[Windows Mobile Development and Hacking General] pai:
from http://www.csharphelp.com/archives/archive52.html

Call Unmanaged Code. Part 1 - Simple DLLImport
By Vyacheslav Biktagirov

Managed world is beautiful, I have all classes I want in FrameWork.. But what if I want call some unmanaged code? For instance, I have DLL written in C++, and want use it from C#.

Let's look some code. Our DLL exports some function, in CDecl convention, that sums two integers:

extern "C" __declspec(dllexport) __cdecl int sum(int a,int b);

And, of course, we want reuse this code in C#. We must recall, that it is no "direct" way to call unmanaged code, but we must inform the compiler, what we want to call, how, and where is needed code located.

[DllImport("TestDll.dll", EntryPoint="sum",
ExactSpelling=false,CallingConvention=CallingConvention.Cdecl)]
static extern int sum(int a,int b);

and now we can call it like normal C# function.

x=5;
y=7;
z=sum(x,y);	// x will receive 12

Here is full C# client code - tested for Beta2.

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;

namespace WindowsApplication6
{
///
/// Summary description for Form1.
///
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.Button button1;
private System.Windows.Forms.TextBox textBox1;
private System.Windows.Forms.Label label1;
private System.Windows.Forms.TextBox textBox2;
private System.Windows.Forms.Label label2;
private System.Windows.Forms.TextBox textBox3;
///
/// Required designer variable.
///
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Required for Windows Form Designer support
//
InitializeComponent();

//
// TODO: Add any constructor code after InitializeComponent call
//
}

///
/// Clean up any resources being used.
///
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows Form Designer generated code
///
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
///
private void InitializeComponent()
{
this.button1 = new System.Windows.Forms.Button();
this.textBox1 = new System.Windows.Forms.TextBox();
this.label1 = new System.Windows.Forms.Label();
this.textBox2 = new System.Windows.Forms.TextBox();
this.label2 = new System.Windows.Forms.Label();
this.textBox3 = new System.Windows.Forms.TextBox();
this.SuspendLayout();
//
// button1
//
this.button1.Location = new System.Drawing.Point(64, 192);
this.button1.Name = "button1";
this.button1.Size = new System.Drawing.Size(144, 64);
this.button1.TabIndex = 0;
this.button1.Text = "call sum";
this.button1.Click += new System.EventHandler(this.button1_Click);
//
// textBox1
//
this.textBox1.Location = new System.Drawing.Point(40, 120);
this.textBox1.Name = "textBox1";
this.textBox1.Size = new System.Drawing.Size(72, 22);
this.textBox1.TabIndex = 1;
this.textBox1.Text = "2";
//
// label1
//
this.label1.Location = new System.Drawing.Point(128, 128);
this.label1.Name = "label1";
this.label1.Size = new System.Drawing.Size(16, 16);
this.label1.TabIndex = 2;
this.label1.Text = "+";
//
// textBox2
//
this.textBox2.Location = new System.Drawing.Point(152, 120);
this.textBox2.Name = "textBox2";
this.textBox2.Size = new System.Drawing.Size(56, 22);
this.textBox2.TabIndex = 3;
this.textBox2.Text = "3";
//
// label2
//
this.label2.Location = new System.Drawing.Point(224, 120);
this.label2.Name = "label2";
this.label2.Size = new System.Drawing.Size(24, 23);
this.label2.TabIndex = 4;
this.label2.Text = "=";
//
// textBox3
//
this.textBox3.Location = new System.Drawing.Point(248, 120);
this.textBox3.Name = "textBox3";
this.textBox3.Size = new System.Drawing.Size(112, 22);
this.textBox3.TabIndex = 5;
this.textBox3.Text = "5";
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 15);
this.ClientSize = new System.Drawing.Size(576, 322);
this.Controls.AddRange(new System.Windows.Forms.Control[] {this.textBox3,this.label2,this.textBox2,this.label1,this.textBox1,this.button1});
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);

}
#endregion

///
/// The main entry point for the application.
///
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

#region My Code
#region Dll Imports
[DllImport("TestDll.dll", EntryPoint="sum",
ExactSpelling=false,CallingConvention=CallingConvention.Cdecl)]
static extern int sum(int a,int b);
#endregion
#region Button Click Events
private void button1_Click(object sender, System.EventArgs e)
{
textBox3.Text=(int.Parse(textBox1.Text)+int.Parse(textBox2.Text)).ToString();
}
#endregion
#endregion
}
}

It sounds very simple, becouse "int" is isomorphic type, says, int in C# and ind C++ is identical. What we can do, when we want operate non-isomorhic types, like String? Recall, that .NET string is some Class, while C++ string is char*,or wchar_t*,or BSTR, .. String may be embedded in a structure, or pointed by pointer, or even something more exotic. Let's call some string function.

[DllImport("Advapi32.dll", EntryPoint="GetUserName", ExactSpelling=false,
SetLastError=true)]
static extern bool GetUserName(
[MarshalAs(UnmanagedType.LPArray)] byte[] lpBuffer,
[MarshalAs(UnmanagedType.LPArray)] Int32[] nSize );

This function receives two parameters: char* and int*. Becouse we must allocate char* buffer and receive string by pointer, we can't use UnmanagedType.LPStr attribute, so we pass ANSI string as byte array. int* is more simple-it's 1-element Int32 array. Let's call it:

private void button2_Click(object sender, System.EventArgs e)
{
byte[] str=new byte[20];
Int32[] len=new Int32[1];
len[0]=20;
GetUserName(str,len);
MessageBox.Show(System.Text.Encoding.ASCII.GetString(str));
}

We allocate 20 bytes for receiving ANSI string,one element in Int32 array, set 20 as max string length and call it. For receiving string from byte array I used Text.Encoding.ASCII class.