1 using System;
 2 using System.Collections.Generic;
 3 using System.ComponentModel;
 4 using System.Data;
 5 using System.Drawing;
 6 using System.Linq;
 7 using System.Text;
 8 using System.Windows.Forms;
 9 using System.Runtime.InteropServices;
10 
11 namespace pinvoke_win32_sum {
12   public partial class Form1 : Form {
13     public Form1() {
14       InitializeComponent();
15     }
16 
17     [DllImport ("win32_sum.dll")]
18     private static extern int sum(int x, int y);
19     private void button1_Click(object sender, EventArgs e)     {
20       textBox3.Text = sum(Convert.ToInt32(textBox1.Text), Convert.ToInt32(textBox2.Text)).ToString();
21     }
22   }
23 }

using System.Runtime.InteropServices;

[DllImport ("win32_sum.dll")]
private static extern int sum(int x, int y);

textBox3.Text = sum(Convert.ToInt32(textBox1.Text), Convert.ToInt32(textBox2.Text)).ToString();