Welcome!

Microsoft Cloud Authors: Liz McMillan, John Basso, Pat Romanski, Elizabeth White, Mihai Corbuleac

Related Topics: Microsoft Cloud

Microsoft Cloud: Article

.NET Makes Window Subclassing Easy - Tap into Windows OS messages

.NET Makes Window Subclassing Easy - Tap into Windows OS messages

Imagine you are coding a .NET application that must perform some logic when a CD is inserted into the drive. Unfortunately, there is no standard event visible to your application that gets fired when a CD is inserted. Windows knows when a user puts a CD in the drive, but how can it notify your application? In fact, Windows does notify your app by sending a Windows message - your app just isn't listening for it. Subclassing gives us the ability to listen for that message and handle it appropriately. In this article, you'll learn how to use subclassing to handle the many Windows messages that are sent by the OS but not exposed through .NET's eventing model. We'll start with the basics of Windows messaging, discuss how to subclass in .NET, and finally we'll look at some more advanced techniques.

Introduction
Subclassing can be defined as intercepting window message processing in an attempt to modify the default behavior. Window subclassing gives us the ability to tap into the various messages that the Windows operating system sends and to use this ability to change our application's behavior. Windows subclassing is different from object-oriented (OO) subclassing. Windows subclassing deals with overriding a window's message handler, while OO subclassing refers to deriving a class from a base class. An implementation of Windows subclassing can be a form of OO subclassing. We'll see more about that when we discuss the various Windows subclassing implementations. But first, to truly understand Windows subclassing (hereafter referred to as simply subclassing) you must understand how the Windows operating system uses messages.

Basics of Windows Messaging
The Windows operating system is a message-based system, which means it communicates by sending messages. The OS communicates with all running applications through messages. When an application should be minimized, the OS sends a message. When an application should be closed, the OS sends a message. When a user moves a scroll bar, the OS sends a message. So what are these messages? The .NET structure created to represent a message looks like this:

public struct System.Windows.Forms.Message
{
public IntPtr HWnd { get; set;
}
public IntPtr LParam { get;
set; }
public int Msg { get; set; }
public IntPtr Result { get;
set; }
public IntPtr WParam { get;
set; }
}
The message is basically a structure used to package up and send data between windows and the OS. Let's take a look at the properties of a message:
  • HWnd: The window handle that identifies which window the message should be sent to. A window handle is basically a unique identifier for a window.
  • LParam: Contains information used to process the message. This data varies by message type.
  • Msg: Identifies the type of message that was sent.
  • Result: The return value. This is what is sent back to the OS after the message is handled.
  • WParam: Contains information used to process the message. This data varies by message type.

    If you wonder why these parameters have strange C-style names such as WParam, it's because this .NET structure is wrapping an existing C structure (MSG). Now that you understand what a message looks like, let's see how a message flows from the OS to a window.

    When a user clicks a button on a window, the mouse's device driver captures the click event. The device driver puts the event into the system message queue, which is basically a FIFO data structure set up by the OS to receive all hardware events. The OS then pulls the hardware event off the system message queue and converts it into a true Windows message. The OS then posts the message to the local message queue on the thread that is running the clicked window. The application that owns that window constantly polls the local queue for messages.

    This constant polling of the local message queue is basically a while loop that looks something like this:

    while(GetMessage(ref msg, null, 0, 0))
    {
    TranslateMessage(ref msg);
    DispatchMessage(ref msg);
    }
    This loop is built into an application by the WinForms framework. It is often referred to as the "message loop" or "message pump" because its job is to send or pump messages to the application. Let's look at some of the win32 API calls used in the message loop:
  • GetMessage: Retrieves a message from the calling thread's message queue
  • TranslateMessage: Translates virtual-key messages into character messages
  • DispatchMessage: Posts the message to the appropriate window procedure

    A message loop is required for any thread that displays a window. A message loop is created by the WinForms framework when the application code calls Application.Run. If you look at the code that is created with a new WinForms project, you'll see something that looks like this:

    static void Main()
    {
    Application.Run(new Form1());
    }
    The one line in the main method starts a message loop for the current thread and shows Form1. A message loop can be destroyed by calling Application.Exit. This method posts a WM_QUIT message to the current thread's message queue. This special message causes the GetMessage function to return false and break out of the loop.

    Now that we have a loop to retrieve messages from our thread's message queue, we have to get those messages to the appropriate window and process them. That is just what the DispatchMessage function does - it takes the message and posts it to the appropriate window. DispatchMessage uses the Hwnd associated with the message to determine which window to post the message to. To be even clearer, the DispatchMessage function actually calls a special function called the window procedure, passing in the message as a parameter. The window procedure is the function that is created to handle window messages sent to the window. It looks something like Listing 1.

    The window procedure usually consists of a big case statement. Different actions are taken depending on the window message. If a WM_PAINT is sent, the window will repaint itself. If a WM_DESTROY is sent, the window will post another message to break out of its message loop and shut down. Finally, after processing the messages, the default window procedure for the class is called. This ensures that all necessary window message handling is performed by the base window class. Subclassing consists of overriding this window procedure and providing our own custom logic to handle messages. I'll show how that is done in the next section.

    How to Subclass in .NET
    .NET truly makes subclassing easy. And for those of you used to subclassing in VB6, you no longer have to worry about General Protection Faults (GPFs) occurring if you make a mistake. In .NET there are three ways to subclass: overriding the Control.WndProc method, creating a class derived from NativeWindow, and implementing IMessageFilter. We will look at all three options, along with their pros and cons. First let's see how to override WndProc.

