Decimal to Hexadecimal Conversion in C# with Example Code

Decimal to Hexadecimal Conversion in C#

Working with RFID cards, hardware devices, or low-level protocols often requires converting decimal values into hexadecimal format. In C#, this is quite straightforward using the built-in formatting options.

Let’s break it down with a practical example.

Why Convert Decimal to Hexadecimal?

• Hexadecimal (base 16) is commonly used in embedded systems, RFID/Wiegand formats, and hardware communication.

• For example, RFID card numbers are stored in decimal format but devices may expect them in hexadecimal (24-bit masked) representation.

C# Example Code

Here’s a working method that fetches user data from SQLite and converts the Card column from decimal to hexadecimal:

public static DataTable SelectAllUserData()
{
    DataTable dt = null;
    SQLiteConnection con = null;
    try
    {
        string query = @"SELECT DGD.[Index], DGD.MachineNo, DGD.DeviceType, 
                         DGD.UserId, UserName, pwd, [Card], InputNo, Template, 
                         Privilege, IsEnabled 
                         FROM tblDemographicsData DGD 
                         LEFT OUTER JOIN tblBiometircsData BMD 
                         ON DGD.UserId = BMD.UserID 
                         ORDER BY CAST(DGD.UserId AS INTEGER), InputNo ASC";
        con = new SQLiteConnection(strCon);
        con.Open();
        SQLiteCommand cmd = new SQLiteCommand(query, con);
        SQLiteDataAdapter sda = new SQLiteDataAdapter(cmd);
        dt = new DataTable();
        sda.Fill(dt);
        // Convert decimal "Card" values into Hexadecimal
        foreach (DataRow row in dt.Rows)
        {
            if (row["Card"] != DBNull.Value)
            {
                string cardValue = row["Card"].ToString();
                if (long.TryParse(cardValue, out long dec))
                {
                    // Keep only lower 24 bits (for RFID/Wiegand format)
                    long masked = dec & 0xFFFFFF;
                    // Convert to Hexadecimal (uppercase)
                    row["Card"] = masked.ToString("X");
                }
                else
                {
                    row["Card"] = string.Empty;
                }
            }
        }
        return dt;
    }
    finally
    {
        if (con != null)
        {
            con.Close();
            con.Dispose();
        }
    }
}

Key Points in Code:

1. long.TryParse → Safely parses decimal values.

2. Bitmask & 0xFFFFFF → Keeps only 24-bit value (common in RFID/Wiegand).

3. ToString("X") → Converts to uppercase hexadecimal.

   • Example: 123456 → 1E240

4. Error Handling → If conversion fails, it stores an empty string.

Quick Conversion Example

long decimalValue = 305419896; // Example: 305419896 in decimal
string hexValue = decimalValue.ToString("X");
Console.WriteLine(hexValue); // Output: 12345678

✅ Output:

12345678

---

Conclusion

Converting decimal to hexadecimal in C# is very simple with .ToString("X").

If you are working with RFID cards, biometrics, or device protocols, applying a 24-bit mask before conversion ensures compatibility with most hardware formats.

This technique is not only useful for card numbers but also for low-level hardware data, memory addresses, and debugging binary protocols.