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) "

试试这个方法

private string removeNestedWhitespaces(char[] st)
{
    StringBuilder sb = new StringBuilder();
    int indx = 0, length = st.Length;
    while (indx < length)
    {
        sb.Append(st[indx]);
        indx++;
        while (indx < length && st[indx] == ' ')
            indx++;
        if(sb.Length > 1  && sb[0] != ' ')
            sb.Append(' ');
    }
    return sb.ToString();
}

像这样使用它:

string test = removeNestedWhitespaces("1 2 3  4    5".toCharArray());

要不耍流氓?

public static string MinimizeWhiteSpace(
    this string _this)
    {
        if (_this != null)
        {
            var returned = new StringBuilder();
            var inWhiteSpace = false;
            var length = _this.Length;
            for (int i = 0; i < length; i++)
            {
                var character = _this[i];
                if (char.IsWhiteSpace(character))
                {
                    if (!inWhiteSpace)
                    {
                        inWhiteSpace = true;
                        returned.Append(' ');
                    }
                }
                else
                {
                    inWhiteSpace = false;
                    returned.Append(character);
                }
            }
            return returned.ToString();
        }
        else
        {
            return null;
        }
    }

老派：

string oldText = "   1 2  3   4    5     ";
string newText = oldText
                    .Replace("  ", " " + (char)22 )
                    .Replace( (char)22 + " ", "" )
                    .Replace( (char)22 + "", "" );

Assert.That( newText, Is.EqualTo( " 1 2 3 4 5 " ) );

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 ));