using System.Diagnostics;


Stopwatch sw = new Stopwatch ();
sw.Start ();

string str = "a";
for (var i = 0; i < 10000; i++) {
    str += "b";
}
sw.Stop ();
Console.WriteLine (sw.Elapsed); // 输出 从 Start() 到 Stop() 之间经过时间。（单位：秒）

long startTime = DateTime.Now.Millisecond;
string str = "abc";
for (var i = 0; i < 1000; i++)
{
    str += "abc";
}
long endTime = DateTime.Now.Millisecond;
Console.Write(str);
Console.WriteLine("str 拼接1000次，用时：{0}毫秒。", endTime - startTime);


startTime = DateTime.Now.Millisecond;
str = "";
for (var i = 0; i < 100; i++)
{
    str += 'a';
}
endTime = DateTime.Now.Millisecond;
Console.Write(str);
Console.WriteLine("str 拼接100次，用时：{0}毫秒。", endTime - startTime);