How to mask string?

Any reason you don't want to just use Substring?

string dashed = text.Substring(0, 2) + "-" +
                text.Substring(2, 3) + "-" +
                text.Substring(7);

Or:

string dashed = string.Format("{0}-{1}-{2}", text.Substring(0, 2),
                              text.Substring(2, 3), text.Substring(7));

(I'm assuming it's deliberate that you've missed out two of the 0s? It's not clear which 0s, admittedly...)

Obviously you should validate that the string is the right length first...

You can try a regular expression and put this inside an extension method ToMaskedString()

public static class StringExtensions
{
    public static string ToMaskedString(this String value)
    {
        var pattern = "^(/d{2})(/d{3})(/d*)$";
        var regExp = new Regex(pattern);
        return regExp.Replace(value, "$1-$2-$3");
    }
}

Then call

respne.Write(value.ToMaskedString());

I wrote a quick extension method for same / similar purpose (similar in a sense that there's no way to skip characters).

Usage:

var testString = "12345";
var maskedString = testString.Mask("##.## #"); // 12.34 5

Method:

public static string Mask(this string value, string mask, char substituteChar = '#')
{
    int valueIndex = 0;
    try
    {
        return new string(mask.Select(maskChar => maskChar == substituteChar ? value[valueIndex++] : maskChar).ToArray());
    }
    catch (IndexOutOfRangeException e)
    {
        throw new Exception("Value too short to substitute all substitute characters in the mask", e);
    }
}

This produces the required result

string result = Int64.Parse(s.Remove(5,2)).ToString("00-000-000000");

assuming that you want to drop 2 characters at the position of the 2 first nulls.