Is C# enum ToString() guaranteed to return enum name?

Question

enum Flags
{
  Foo,
  Bar
}

Is Flags.Foo.ToString() guaranteed to return "Foo"? Or do I have to use Enum.GetName(...)?

Answer 1

If the enum value happens to match an enum item, then yes.

But beware of cases like this:

var test = (Flags)(-1);
// test.ToString() == "-1"

If the value doesn't match an enum item, it will just return the underlying value as a string. By default, the underlying datatype of an enum is int.

Also, if your enum is defined with [Flags] like this:

[Flags]
enum Flags
{
    Foo = 1,
    Bar = 2
}

Then ToString() can return a comma-separated list of flags:

var test = Flags.Foo | Flags.Bar;
// test.ToString() == "Foo, Bar"

Like Orace points out in the comments, if the value is ambiguous, that is if multiple enum items can match the value, you should not make any assumptions about which one will be chosen.

Answer 2

The return value is formatted with the general format specifier ("G"). That is, if the FlagsAttribute is not applied to this enumerated type and there is a named constant equal to the value of this instance, then the return value is a string containing the name of the constant. If the FlagsAttribute is applied and there is a combination of one or more named constants equal to the value of this instance, then the return value is a string containing a delimiter-separated list of the names of the constants. Otherwise, the return value is the string representation of the numeric value of this instance.

from MSDN