Notes

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Text;

public class BasicInject
{
[DllImport("kernel32.dll")]
public static extern IntPtr OpenProcess(int dwDesiredAccess, bool bInheritHandle, int dwProcessId);

[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
public static extern IntPtr GetModuleHandle(string lpModuleName);

[DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
static extern IntPtr GetProcAddress(IntPtr hModule, string procName);

[DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
static extern IntPtr VirtualAllocEx(IntPtr hProcess, 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 UIntPtr lpNumberOfBytesWritten);

[DllImport("kernel32.dll")]
static extern IntPtr CreateRemoteThread(IntPtr hProcess,
IntPtr lpThreadAttributes, uint dwStackSize, IntPtr lpStartAddress, IntPtr lpParameter, uint dwCreationFlags, IntPtr lpThreadId);

// privileges
const int PROCESS_CREATE_THREAD = 0x0002;
const int PROCESS_QUERY_INFORMATION = 0x0400;
const int PROCESS_VM_OPERATION = 0x0008;
const int PROCESS_VM_WRITE = 0x0020;
const int PROCESS_VM_READ = 0x0010;

// used for memory allocation
const uint MEM_COMMIT = 0x00001000;
const uint MEM_RESERVE = 0x00002000;
const uint PAGE_READWRITE = 4;

public static int Main()
{
// the target process - I'm using a dummy process for this
// if you don't have one, open Task Manager and choose wisely
Process targetProcess = Process.GetProcessesByName("testApp")[0];

// geting the handle of the process - with required privileges
IntPtr procHandle = OpenProcess(PROCESS_CREATE_THREAD | PROCESS_QUERY_INFORMATION | PROCESS_VM_OPERATION | PROCESS_VM_WRITE | PROCESS_VM_READ, false, targetProcess.Id);

// searching for the address of LoadLibraryA and storing it in a pointer
IntPtr loadLibraryAddr = GetProcAddress(GetModuleHandle("kernel32.dll"), "LoadLibraryA");

// name of the dll we want to inject
string dllName = "test.dll";

// alocating some memory on the target process - enough to store the name of the dll
// and storing its address in a pointer
IntPtr allocMemAddress = VirtualAllocEx(procHandle, IntPtr.Zero, (uint)((dllName.Length + 1) * Marshal.SizeOf(typeof(char))), MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);

// writing the name of the dll there
UIntPtr bytesWritten;
WriteProcessMemory(procHandle, allocMemAddress, Encoding.Default.GetBytes(dllName), (uint)((dllName.Length + 1) * Marshal.SizeOf(typeof(char))), out bytesWritten);

// creating a thread that will call LoadLibraryA with allocMemAddress as argument
CreateRemoteThread(procHandle, IntPtr.Zero, 0, loadLibraryAddr, allocMemAddress, 0, IntPtr.Zero);

return 0;
}
}