ObscureEmailAddress

StringHelper.cs

public static class StringHelper
{
    public const byte DefaultUnobscuredLength = 3;

    public enum Keep { Start, End };

    public static string? ObscureEmailAddress(string? value)
    {
        if (string.IsNullOrEmpty(value))
            return value;

        string?[] parts = value.Split('@');
        Keep keep = Keep.Start;
        for(int i = 0; i < parts.Length; i++)
        {
            parts[i] = Obscure(parts[i], keep, 3);
            keep = Keep.End;
        }
        return String.Join('@', parts);
    }

    public static string? Obscure(string? value, Keep keep, int unobscuredLength = DefaultUnobscuredLength)
    {
        if (string.IsNullOrEmpty(value))
            return value;
        if (unobscuredLength < 0)
            throw new ArgumentOutOfRangeException(message: "Must be at least 0", paramName: nameof(unobscuredLength));

        // Ensure we obscure at least half of the string, and reveal at most DefaultUnobscuredLength (3) chars.
        unobscuredLength = Math.Min(unobscuredLength, value.Length / 2);

        return keep switch
        {
            Keep.Start => value[0..unobscuredLength] + new string('*', value.Length - unobscuredLength),
            _ => new string('*', value.Length - unobscuredLength) + value[^unobscuredLength..]
        };
    }
}