Mask String

Copy 
1/// <summary>
2/// Mask the string.
3/// </summary>
4/// <param name="value">String that need to be masked</param>
5/// <param name="startIndex">zero index indicating mask start position</param>
6/// <param name="mask">mask that need to be applied, eg. ***</param>
7/// <returns>Usage: "123456789".Mask(3, "****") => "123****89"</returns>
8public static string Mask(this string value, int startIndex, string mask)
9{
10    if (string.IsNullOrEmpty(value))
11        return string.Empty;
12
13    var result = value;
14    var starLength = mask.Length;
15
16    if (value.Length < startIndex) return result;
17
18    result = value.Insert(startIndex, mask);
19
20    result = result.Length >= (startIndex + (starLength * 2)) ? result.Remove(startIndex + starLength, starLength) : result.Remove(startIndex + starLength, result.Length - (startIndex + starLength));
21
22    return result;
23}

Copy 
1"123456789".Mask(3, "****").Should().Be("123****89");
2"123456789".Mask(3, "****---").Should().Be("123****---");