qr code generator asp net c# Lesson 3: Using Unmanaged Code in Visual C#.NET

Drawer QR Code 2d barcode in Visual C#.NET Lesson 3: Using Unmanaged Code

As mentioned previously, the .NET Framework provides an abundance of functionality, but there are areas that have not yet been wrapped for .NET. This is the case with legacy code obviously, but it is also the case with Windows API functionality. With each new version of the Framework, the number of Windows APIs that aren t wrapped gets smaller, but gaps still exist. In this lesson, you will learn how to bridge those gaps.

After this lesson, you will be able to:

Use Platform Invoke (P/Invoke) to access unmanaged code. Encapsulate DLL functions. Convert data types between managed and unmanaged code.

Platform Invoke, or P/Invoke as it s commonly called, is critical in many instances where, for example, you need to call an unmanaged Windows API. If the .NET Framework doesn t have an existent wrapper and you don t have legacy code to do something, P/Invoke is often the only viable solution.

With respect to the Windows API, the following list provides typical use case scenarios:

Functionality specific to the Windows operating system, context switching, and file I/O. Advanced manipulation of windows, menus, dialog boxes, and icons. For example, if you want to customize a MessageBox outside of what the .NET Framework provides, the only way to do it is through the Windows API. Advanced drawing functionality.

You manage P/Invoke through the System.Runtime.InteropServices namespace, just as you would manage other unmanaged code. To use P/Invoke, you do the following: 1. Create a new static/shared external method with the name of the function you want to call. 2. Decorate it with the DllImport attribute specifying the library that it should call. 3. Call the method from your code.

13

In the following example, we re going to use the GetWindowText Windows API. To do this, we need to ensure that we are accurately referencing the active window running on the operating system, which might not be our application.

' VB Imports System.Text Imports System.Runtime.InteropServices Public Class WindowExample Private Const BufferSize As Int32 = 256 <DllImport("user32.dll")> _ Private Shared Function GetForegroundWindow() As IntPtr End Function <DllImport("user32.dll")> _ Private Shared Function GetWindowText(ByVal hWnd As IntPtr, _ ByVal textValue As StringBuilder, ByVal counter As Int32) As Int32 End Function Public Shared Sub GetScreenDemo() Dim DemoBuilder As New StringBuilder(BufferSize) Dim DemoHandle = GetForegroundWindow() If GetWindowText(DemoHandle, DemoBuilder, BufferSize) > 0 Then Console.WriteLine(DemoBuilder.ToString()) End If End Sub End Class // C# using System.Runtime.InteropServices; namespace OptionalCS { class WindowExample { private const Int32 BufferSize = 256; [DllImport("user32.dll")] private static extern IntPtr GetForegroundWindow(); [DllImport("user32.dll")] private static extern Int32 GetWindowText(IntPtr hWnd, StringBuilder textValue, Int32 counter); public static void GetScreenDemo() { StringBuilder DemoBuilder = new StringBuilder(BufferSize); IntPtr DemoHandle = GetForegroundWindow(); if (GetWindowText(DemoHandle, DemoBuilder, BufferSize) > 0)

{ Console.WriteLine(DemoBuilder.ToString()); } } } }

When using Platform Invoke, use a StringBuilder object instead of a String. A StringBuilder is a reference type with no atypical behavior and is needed because of the way P/Invoke internals work.

Because P/Invoke calls can be less than elegant, at least in terms of what most .NET developers are accustomed to doing, it s often beneficial to create a class that exposes them and wraps this functionality. After all, much of the .NET Framework is composed of precisely this methodology. This approach has the following benefits:

Consumers of your class will not know this code from any other normal code they are used to dealing with. It relieves developers of having to remember the API call names and their respective parameters. You can create it once and then use it like any other .NET method. It will be less error prone. Even slight typing differences can cause P/Invoke calls to break. Even if you are a perfect typist and never forget anything, it s doubtful that everyone you work with will have the same capability. And if they don t, they will invariable type something incorrectly, miss a parameter, or forget the name of something.

Assume that we are using the WindowsExample class shown in the previous section. Wrapping it so that it can be used like a normal .NET class is already taken care of. Instead of repetitiously writing the code in the GetScreenDemo method each time you need that functionality, you can simply do the following:

' VB WindowExample.GetScreenDemo() // C# WindowExample.GetScreenDemo();