Understanding Byte Representation of Strings in C#

Introduction

In C#, strings are represented internally using UTF-16 encoding, which means each character (or code unit) is typically two bytes. When you need to convert a string to its byte representation—for instance, for purposes like encryption—it’s essential to understand the role of encodings in this process and how you can achieve your goals.

Why Consider Encoding?

Encoding determines how characters are mapped to byte sequences. It becomes relevant when converting strings to bytes because different encoding schemes represent characters differently:

UTF-8: Variable-length encoding; each character can be 1 to 4 bytes.
ASCII: Fixed-width, 7-bit encoding, suitable for English letters and symbols.
UTF-16: Used internally by .NET for strings; typically two bytes per character.

Encoding matters because converting a string to a byte array using a specific encoding might change the content if certain characters are not representable in that encoding. This is crucial for understanding how data can be accurately stored or transmitted across different systems.

Converting Strings to Bytes Without Encoding

If your objective is simply to obtain the bytes as they are represented internally without worrying about interpretation, you can bypass traditional encoding mechanisms. Here’s a method to achieve this:

Using Unsafe Code and Memory Manipulation

To directly access the byte representation of a string in C#, you need to employ unsafe code, which involves pointers. This approach requires enabling unsafe code blocks in your project settings.

using System;
using System.Runtime.InteropServices;

public static class StringByteConverter
{
    public static unsafe byte[] GetRawBytes(string str)
    {
        if (str == null) return null;
        
        int charCount = str.Length;
        // UTF-16 uses 2 bytes per character
        int byteCount = charCount * 2; 
        byte[] bytes = new byte[byteCount];

        fixed (char* pChars = str)
        {
            Marshal.Copy((IntPtr)pChars, bytes, 0, byteCount);
        }

        return bytes;
    }
}

Explanation:

Fixed Statement: The fixed statement is used to pin the string in memory so that it cannot be moved by garbage collection.
Marshal.Copy: This method copies data from a pointer (in this case, pointing to the string’s character buffer) into a byte array.

Important Considerations

Endianness: The output depends on the architecture of the machine (little-endian or big-endian). Be cautious when transferring bytes across different systems.
Safety and Portability: Using unsafe code can introduce security risks and reduce portability. Ensure that your application logic accounts for these limitations.

Converting Bytes Back to Strings

To convert the byte array back into a string, you need to reverse the process:

public static unsafe string GetStringFromRawBytes(byte[] bytes)
{
    if (bytes == null) return null;
    
    int charCount = bytes.Length / 2; // Each UTF-16 character is 2 bytes
    char[] chars = new char[charCount];
    
    fixed (char* pChars = chars)
    {
        Marshal.Copy(bytes, 0, (IntPtr)pChars, bytes.Length);
    }

    return new string(chars);
}