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[MrAksel]

When calling native code in .NET you need to use Platform Invoke. It can be done in two ways with Visual Basic .NET. All examples expect that you have imported System.Runtime.InteropServices

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Public Declare Function ShowWindow Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal nCmdShow As Integer) As Boolean

<DllImport("user32.dll")>
Public Shared Function ShowWindow(ByVal hWnd As IntPtr, ByVal nCmdShow As Integer) As Boolean
End Function

Both methods work just as fine. But it is also a dynamic way to load functions, this example will minimize the foreground window

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Public Class Form1

    <DllImport("kernel32.dll")> Shared Function LoadLibrary(ByVal lpFileName As String) As IntPtr
    End Function

    <DllImport("kernel32.dll")> Shared Function FreeLibrary(ByVal hModule As IntPtr) As Boolean
    End Function

    <DllImport("kernel32.dll")> Shared Function GetProcAddress(ByVal hModule As IntPtr, ByVal lpProcName As String) As IntPtr
    End Function

    <UnmanagedFunctionPointer(CallingConvention.StdCall)> _
    Delegate Function GetForegroundWindow() As IntPtr

    <UnmanagedFunctionPointer(CallingConvention.StdCall)> _
    Delegate Function ShowWindow(ByVal hWnd As IntPtr, ByVal nCmdShow As Integer) As Boolean

    Private Sub HideForegroundWindow()
        Dim user32 As IntPtr = LoadLibrary("user32.dll")
        Dim GetForegroundWindowAddress As IntPtr = GetProcAddress(user32, "GetForegroundWindow")
        Dim ShowWindowAddress As IntPtr = GetProcAddress(user32, "ShowWindow")

        Dim GetForegroundWindowInvoker As GetForegroundWindow = Marshal.GetDelegateForFunctionPointer(GetForegroundWindowAddress, GetType(GetForegroundWindow))

        Dim ShowWindowInvoker As ShowWindow = Marshal.GetDelegateForFunctionPointer(ShowWindowAddress, GetType(ShowWindow))

        Dim ForegroundWindowHandle As IntPtr = GetForegroundWindowInvoker.Invoke()

        ShowWindowInvoker.Invoke(ForegroundWindowHandle, 6)

        FreeLibrary(user32)
    End Sub

End Class

This covers the basics of PInvoke, but when you start having types not built in in the .NET framework, you need to write them yourself and make sure memory and such is alright. A great resource to find the right way to declare a function is at http://www.pinvoke.net