测试环境：https://try.dot.net/
Console.WriteLine(string.IsNullOrWhiteSpace("")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(null)); --> True
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> False 空字符 所以如果输入字符串中含有 这样的单独串，使用 string.IsNullOrWhiteSpace 是应该注意
Console.WriteLine(string.IsNullOrWhiteSpace(" ")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace("v")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace("
")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace("
")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace("f")); --> True
Console.WriteLine(string.IsNullOrWhiteSpace("a")); --> False a 感叹号
Console.WriteLine(string.IsNullOrWhiteSpace("")); --> False 退格
Console.WriteLine("123 "+'d'); --> 123 d
Console.WriteLine(("123 "+'d').Length); --> 5
Console.WriteLine("\0"+'d'); --> d
Console.WriteLine(@" "+'d'); --> d