using System.Drawing;
using System.Reflection;
using System.Runtime.InteropServices;

public class Form1 : System.Windows.Forms.Form
{

    // アプリケーションに関連づけられたアイコンを取得するためのAPI関数
    [DllImport( "shell32.dll", EntryPoint = "ExtractAssociatedIcon" )]
    private extern static IntPtr ExtractAssociatedIcon
        (
        IntPtr hInst,
        [MarshalAs(UnmanagedType.LPStr)] string lpIconPath,
        ref int lpiIcon
        );

    Icon ico;

    private void Form1_Load( object sender, System.EventArgs e )
    {
        string iconPath = "E:\\Test.exe";
        IntPtr hInst = Marshal.GetHINSTANCE( Assembly.GetExecutingAssembly().GetModules()[0] );
        Int32  iIcon = 0;
        IntPtr hIcon;

        // ファイルに関連付けられたアイコンのハンドルを取得する
        hIcon = ExtractAssociatedIcon( hInst, iconPath, ref iIcon );

        // アイコンハンドルからIconオブジェクトを作成する
        ico = Icon.FromHandle( hIcon );
    }

    private void Form1_Paint( object sender, System.Windows.Forms.PaintEventArgs e )
    {
        e.Graphics.DrawIcon( ico, 0, 0 );
    }
}

Imports System.Drawing
Imports System.Reflection
Imports System.Runtime.InteropServices


Public Class Form1
    Inherits System.Windows.Forms.Form

    ' アプリケーションに関連づけられたアイコンを取得するためのAPI関数
    <DllImport("shell32.dll", EntryPoint:="ExtractAssociatedIcon")> _
    Private Shared Function ExtractAssociatedIcon _
        ( _
         ByVal hInst As System.IntPtr, _
         <MarshalAs(UnmanagedType.LPStr)> ByVal lpIconPath As String, _
         ByRef lpiIcon As Integer _
        ) _
        As System.IntPtr
    End Function

    Dim ico As Icon

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim iconPath As String = "E:\Test.exe"
        Dim hInst As IntPtr = Marshal.GetHINSTANCE([Assembly].GetExecutingAssembly().GetModules()(0))
        Dim iIcon As Int32
        Dim hIcon As IntPtr

        ' ファイルに関連付けられたアイコンのハンドルを取得する
        hIcon = ExtractAssociatedIcon(hInst, iconPath, iIcon)

        ' アイコンハンドルからIconオブジェクトを作成する
        ico = Icon.FromHandle(hIcon)

    End Sub

    Private Sub Form1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint

        e.Graphics.DrawIcon(ico, 0, 0)

    End Sub

End Class