首先看测试代码：
public class StringSpeedTest { private readonly static string _testStr = "0123456789"; public string StringAdd(int count) { string str = string.Empty; for (int i = 0; i < count; i++) { str += _testStr; } return str; } public string UseStringBuilder(int count) { StringBuilder sb = new StringBuilder(); for (int i = 0; i < count; i++) { sb.Append(_testStr); } return sb.ToString(); } public string UseStringBuilderWithCapacity(int count) { StringBuilder sb = new StringBuilder(count * _testStr.Length); for (int i = 0; i < count; i++) { sb.Append(_testStr); } return sb.ToString(); } public string UseStringConcat(int count) { string[] list = new string[count]; for (int i = 0; i < count; i++) { list[i] = _testStr; } return string.Concat(list); } public string UseStringConcatWithList(int count) { List<string> list = new List<string>(count); for (int i = 0; i < count; i++) { list.Add(_testStr); } return string.Concat(list); } public string UseStringConcatWithListToArray(int count) { List<string> list = new List<string>(count); for (int i = 0; i < count; i++) { list.Add(_testStr); } return string.Concat(list.ToArray()); } }
Stopwatch watch = new Stopwatch(); StringSpeedTest strSpeedTest = new StringSpeedTest(); int count = 10000; TimeSpan time = new TimeSpan(0, 0, 0); watch.Start(); for (int i = 0; i < 10; i++) { Console.WriteLine("-------------------------------------------------------------------"); strSpeedTest.StringAdd(count); Console.WriteLine($"StringAdd: {watch.Elapsed - time}"); time = watch.Elapsed; strSpeedTest.UseStringBuilder(count); Console.WriteLine($"UseStringBuilder: {watch.Elapsed - time}"); time = watch.Elapsed; strSpeedTest.UseStringBuilderWithCapacity(count); Console.WriteLine($"UseStringBuilderWithCapacity: {watch.Elapsed - time}"); time = watch.Elapsed; strSpeedTest.UseStringConcatWithList(count); Console.WriteLine($"UseStringConcatWithList: {watch.Elapsed - time}"); time = watch.Elapsed; strSpeedTest.UseStringConcatWithListToArray(count); Console.WriteLine($"UseStringConcatWithListToArray: {watch.Elapsed - time}"); time = watch.Elapsed; strSpeedTest.UseStringConcat(count); Console.WriteLine($"UseStringConcat: {watch.Elapsed - time}"); Console.WriteLine("*******************************************************************"); } watch.Stop();
测试结果：
------------------------------------------------------------------- StringAdd: 00:00:00.2908582 UseStringBuilder: 00:00:00.0008419 UseStringBuilderWithCapacity: 00:00:00.0008429 UseStringConcatWithList: 00:00:00.0101701 UseStringConcatWithListToArray: 00:00:00.0032169 UseStringConcat: 00:00:00.0005971 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1373059 UseStringBuilder: 00:00:00.0003654 UseStringBuilderWithCapacity: 00:00:00.0003533 UseStringConcatWithList: 00:00:00.0006684 UseStringConcatWithListToArray: 00:00:00.0003922 UseStringConcat: 00:00:00.0003371 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1313924 UseStringBuilder: 00:00:00.0003778 UseStringBuilderWithCapacity: 00:00:00.0010882 UseStringConcatWithList: 00:00:00.0010397 UseStringConcatWithListToArray: 00:00:00.0007571 UseStringConcat: 00:00:00.0004528 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1282163 UseStringBuilder: 00:00:00.0003770 UseStringBuilderWithCapacity: 00:00:00.0003452 UseStringConcatWithList: 00:00:00.0006388 UseStringConcatWithListToArray: 00:00:00.0003873 UseStringConcat: 00:00:00.0004068 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1349978 UseStringBuilder: 00:00:00.0004084 UseStringBuilderWithCapacity: 00:00:00.0003698 UseStringConcatWithList: 00:00:00.0006802 UseStringConcatWithListToArray: 00:00:00.0004059 UseStringConcat: 00:00:00.0003503 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1290642 UseStringBuilder: 00:00:00.0003813 UseStringBuilderWithCapacity: 00:00:00.0003674 UseStringConcatWithList: 00:00:00.0006886 UseStringConcatWithListToArray: 00:00:00.0004115 UseStringConcat: 00:00:00.0003509 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1363654 UseStringBuilder: 00:00:00.0003885 UseStringBuilderWithCapacity: 00:00:00.0004270 