Managed resources are those that are pure.NET code and managed by the runtime and are under its direct control. Unmanaged resources are those that are not. File handles, pinned memory, COM objects, database connections etc
// using unmanaged code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Runtime.InteropServices;
namespace DllImportSample
{
 public partial class frmDllImportSample : Form
 { 
 [DllImport("user32.dll")]
 private static extern int MessageBox(IntPtr hWnd, String lpText, String lpCaption, uint uType);
 public frmDllImportSample()
 {
 InitializeComponent();
 }
 private void btnMessageBox_Click(object sender, EventArgs e)
 {
 MessageBox(new IntPtr(this.Handle.Int32()), @"Calling MessageBox Win32 API from C#", @"MessageBox", 0);
 System.Windows.Forms.MessageBox.Show(@"This is C# MessageBox!", @"MessageBox");
 }
 }
}