An example of an unsafe use of Enum.IsDefined, checking enum values in a non-backwards compatible way.

Enum_IsDefined_example.cs

using System;
using System.Linq;

public enum MyEnum
{
    A,
    B
    // introducing a new value C here would cause an argument exception to be thrown in SafeMethod, but not in UnsafeMethod
}

public class MyClass
{
    public void SafeMethod(int value)
    {
        // safe, as it explicitly specifies supported values
        if (!(new[] { MyEnum.A, MyEnum.B }).Contains((MyEnum)value))
        {
            throw new ArgumentException("Supplied value is not supported", nameof(value));
        }

        ...
    }

    public void UnsafeMethod(int value)
    {
        // unsafe, as values introduced in the future would also pass this check
        if (!Enum.IsDefined(typeof(MyEnum), value))
        {
            throw new ArgumentException("Supplied value is not supported", nameof(e));
        }

        ...
    }
}