首先看测试代码：
public class StringSpeedTest
{
private readonly static string _testStr = ""; public string StringAdd(int count)
{
string str = string.Empty;
for (int i = ; i < count; i++)
{
str += _testStr;
}
return str;
} public string UseStringBuilder(int count)
{
StringBuilder sb = new StringBuilder(); for (int i = ; i < count; i++)
{
sb.Append(_testStr);
} return sb.ToString();
}
public string UseStringBuilderWithCapacity(int count)
{
StringBuilder sb = new StringBuilder(count * _testStr.Length); for (int i = ; i < count; i++)
{
sb.Append(_testStr);
} return sb.ToString();
}
public string UseStringConcat(int count)
{
string[] list = new string[count];
for (int i = ; 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 = ; 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 = ; i < count; i++)
{
list.Add(_testStr);
}
return string.Concat(list.ToArray());
}
}
Stopwatch watch = new Stopwatch();
StringSpeedTest strSpeedTest = new StringSpeedTest(); int count = ;
TimeSpan time = new TimeSpan(, , );
watch.Start();
for (int i = ; i < ; i++)
{
Console.WriteLine("-------------------------------------------------------------------");
strSpeedTest.StringAdd(count); Console.WriteLine($"StringAdd:\t\t\t{watch.Elapsed - time}");
time = watch.Elapsed; strSpeedTest.UseStringBuilder(count);
Console.WriteLine($"UseStringBuilder:\t\t{watch.Elapsed - time}");
time = watch.Elapsed; strSpeedTest.UseStringBuilderWithCapacity(count);
Console.WriteLine($"UseStringBuilderWithCapacity:\t{watch.Elapsed - time}");
time = watch.Elapsed; strSpeedTest.UseStringConcatWithList(count);
Console.WriteLine($"UseStringConcatWithList:\t{watch.Elapsed - time}");
time = watch.Elapsed; strSpeedTest.UseStringConcatWithListToArray(count);
Console.WriteLine($"UseStringConcatWithListToArray:\t{watch.Elapsed - time}");
time = watch.Elapsed; strSpeedTest.UseStringConcat(count);
Console.WriteLine($"UseStringConcat:\t\t{watch.Elapsed - time}"); Console.WriteLine("*******************************************************************");
} watch.Stop();
测试结果：
-------------------------------------------------------------------
StringAdd: ::00.2908582
UseStringBuilder: ::00.0008419
UseStringBuilderWithCapacity: ::00.0008429
UseStringConcatWithList: ::00.0101701
UseStringConcatWithListToArray: ::00.0032169
UseStringConcat: ::00.0005971
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1373059
UseStringBuilder: ::00.0003654
UseStringBuilderWithCapacity: ::00.0003533
UseStringConcatWithList: ::00.0006684
UseStringConcatWithListToArray: ::00.0003922
UseStringConcat: ::00.0003371
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1313924
UseStringBuilder: ::00.0003778
UseStringBuilderWithCapacity: ::00.0010882
UseStringConcatWithList: ::00.0010397
UseStringConcatWithListToArray: ::00.0007571
UseStringConcat: ::00.0004528
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1282163
UseStringBuilder: ::00.0003770
UseStringBuilderWithCapacity: ::00.0003452
UseStringConcatWithList: ::00.0006388
UseStringConcatWithListToArray: ::00.0003873
UseStringConcat: ::00.0004068
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1349978
UseStringBuilder: ::00.0004084
UseStringBuilderWithCapacity: ::00.0003698
UseStringConcatWithList: ::00.0006802
UseStringConcatWithListToArray: ::00.0004059
UseStringConcat: ::00.0003503
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1290642
UseStringBuilder: ::00.0003813
UseStringBuilderWithCapacity: ::00.0003674
UseStringConcatWithList: ::00.0006886
UseStringConcatWithListToArray: ::00.0004115
UseStringConcat: ::00.0003509
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1363654
UseStringBuilder: ::00.0003885
UseStringBuilderWithCapacity: ::00.0004270
UseStringConcatWithList: ::00.0007209
UseStringConcatWithListToArray: ::00.0004618
UseStringConcat: ::00.0003863
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1326732
UseStringBuilder: ::00.0003978
UseStringBuilderWithCapacity: ::00.0003965
UseStringConcatWithList: ::00.0006867
UseStringConcatWithListToArray: ::00.0004012
UseStringConcat: ::00.0003480
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1275684
UseStringBuilder: ::00.0003673
UseStringBuilderWithCapacity: ::00.0004190
UseStringConcatWithList: ::00.0007201
UseStringConcatWithListToArray: ::00.0004572
UseStringConcat: ::00.0003848
*******************************************************************
-------------------------------------------------------------------
StringAdd: ::00.1606576
UseStringBuilder: ::00.0004435
UseStringBuilderWithCapacity: ::00.0005415
UseStringConcatWithList: ::00.0007263
UseStringConcatWithListToArray: ::00.0004205
UseStringConcat: ::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: ::00.1400624
UseStringBuilderWithCapacity: ::00.0057860
UseStringConcatWithList: ::00.0186478
UseStringConcatWithListToArray: ::00.0074943
UseStringConcat: ::00.0054871
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0115386
UseStringBuilderWithCapacity: ::00.0057336
UseStringConcatWithList: ::00.0080408
UseStringConcatWithListToArray: ::00.0052401
UseStringConcat: ::00.0037864
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0085633
UseStringBuilderWithCapacity: ::00.0049141
UseStringConcatWithList: ::00.0179742
UseStringConcatWithListToArray: ::00.0045335
UseStringConcat: ::00.0042053
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0085083
UseStringBuilderWithCapacity: ::00.0041245
UseStringConcatWithList: ::00.0080081
UseStringConcatWithListToArray: ::00.0043282
UseStringConcat: ::00.0037264
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0087733
UseStringBuilderWithCapacity: ::00.0086311
UseStringConcatWithList: ::00.0080520
UseStringConcatWithListToArray: ::00.0056323
UseStringConcat: ::00.0038094
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0086884
UseStringBuilderWithCapacity: ::00.0063233
UseStringConcatWithList: ::00.0081824
UseStringConcatWithListToArray: ::00.0055393
UseStringConcat: ::00.0040967
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0108527
UseStringBuilderWithCapacity: ::00.0048482
UseStringConcatWithList: ::00.0079176
UseStringConcatWithListToArray: ::00.0043536
UseStringConcat: ::00.0042921
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0119569
UseStringBuilderWithCapacity: ::00.0043067
UseStringConcatWithList: ::00.0091552
UseStringConcatWithListToArray: ::00.0045956
UseStringConcat: ::00.0041058
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0096342
UseStringBuilderWithCapacity: ::00.0055840
UseStringConcatWithList: ::00.0073000
UseStringConcatWithListToArray: ::00.0056438
UseStringConcat: ::00.0038800
*******************************************************************
-------------------------------------------------------------------
UseStringBuilder: ::00.0090576
UseStringBuilderWithCapacity: ::00.0047266
UseStringConcatWithList: ::00.0076383
UseStringConcatWithListToArray: ::00.0048706
UseStringConcat: ::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。
参考：
http://blog.zhaojie.me/2009/11/string-concat-perf-1-benchmark.html