Tutorial Reinterpreting Byte Arrays to Structs at Zero Cost with MemoryMarshal.Cast - UnreliableCode Forums

BytePusher

Memory & Performance

MEMBER

Rep: 209

Join Date: Dec 2025

Posts: 13

Thanks: 32

2y ago · Jan 8, 2024 5:15 PM

#1

When reading binary packet streams or file headers, using

CODE

BinaryReader

copies fields one by one.

Using
CODE
MemoryMarshal.Cast
to cast raw byte spans directly into structured types in O(0) time:

CSHARP

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public readonly record struct PacketHeader(uint Magic, ushort PacketType, uint PayloadLength, ulong Timestamp);

public static class FastPacketParser
{
    public static bool TryParseHeader(ReadOnlySpan<byte> rawBytes, out PacketHeader header)
    {
        // Cast ReadOnlySpan<byte> into ReadOnlySpan<PacketHeader>
        ReadOnlySpan<PacketHeader> headerSpan = MemoryMarshal.Cast<byte, PacketHeader>(rawBytes);
        
        if (headerSpan.IsEmpty)
        {
            header = default;
            return false;
        }

        header = headerSpan[0];
        return header.Magic == 0x504B5431; // 'PKT1'
    }
}

CODE

MemoryMarshal.Cast

performs bounds checking and reinterprets the span memory layout without allocating or copying a single byte!

When reading binary packet streams or file headers, using [code]BinaryReader[/code] copies fields one by one.

[b]Using [code]MemoryMarshal.Cast[/code] to cast raw byte spans directly into structured types in O(0) time:[/b]

[code=csharp]using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public readonly record struct PacketHeader(uint Magic, ushort PacketType, uint PayloadLength, ulong Timestamp);

public static class FastPacketParser
{
    public static bool TryParseHeader(ReadOnlySpan<byte> rawBytes, out PacketHeader header)
    {
        // Cast ReadOnlySpan<byte> into ReadOnlySpan<PacketHeader>
        ReadOnlySpan<PacketHeader> headerSpan = MemoryMarshal.Cast<byte, PacketHeader>(rawBytes);
        
        if (headerSpan.IsEmpty)
        {
            header = default;
            return false;
        }

header = headerSpan[0];
        return header.Magic == 0x504B5431; // 'PKT1'
    }
}[/code]

[code]MemoryMarshal.Cast[/code] performs bounds checking and reinterprets the span memory layout without allocating or copying a single byte!

BytePusher · Memory & Performance
Zero-allocation C# code using Span<T>, Memory<T>, and Unsafe...

The following users thanked BytePusher for this post:

NativeCoder

Low-Level C++ / ASM

VIP

Rep: 374

Join Date: Jun 2025

Posts: 12

Thanks: 20

2y ago · Jan 8, 2024 6:33 PM

#2

Just make sure your struct is marked

CODE

[StructLayout(LayoutKind.Sequential)]

and does not contain managed reference types (like string or class objects), otherwise

CODE

MemoryMarshal.Cast

will throw a runtime exception.

NativeCoder · Low-Level C++ / ASM
Passionate about cache locality, SIMD instructions, and raw ...

StructPacker

Binary Protocol Dev

MEMBER

Rep: 314

Join Date: May 2024

Posts: 7

Thanks: 19

2y ago · Jan 9, 2024 7:33 PM

#3

We use this in our custom binary asset loader. Reading 100,000 vertex mesh buffers takes literally 0.05 milliseconds because it is just a pointer reinterpret!

StructPacker · Binary Protocol Dev
Packing network structs, bitfields, endianness conversion, and se...

UnreliableCode Community

Reinterpreting Byte Arrays to Structs at Zero Cost with MemoryMarshal.Cast

Create Forum Thread

Add Reputation