UseStringConcatWithList: 00:00:00.0007209 UseStringConcatWithListToArray: 00:00:00.0004618 UseStringConcat: 00:00:00.0003863 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1326732 UseStringBuilder: 00:00:00.0003978 UseStringBuilderWithCapacity: 00:00:00.0003965 UseStringConcatWithList: 00:00:00.0006867 UseStringConcatWithListToArray: 00:00:00.0004012 UseStringConcat: 00:00:00.0003480 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1275684 UseStringBuilder: 00:00:00.0003673 UseStringBuilderWithCapacity: 00:00:00.0004190 UseStringConcatWithList: 00:00:00.0007201 UseStringConcatWithListToArray: 00:00:00.0004572 UseStringConcat: 00:00:00.0003848 ******************************************************************* ------------------------------------------------------------------- StringAdd: 00:00:00.1606576 UseStringBuilder: 00:00:00.0004435 UseStringBuilderWithCapacity: 00:00:00.0005415 UseStringConcatWithList: 00:00:00.0007263 UseStringConcatWithListToArray: 00:00:00.0004205 UseStringConcat: 00:00:00.0003707 *******************************************************************
从结果中可以看到，StringBuilder 速度不是永远最快的。当把cout调整为100000时 结果如下，由于string直接+速度太慢直接去除比较梯队。
------------------------------------------------------------------- UseStringBuilder: 00:00:00.1400624 UseStringBuilderWithCapacity: 00:00:00.0057860 UseStringConcatWithList: 00:00:00.0186478 UseStringConcatWithListToArray: 00:00:00.0074943 UseStringConcat: 00:00:00.0054871 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0115386 UseStringBuilderWithCapacity: 00:00:00.0057336 UseStringConcatWithList: 00:00:00.0080408 UseStringConcatWithListToArray: 00:00:00.0052401 UseStringConcat: 00:00:00.0037864 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0085633 UseStringBuilderWithCapacity: 00:00:00.0049141 UseStringConcatWithList: 00:00:00.0179742 UseStringConcatWithListToArray: 00:00:00.0045335 UseStringConcat: 00:00:00.0042053 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0085083 UseStringBuilderWithCapacity: 00:00:00.0041245 UseStringConcatWithList: 00:00:00.0080081 UseStringConcatWithListToArray: 00:00:00.0043282 UseStringConcat: 00:00:00.0037264 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0087733 UseStringBuilderWithCapacity: 00:00:00.0086311 UseStringConcatWithList: 00:00:00.0080520 UseStringConcatWithListToArray: 00:00:00.0056323 UseStringConcat: 00:00:00.0038094 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0086884 UseStringBuilderWithCapacity: 00:00:00.0063233 UseStringConcatWithList: 00:00:00.0081824 UseStringConcatWithListToArray: 00:00:00.0055393 UseStringConcat: 00:00:00.0040967 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0108527 UseStringBuilderWithCapacity: 00:00:00.0048482 UseStringConcatWithList: 00:00:00.0079176 UseStringConcatWithListToArray: 00:00:00.0043536 UseStringConcat: 00:00:00.0042921 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0119569 UseStringBuilderWithCapacity: 00:00:00.0043067 UseStringConcatWithList: 00:00:00.0091552 UseStringConcatWithListToArray: 00:00:00.0045956 UseStringConcat: 00:00:00.0041058 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0096342 UseStringBuilderWithCapacity: 00:00:00.0055840 UseStringConcatWithList: 00:00:00.0073000 UseStringConcatWithListToArray: 00:00:00.0056438 UseStringConcat: 00:00:00.0038800 ******************************************************************* ------------------------------------------------------------------- UseStringBuilder: 00:00:00.0090576 UseStringBuilderWithCapacity: 00:00:00.0047266 UseStringConcatWithList: 00:00:00.0076383 UseStringConcatWithListToArray: 00:00:00.0048706 UseStringConcat: 00:00:00.0048964 *******************************************************************
从结果中可以看到再没有使用初始化StringBuilder 容器容量的时候，性能与使用string.Concat相差已经很大。在使用初始化容量之后 性能有较大的提升。
总结如下：
1、在只知道字符串最终长度的时候，可以使用 StringBuilder 并初始化容量，可以有较大的提升；
2、在不知道最终字符串长度时候，但是知道拼接字符串次数的时候，可以将每次字符串保存在一个字符串数组中，可以有较大的性能提升;
3、在不知道字符串长度以及拼接的次数的时候，可以使用LIst<string> 保存，可以有较大的性能提升。当然也可以大约估计最终字符串长度，然后初始化StringBuilder 时候使用较大的容量，以空间来换取速度
当然这些都是在拼接字符串的次数很大的情况下，会有较大的性能提升。
