(typeof(Dummy), 123456)); //False 