为什么我不能"看到"这个枚举扩展方法？

public enum MyEnum
{
    Value1,
    Value2,
    Value3
}

public static class MyEnumExtensions
{
    public static string GetDescription(this MyEnum value)
    {
        switch (value)
        {
            case MyEnum.Value1:
                return "This is Value1";
            case MyEnum.Value2:
                return "This is Value2";
            case MyEnum.Value3:
                return "This is Value3";
            default:
                throw new ArgumentOutOfRangeException(nameof(value), value, null);
        }
    }
}

MyEnum myValue = MyEnum.Value1;
string description = myValue.GetDescription();
Console.WriteLine(description);