Enum.IsDefined returns false for strings

Question

I used the Enum.IsDefined() method for a string, but I get False in a situation where I think I should get True. Please check the below code:

public enum YourEnum : int
{
    Zero = 0,
    One = 1
}

public class Program
{

    public static void Main(string[] args)
    {
        Console.WriteLine(Enum.IsDefined(typeof(YourEnum), 1));
        Console.WriteLine(Enum.IsDefined(typeof(YourEnum), 1.ToString()));
    }
} 

C# Fiddle Demo
Result:

True
False

I don't know why should I get False in the second case. Any help is appreciated.

Answer 1

When you pass a string to the IsDefined() method, you are asking whether there is a value in the enum having that name. The documentation reads:

The value parameter can be any of the following:
• Any member of type enumType.
• A variable whose value is an enumeration member of type enumType.
• The string representation of the name of an enumeration member. The characters in the string must have the same case as the enumeration member name.
• A value of the underlying type of enumType.

(emphasis mine)

It's a bit confusing to read, because the first, second, and fourth options above all result in the same thing: a value of the enum type being passed (boxed, of course).

But the third option is what's going on your scenario, and the string needs to match the name of an enum member. You're passing the string "1", and the only valid names in the enum are "Zero" and "One". The string "1" doesn't match either of those, so IsDefined() returns false.