不使用正则表达式:

while (myString.IndexOf("  ", StringComparison.CurrentCulture) != -1)
{
    myString = myString.Replace("  ", " ");
}

可以在短弦上使用，但在有很多空格的长弦上表现不佳。

根据Joel的说法，巩固其他答案，并希望随着我的前进略有改善:

你可以用Regex.Replace()来实现:

string s = Regex.Replace (
    "   1  2    4 5", 
    @"[ ]{2,}", 
    " "
    );

或者使用String.Split():

static class StringExtensions
{
    public static string Join(this IList<string> value, string separator)
    {
        return string.Join(separator, value.ToArray());
    }
}

//...

string s = "     1  2    4 5".Split (
    " ".ToCharArray(), 
    StringSplitOptions.RemoveEmptyEntries
    ).Join (" ");

下面的代码将所有多个空格删除为一个空格

    public string RemoveMultipleSpacesToSingle(string str)
    {
        string text = str;
        do
        {
            //text = text.Replace("  ", " ");
            text = Regex.Replace(text, @"\s+", " ");
        } while (text.Contains("  "));
        return text;
    }

其实比这简单得多:

while(str.Contains("  ")) str = str.Replace("  ", " ");

我可以用这个删除空格

while word.contains("  ")  //double space
   word = word.Replace("  "," "); //replace double space by single space.
word = word.trim(); //to remove single whitespces from start & end.

myString = Regex.Replace(myString, " {2,}", " ");