Mask

Replaces all but the last n characters in a string with the specified mask character.

• Use string.Substring() to get the last n characters of the passed string, str.
• Use string.PadLeft() to add as many mask characters as necessary to the start of the string to return a string of the same length.
• Omit the third argument, mask, to use a default character of '*'.
• Omit the second argument, n, to keep a default of 4 characters unmasked.

public static partial class _30s 
{
  public static string Mask(string str, int n = 4, char mask = '*') 
  {
    return str.Substring(str.Length - n).PadLeft(str.Length, mask);
  }
}

string s = "1234567890";

_30s.Mask(s); // "******7890"
_30s.Mask(s, 3); // "*******890"
_30s.Mask(s, 2, '/$'); // "$$$$$$$$90"