混合StringBuilder和Enumerable.Aggregate()作为字符串的扩展方法:

using System;
using System.Linq;
using System.Text;

public static class StringExtension
{
    public static string CondenseSpaces(this string s)
    {
        return s.Aggregate(new StringBuilder(), (acc, c) =>
        {
            if (c != ' ' || acc.Length == 0 || acc[acc.Length - 1] != ' ')
                acc.Append(c);
            return acc;
        }).ToString();
    }

    public static void Main()
    {
        const string input = "     (five leading spaces)     (five internal spaces)     (five trailing spaces)     ";
        
        Console.WriteLine(" Input: \"{0}\"", input);
        Console.WriteLine("Output: \"{0}\"", StringExtension.CondenseSpaces(input));
    }
}

执行这个程序产生以下输出:

 Input: "     (five leading spaces)     (five internal spaces)     (five trailing spaces)     "
Output: " (five leading spaces) (five internal spaces) (five trailing spaces) "

我认为Matt的答案是最好的，但我不认为它是完全正确的。如果你想替换换行符，你必须使用:

myString = Regex.Replace(myString, @"\s+", " ", RegexOptions.Multiline);

您可以简单地在一行解决方案中做到这一点!

string s = "welcome to  london";
s.Replace(" ", "()").Replace(")(", "").Replace("()", " ");

如果喜欢，可以选择其他括号(甚至其他字符)。

以下是对Nolonar原始答案的轻微修改。

检查字符是否只是一个空格，而是任何空白，使用这个:

它将用一个空格替换任何多个空白字符。

public static string FilterWhiteSpaces(string input)
{
    if (input == null)
        return string.Empty;

    var stringBuilder = new StringBuilder(input.Length);
    for (int i = 0; i < input.Length; i++)
    {
        char c = input[i];
        if (i == 0 || !char.IsWhiteSpace(c) || (char.IsWhiteSpace(c) && 
            !char.IsWhiteSpace(strValue[i - 1])))
            stringBuilder.Append(c);
    }
    return stringBuilder.ToString();
}

string sentence = "This is a sentence with multiple    spaces";
RegexOptions options = RegexOptions.None;
Regex regex = new Regex("[ ]{2,}", options);     
sentence = regex.Replace(sentence, " ");

string xyz = "1   2   3   4   5";
xyz = string.Join( " ", xyz.Split( new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries ));