你如何为一般类型的类编写C#扩展方法

public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }

    public static bool IsNullOrWhiteSpace(this string str)
    {
        return string.IsNullOrWhiteSpace(str);
    }

    public static string Reverse(this string str)
    {
        char[] charArray = str.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }
}

string myString = "Hello, world!";

if (!myString.IsNullOrEmpty())
{
    Console.WriteLine(myString.Reverse());
}