是的，性能差异是显著的。请参阅KB文章“如何在Visual c#中提高字符串连接性能”。

我总是试着先编写清晰的代码，然后再优化性能。这比反过来做要容易得多!但是，看到了两者在我的应用程序中的巨大性能差异后，我现在更仔细地考虑了一下。

幸运的是，在代码上运行性能分析相对简单，可以看到您在哪些地方花费了时间，然后在需要的地方修改它以使用StringBuilder。

The performance of a concatenation operation for a String or StringBuilder object depends on how often a memory allocation occurs. A String concatenation operation always allocates memory, whereas a StringBuilder concatenation operation only allocates memory if the StringBuilder object buffer is too small to accommodate the new data. Consequently, the String class is preferable for a concatenation operation if a fixed number of String objects are concatenated. In that case, the individual concatenation operations might even be combined into a single operation by the compiler. A StringBuilder object is preferable for a concatenation operation if an arbitrary number of strings are concatenated; for example, if a loop concatenates a random number of strings of user input.

来源:MSDN

除了前面的答案，当我想到这样的问题时，我总是做的第一件事是创建一个小型测试应用程序。在这个应用程序中，为这两种情况执行一些计时测试，看看哪个更快。

恕我直言，添加500+字符串条目绝对应该使用StringBuilder。

StringBuilder减少了分配和赋值的数量，代价是使用了额外的内存。如果使用得当，它可以完全消除编译器需要一遍又一遍地分配越来越大的字符串，直到找到结果为止。

string result = "";
for(int i = 0; i != N; ++i)
{
   result = result + i.ToString();   // allocates a new string, then assigns it to result, which gets repeated N times
}

vs.

String result;
StringBuilder sb = new StringBuilder(10000);   // create a buffer of 10k
for(int i = 0; i != N; ++i)
{
   sb.Append(i.ToString());          // fill the buffer, resizing if it overflows the buffer
}

result = sb.ToString();   // assigns once

这个基准测试表明，当组合3个或更少字符串时，常规连接更快。

http://www.chinhdo.com/20070224/stringbuilder-is-not-always-faster/

StringBuilder可以在内存使用方面做出非常显著的改进，特别是在将500个字符串加在一起的情况下。

考虑下面的例子:

string buffer = "The numbers are: ";
for( int i = 0; i < 5; i++)
{
    buffer += i.ToString();
}
return buffer;

内存中发生了什么?创建以下字符串:

1 - "The numbers are: "
2 - "0"
3 - "The numbers are: 0"
4 - "1"
5 - "The numbers are: 01"
6 - "2"
7 - "The numbers are: 012"
8 - "3"
9 - "The numbers are: 0123"
10 - "4"
11 - "The numbers are: 01234"
12 - "5"
13 - "The numbers are: 012345"

通过将这5个数字加到字符串的末尾，我们创建了13个字符串对象!其中12个是无用的!哇!

StringBuilder fixes this problem. It is not a "mutable string" as we often hear (all strings in .NET are immutable). It works by keeping an internal buffer, an array of char. Calling Append() or AppendLine() adds the string to the empty space at the end of the char array; if the array is too small, it creates a new, larger array, and copies the buffer there. So in the example above, StringBuilder might only need a single array to contain all 5 additions to the string-- depending on the size of its buffer. You can tell StringBuilder how big its buffer should be in the constructor.

StringBuilder是可取的，如果你正在做多个循环，或分叉在你的代码传递…然而，对于PURE性能，如果你可以使用SINGLE字符串声明，那么性能就会更好。

例如:

string myString = "Some stuff" + var1 + " more stuff"
                  + var2 + " other stuff" .... etc... etc...;

更有表现力吗

StringBuilder sb = new StringBuilder();
sb.Append("Some Stuff");
sb.Append(var1);
sb.Append(" more stuff");
sb.Append(var2);
sb.Append("other stuff");
// etc.. etc.. etc..

在这种情况下，可以认为StringBuild更易于维护，但性能并不比单个字符串声明更好。

但是十有八九……使用字符串构建器。

另一方面:string + var也比string的性能更好。格式方法(通常)在内部使用StringBuilder(如果有疑问…检查反射器!)