C# code that sets up an x64 syscall stub as a byte array, allocate that stub into memory, replace the syscall ID with a user-defined one, create a delegate for the stub, and then execute the syscall

calls.cs

using System;
using System.Runtime.InteropServices;

namespace SyscallStub
{
    // Define the syscall stub as a byte array
    static readonly byte[] syscallStub = new byte[] {
        0x48, 0x31, 0xc0,                         // xor rax, rax
        0x48, 0xbb, 0x01, 0x00, 0x00, 0x00, 0x00, // mov rbx, user-defined syscall ID
        0x0f, 0x05                                // syscall
    };

    // Define a delegate type for the syscall stub
    delegate void SyscallStubDelegate();

    class Program
    {
        static void Main(string[] args)
        {
            // Allocate the syscall stub into memory
            var ptr = Marshal.AllocHGlobal(syscallStub.Length);
            Marshal.Copy(syscallStub, 0, ptr, syscallStub.Length);

            // Replace the syscall ID in the stub with the user-defined one
            Marshal.WriteInt64(ptr + 2, userDefinedSyscallId);

            // Create a delegate for the syscall stub
            var syscallStubDelegate = (SyscallStubDelegate)Marshal.GetDelegateForFunctionPointer(ptr, typeof(SyscallStubDelegate));

            // Execute the syscall
            syscallStubDelegate();
        }
    }
}