Managed Dll Injection with C#

Library.cs

using System.Diagnostics;
using System.Windows.Forms;

namespace Loader {
	public static class Library
    {
		[DllExport]
		static void ShowMessage() {
			using(var p = Process.GetCurrentProcess()) {
				// Add System.Windows.Forms reference
				MessageBox.Show("Hello From " + p.ProcessName);
			}
		}
    }
}

Main.cs

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

class Program {

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

	[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Ansi)]
	static extern IntPtr LoadLibrary([MarshalAs(UnmanagedType.LPStr)]string lpFileName);

	static void Main(string[] args) {
		// Get our process
		using (var p = Process.GetProcessesByName("notepad++")[0]) {
			// Must give full path of library
			// Or relative to target process
			var path = @"c:\fakepath\Loader.dll";
			var ptr = p.LoadLibrary(path);
			// In order to get function address
			// we must load library in our process
			var lib = LoadLibrary(path);
			var addr = GetProcAddress(lib, "ShowMessage");
			// Lets call target function
			p.Call(addr, IntPtr.Zero);
		}

	}
}

Works with 64-bit processes.