c# - 优化代码，使其执行得更快

Question

如何优化以下代码以使其执行得更快？

static void Main(string[] args) 
{
    String a = "Hello ";
    String b = " World! ";
    for (int i=0; i<20000; i++) 
    {
        a = a + b;
    }
    Console.WriteLine(a);
} 

Answer 1

来自StringBuilder文档:

Performance Considerations

The Concat and AppendFormat methods both concatenate new data to an existing String or StringBuilder object. A String object concatenation operation always creates a new object from the existing string and the new data. A StringBuilder object maintains a buffer to accommodate the concatenation of new data. New data is appended to the end of the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, then the new data is appended to the new buffer.

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.

static void Main(string[] args) {
    String a = "Hello ";
    String b = " World! ";
    StringBuilder result = new StringBuilder(a.Length + b.Length * 20000);
    result.Append(a);
    for (int i=0; i<20000; i++) {
        result.Append(b);
    }
    Console.WriteLine(result.ToString());
}