How do I replace multiple spaces with a single space in C#?

Question

How can I replace multiple spaces in a string with only one space in C#?

Example:

1 2 3  4    5

would be:

1 2 3 4 5         


        

Answer 1

Many answers are providing the right output but for those looking for the best performances, I did improve Nolanar's answer (which was the best answer for performance) by about 10%.

public static string MergeSpaces(this string str)
{

    if (str == null)
    {
        return null;
    }
    else
    {
        StringBuilder stringBuilder = new StringBuilder(str.Length);

        int i = 0;
        foreach (char c in str)
        {
            if (c != ' ' || i == 0 || str[i - 1] != ' ')
                stringBuilder.Append(c);
            i++;
        }
        return stringBuilder.ToString();
    }

}