Mask String

/// <summary>
/// Mask the string.
/// </summary>
/// <param name="value">String that need to be masked</param>
/// <param name="startIndex">zero index indicating mask start position</param>
/// <param name="mask">mask that need to be applied, eg. ***</param>
/// <returns>Usage: "123456789".Mask(3, "****") => "123****89"</returns>
public static string Mask(this string value, int startIndex, string mask)
{
    if (string.IsNullOrEmpty(value))
        return string.Empty;

    var result = value;
    var starLength = mask.Length;

    if (value.Length < startIndex) return result;

    result = value.Insert(startIndex, mask);

    result = result.Length >= (startIndex + (starLength * 2)) ? result.Remove(startIndex + starLength, starLength) : result.Remove(startIndex + starLength, result.Length - (startIndex + starLength));

    return result;
}

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