1 string a1 = null;　　// 仅仅是一个声明，内存中并没有分配用来存储值的空间。
 2 
 3 string a1 = String.Empty;　　// 等价于 ""，以新建字符串长度为零的 String 对象，可以减少 NullReferenceException 发生的可能性。
 4 
 5 System.String greeting = "Hello World!";　　// 在内存中分配了一个连续性的不可变的储存空间。
 6 
 7 // 长字符串字面量拆分为较短的字符串，从而提高源代码的可读性。
 8 // 以下代码将较短的字符串连接起来，以创建长字符串字面量。 在编译时将这些部分连接成一个字符串。 无论涉及到多少个字符串，均不产生运行时性能开销。
 9 string text = "Historically, the world of data and the world of objects " +
10 "have not been well integrated. Programmers work in C# or Visual Basic " +
11 "and also in SQL or XQuery. On the one side are concepts such as classes, " +
12 "objects, fields, inheritance, and .NET Framework APIs. On the other side " +
13 "are tables, columns, rows, nodes, and separate languages for dealing with " +
14 "them. Data types often require translation between the two worlds; there are " +
15 "different standard functions. Because the object world has no notion of query, a " +
16 "query can only be represented as a string without compile-time type checking or " +
17 "IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to " +
18 "objects in memory is often tedious and error-prone.";

 1 // 初始化 s1
 2 // 内存为 s1 分配了存放字符的空间
 3 string s1 = "A string is more ";
 4 
 5 // 初始化 s2
 6 // 内存为 s2 分配了存放字符的空间
 7 string s2 = "than the sum of its chars.";
 8 
 9 // 内存为 s1 重新分配了新值的空间
10 // 并指向新空间的地址，所以 s1 有了新的值
11 // 无用的s1旧值等待GC垃圾回收
12 s1 += s2;
13 
14 System.Console.WriteLine(s1);

StringBuilder strb = new StringBuilder();
strb.Append('*', 10).Append(" Adding Text to a StringBuilder Object ").Append('*', 10);
strb.AppendLine("\n");
strb.AppendLine("Some code points and their corresponding characters:");
for (int ctr = 50; ctr <= 60; ctr++) {
    strb.AppendFormat("{0,12:X4} {1,12}", ctr,Convert.ToChar(ctr));
    strb.AppendLine();
}