Count the spaces at start of a string

Use Enumerable.TakeWhile, Char.IsWhiteSpace and Enumerable.Count

int count = str.TakeWhile(Char.IsWhiteSpace).Count();

Note that not only " " is a white-space but:

White space characters are the following Unicode characters:

• Members of the SpaceSeparator category, which includes the characters SPACE (U+0020), OGHAM SPACE MARK (U+1680), MONGOLIAN VOWEL SEPARATOR (U+180E), EN QUAD (U+2000), EM QUAD (U+2001), EN SPACE (U+2002), EM SPACE (U+2003), THREE-PER-EM SPACE (U+2004), FOUR-PER-EM SPACE (U+2005), SIX-PER-EM SPACE (U+2006), FIGURE SPACE (U+2007), PUNCTUATION SPACE (U+2008), THIN SPACE (U+2009), HAIR SPACE (U+200A), NARROW NO-BREAK SPACE (U+202F), MEDIUM MATHEMATICAL SPACE (U+205F), and IDEOGRAPHIC SPACE (U+3000).
• Members of the LineSeparator category, which consists solely of the LINE SEPARATOR character (U+2028).
• Members of the ParagraphSeparator category, which consists solely of the PARAGRAPH SEPARATOR character (U+2029). The characters CHARACTER TABULATION (U+0009), LINE FEED (U+000A), LINE TABULATION (U+000B), FORM FEED (U+000C), CARRIAGE RETURN (U+000D), NEXT LINE (U+0085), and NO-BREAK SPACE (U+00A0).

You can use LINQ, because string implements IEnumerable<char>:

var numberOfSpaces = input.TakeWhile(c => c == ' ').Count();

input.TakeWhile(c => c == ' ').Count()

Or

input.Length - input.TrimStart(' ').Length

Try this:

        static void Main(string[] args)
        {
            string s = "    this is a string";
            Console.WriteLine(count(s));
        }

        static int count(string s)
        {
            int total = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == ' ')
                    total++;
                else
                    break;
            }
            return total;
        }

While I like the Linq based answers here's a boring unsafe method that should be pretty fast

    private static unsafe int HowManyLeadingSpaces(string input)
    {
        if (input == null)
            return 0;
        if (input.Length == 0)
            return 0;
        if (string.IsNullOrWhiteSpace(input))
            return input.Length;

        int count = 0;
        fixed (char* unsafeChar = input)
        {

            for (int i = 0; i < input.Length; i++)
            {
                if (char.IsWhiteSpace((char)(*(unsafeChar + i))))
                    count++;
                else
                    break;
            }
        }

        return count;           


    }

 int count = 0, index = 0, lastIndex = 0;
  string s = "    this is a string";

  index = s.IndexOf(" ");
  while (index > -1) 
  {
    count++;
    index = s.IndexOf(" ", index + 1);

    if ((index - lastIndex) > 1)
      break;

    lastIndex = index;
  } 

  Console.WriteLine(count);