Mask

using System;
using System.Text;

/// <summary>
/// An enumeration of the types of masking styles for the Mask() extension method
/// of the string class.
/// </summary>
public enum MaskStyle
{
	/// <summary>
	/// Masks all characters within the masking region, regardless of type.
	/// </summary>
	All,

	/// <summary>
	/// Masks only alphabetic and numeric characters within the masking region.
	/// </summary>
	AlphaNumericOnly,
}

/// <summary>
/// Utility class for string manipulation.
/// </summary>
public static class StringExtensions
{
	/// <summary>
	/// Default masking character used in a mask.
	/// </summary>
	public static readonly char DefaultMaskCharacter = '*';


	/// <summary>
	/// Returns true if the string is non-null and at least the specified number of characters.
	/// </summary>
	/// <param name="value">The string to check.</param>
	/// <param name="length">The minimum length.</param>
	/// <returns>True if string is non-null and at least the length specified.</returns>
	/// <exception>throws ArgumentOutOfRangeException if length is not a non-negative number.</exception>
	public static bool IsLengthAtLeast(this string value, int length)
	{
		if (length < 0)
		{
			throw new ArgumentOutOfRangeException("length", length,
													"The length must be a non-negative number.");
		}

		return value != null
					? value.Length >= length
					: false;
	}

	/// <summary>
	/// Mask the source string with the mask char except for the last exposed digits.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="maskChar">The character to use to mask the source.</param>
	/// <param name="numExposed">Number of characters exposed in masked value.</param>
	/// <param name="style">The masking style to use (all characters or just alpha-nums).</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, char maskChar, int numExposed, MaskStyle style)
	{
		var maskedString = sourceValue;

		if (sourceValue.IsLengthAtLeast(numExposed))
		{
			var builder = new StringBuilder(sourceValue.Length);
			int index = maskedString.Length - numExposed;

			if (style == MaskStyle.AlphaNumericOnly)
			{
				CreateAlphaNumMask(builder, sourceValue, maskChar, index);
			}
			else
			{
				builder.Append(maskChar, index);
			}

			builder.Append(sourceValue.Substring(index));
			maskedString = builder.ToString();
		}

		return maskedString;
	}

	/// <summary>
	/// Mask the source string with the mask char except for the last exposed digits.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="maskChar">The character to use to mask the source.</param>
	/// <param name="numExposed">Number of characters exposed in masked value.</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, char maskChar, int numExposed)
	{
		return Mask(sourceValue, maskChar, numExposed, MaskStyle.All);
	}

	/// <summary>
	/// Mask the source string with the mask char.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="maskChar">The character to use to mask the source.</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, char maskChar)
	{
		return Mask(sourceValue, maskChar, 0, MaskStyle.All);
	}

	/// <summary>
	/// Mask the source string with the default mask char except for the last exposed digits.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="numExposed">Number of characters exposed in masked value.</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, int numExposed)
	{
		return Mask(sourceValue, DefaultMaskCharacter, numExposed, MaskStyle.All);
	}

	/// <summary>
	/// Mask the source string with the default mask char.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue)
	{
		return Mask(sourceValue, DefaultMaskCharacter, 0, MaskStyle.All);
	}

	/// <summary>
	/// Mask the source string with the mask char.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="maskChar">The character to use to mask the source.</param>
	/// <param name="style">The masking style to use (all characters or just alpha-nums).</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, char maskChar, MaskStyle style)
	{
		return Mask(sourceValue, maskChar, 0, style);
	}

	/// <summary>
	/// Mask the source string with the default mask char except for the last exposed digits.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="numExposed">Number of characters exposed in masked value.</param>
	/// <param name="style">The masking style to use (all characters or just alpha-nums).</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, int numExposed, MaskStyle style)
	{
		return Mask(sourceValue, DefaultMaskCharacter, numExposed, style);
	}

	/// <summary>
	/// Mask the source string with the default mask char.
	/// </summary>
	/// <param name="sourceValue">Original string to mask.</param>
	/// <param name="style">The masking style to use (all characters or just alpha-nums).</param>
	/// <returns>The masked account number.</returns>
	public static string Mask(this string sourceValue, MaskStyle style)
	{
		return Mask(sourceValue, DefaultMaskCharacter, 0, style);
	}

	/// <summary>
	/// Masks all characters for the specified length.
	/// </summary>
	/// <param name="buffer">String builder to store result in.</param>
	/// <param name="source">The source string to pull non-alpha numeric characters.</param>
	/// <param name="mask">Masking character to use.</param>
	/// <param name="length">Length of the mask.</param>
	private static void CreateAlphaNumMask(StringBuilder buffer, string source, char mask, int length)
	{
		for (int i = 0; i < length; i++)
		{
			buffer.Append(char.IsLetterOrDigit(source[i])
							? mask
							: source[i]);
		}
	}
}

var someInput = "123-45-6789"

var maskedId = someInput.Mask('X', 3);

// outputs: XXXXXXXX789
Console.WriteLine(maskedId);

var maskedWithDashes = someInput.Mask('*', 3, MaskStyle.AlphaNumericOnly);

// outputs: ***-**-*789
Console.WriteLine(maskedWithDashes);