Hex String to Byte Array Conversion in C#

Introduction

Converting a hexadecimal string to a byte array is a common task in programming, especially when dealing with low-level data processing such as cryptography or communication protocols. Hexadecimal strings represent binary data in a more human-readable form. In this tutorial, we’ll explore how to perform this conversion efficiently using C#. We will cover several methods, each showcasing different techniques and performance considerations.

Understanding the Problem

A hexadecimal string consists of characters ranging from 0-9 and A-F (or a-f), representing values in base-16. Each pair of hex digits corresponds to one byte (8 bits) of data. Therefore, a valid hex string should have an even number of characters.

For example:

• Hex string: "4A3B2C1D"
• Byte array: [74, 59, 44, 29]

Method 1: Using LINQ

This method leverages LINQ to transform the hexadecimal string into a byte array. It is concise and expressive but may not be the fastest for large datasets.

using System;
using System.Linq;

public class HexConverter
{
    public static byte[] StringToByteArray(string hex)
    {
        return Enumerable.Range(0, hex.Length)
                         .Where(x => x % 2 == 0)
                         .Select(x => Convert.ToByte(hex.Substring(x, 2), 16))
                         .ToArray();
    }
}

Method 2: Optimized Loop with Bit Manipulation

This approach uses a loop and bit manipulation to achieve faster performance. It directly constructs the byte array without intermediate collections.

using System;

public class HexConverter
{
    public static byte[] StringToByteArrayFastest(string hex)
    {
        if (hex.Length % 2 == 1)
            throw new ArgumentException("The binary key cannot have an odd number of digits");

        byte[] arr = new byte[hex.Length >> 1];

        for (int i = 0; i < arr.Length; ++i)
        {
            int high = GetHexVal(hex[i << 1]);
            int low = GetHexVal(hex[(i << 1) + 1]);
            arr[i] = (byte)((high << 4) + low);
        }

        return arr;
    }

    private static int GetHexVal(char hex)
    {
        return hex - (hex < 58 ? 48 : (hex < 97 ? 55 : 87));
    }
}

Method 3: Using Byte.Parse

This method uses Byte.Parse with NumberStyles.HexNumber to convert each pair of hexadecimal characters into a byte. It is straightforward and leverages built-in parsing capabilities.

using System;
using System.Globalization;

public class HexConverter
{
    public static byte[] ConvertHexStringToByteArray(string hexString)
    {
        if (hexString.Length % 2 != 0)
            throw new ArgumentException($"The binary key cannot have an odd number of digits: {hexString}");

        byte[] data = new byte[hexString.Length / 2];
        for (int index = 0; index < data.Length; index++)
        {
            string byteValue = hexString.Substring(index * 2, 2);
            data[index] = byte.Parse(byteValue, NumberStyles.HexNumber, CultureInfo.InvariantCulture);
        }

        return data;
    }
}

Method 4: Using a Dictionary for Lookup

This method precomputes a dictionary of all possible byte values from hexadecimal strings. It is less efficient in terms of memory but can be faster if the same conversion logic is used repeatedly.

using System;
using System.Collections.Generic;

public class HexConverter
{
    public static byte[] StrToByteArray(string str)
    {
        var hexIndex = new Dictionary<string, byte>();
        for (int i = 0; i <= 255; i++)
            hexIndex.Add(i.ToString("X2"), (byte)i);

        List<byte> hexRes = new List<byte>();
        for (int i = 0; i < str.Length; i += 2)
            hexRes.Add(hexIndex[str.Substring(i, 2)]);

        return hexRes.ToArray();
    }
}

Conclusion

Each method has its advantages and trade-offs. LINQ offers readability, the optimized loop provides speed, Byte.Parse is simple and leverages built-in parsing, and the dictionary approach can be efficient for repeated conversions. Choose the method that best fits your performance needs and coding style.

Best Practices

• Ensure the input hex string has an even length to avoid exceptions.
• Consider using StringToByteArrayFastest for performance-critical applications.
• For readability and maintainability, prefer methods like ConvertHexStringToByteArray.
• Test each method with edge cases, such as empty strings or invalid characters.

By understanding these techniques, you can efficiently handle hexadecimal data in your C# applications.