Here’s a C# function that accepts a credit card number and returns a string where all characters are replaced with asterisks except for the last four sprunki digits:
using System;

public class CreditCardHider
{
public static string HideCreditCardNumber(string creditCardNumber)
{
if (string.IsNullOrEmpty(creditCardNumber))
{
throw new ArgumentException(“Credit card number cannot be null or empty.”);
}

 // Get the length of the credit card number
 int length = creditCardNumber.Length;
 // Check if the number has at least 4 digits
 if (length < 4)
 {
 throw new ArgumentException("Credit card number must have at least 4 digits.");
 }
 // Create a string of asterisks for all but the last four characters
 string hiddenPart = new string('*', length - 4);
 string visiblePart = creditCardNumber.Substring(length - 4);
 // Combine and return the result
 return hiddenPart + visiblePart;
}
public static void Main(string[] args)
{
 string creditCard = "1234567812345678";
 string hiddenCard = HideCreditCardNumber(creditCard);
 Console.WriteLine(hiddenCard); // Output: ************5678
}

}