.NET string.IsNullOrWhiteSpace实现

public static bool IsNullOrWhiteSpace(string value);

string str1 = null;
string str2 = "";
string str3 = "   ";
string str4 = "Hello World!";

bool result1 = string.IsNullOrWhiteSpace(str1); // true
bool result2 = string.IsNullOrWhiteSpace(str2); // true
bool result3 = string.IsNullOrWhiteSpace(str3); // true
bool result4 = string.IsNullOrWhiteSpace(str4); // false