    One of the easiest ways to subclass in .NET is to override the WndProc method of the form you are working on. Listing 2 shows how to do this. That's it!

    .NET makes it easy for you because it exposes the virtual WndProc method on the Control class, which all windows inherit from. In order to handle a message you need to know the unique number that identifies it. The definitions for windows messages and their ID numbers can be found in the <install drive>:Program FilesMicrosoft Visual Studio .NETVc7PlatformSDKIncludeWinUser.h file. Once you know the number, it is trivial to create a constant and hook that message the way the sample code does for the double-click message. (All of the code for this article is available for download from www.sys-con.com/dotnet/sourcec.cfm.) As a side note, apart from demonstration purposes, you don't really want to show message boxes in your WndProc because that will stop the procedure from processing messages until the user clicks OK. Any messages that are not handled by your custom code are passed on to the Control's WndProc to be handled by calling base.WndProc. This allows you to handle only the messages you are interested in without having to re-implement the base Windows procedure and handle all possible messages sent to the window. The call to the base WndProc is critical, as this window will not function if key messages are not handled.

    For demonstration purposes I handled the double-click, but you may wonder what the point is of handling a message that .NET already exposes as a form event. There would be no reason to handle the double-click other than to show how subclassing works. The real power of subclassing comes from handling messages that are not exposed through the .NET eventing model. We'll look at some of those messages later, but first let's look at the second way to subclass in .NET - by using NativeWindow.

    Overriding WndProc is definitely the easiest way to subclass in .NET but it does have its drawbacks, mainly the fact that your subclassing code is now tied to one window. What if you want to use the same subclassing code on two other windows? Wouldn't it be better if you could define subclassing logic on a generic window and then use that code with other windows?

    The System.Windows.Forms .NativeWindow class does just that. It is basically an encapsulation of a Windows handle and a window procedure. It represents the roots of a window. Listing 3 shows how we use the NativeWindow class to achieve our goal of reusable message-handling code.

    Note that this code is very similar to the subclassing code shown in Listings 1 and 2. We still override WndProc, but the main difference is that we do it inside a class that inherits from NativeWindow. Remember, the NativeWindow class consists of a window procedure that we have overridden and a window handle that we assign via the AssignHandle method. We can now associate our subclassing code with any window by using the AssignHan dle method.

    There is one other way to assign subclassing code to multiple windows. We can implement the System.Windows.Forms.IMessageFilter interface. This will allow us to filter and react to all messages sent to a thread's message loop. The difference between this method and the other two is that this method intercepts messages right after they leave the thread's message loop, while the others do not intercept the messages until they are dispatched to the appropriate window. This gives us a much more global approach in that we can capture all messages sent to all windows on a thread (see Listing 4).

    Again the code looks very similar to the other examples, but instead of deriving from a base or overriding a method, we implement an interface. The one method of the interface, PreFilterMessage, passes us the message that was sent to the thread's message queue. We now have a chance to process it before this message is sent to any window. We can take action based on the message, modify it or even discard it. If the PreFilterMessage method returns true, the message is sent to the appropriate window(s). If the method returns false, then the message is discarded without being sent to any windows.

    One thing to note in regard to this method of subclassing is that not all messages go through the thread's message queue and therefore through this method. Messages can be sent in one of two ways - by being dispatched through the message queue or by being sent directly to the window and bypassing the queue. The latter type will not flow through our filter because they will go directly to the window. The way a message is sent is determined by the sender. They can use a variant of two win32 API calls:

  • SendMessage: Sends the message directly to the window
  • PostMessage: Dispatches the message to the window thread's message queue

    So the filter intercepts messages earlier than the other two subclassing methods, but as a consequence it will not receive messages that are sent directly to the window.

    Overriding WndProc is the easiest way to subclass and will intercept all messages whether posted or sent, but the subclassing code is not reusable. Inheriting from NativeWindow provides a way to receive all messages and reuse code but requires you to pass the handle of all windows whose messages should be intercepted. Finally, implementing IMessageFilter intercepts the messages for all windows created on a thread, but does not receive messages that are sent directly to the window. Now that you know the various ways to subclass in .NET, let's look at a practical example of what we can do with subclassing.

    Adding New Events Through Subclassing
    The true power in subclassing can be seen when you begin to handle messages that are not already exposed as events. For example, the DoubleClick event is already exposed as an event of a Form, but there is no event that is raised when a CD is inserted in the drive and ready for use. A good use of subclassing is to capture these kind of events that the OS communicates through Windows messages but the WinForms framework does not expose as events. Listing 5 shows how we would capture device changes such as CD inserted and removed:

    For this example we've taken the simplest approach of overriding WndProc. I want to intercept only the messages for the application's main window, so there's no need to use NativeWindow or IMessageFilter. In my overridden WndProc I simply look for the WM_DEVICECHANGE message that the OS sends to all windows when a CD is inserted or removed. I then check the message's WParam, which provides additional data about the message. In this case it specifies whether a CD was inserted or removed. That's all there is to it.

    Conclusion
    With this example, you can begin to see the possibilities of subclassing. The OS sends many messages that are not incorporated into the WinForms framework. By handling them via subclassing you can easily add new features to an application without resorting to Win32 API calls or unmanaged code.

  • More Stories By Bryant Hankins

    Bryant Hankins is a consultant at Clarity Consulting Inc., a Chicago-based information technology firm and Microsoft Gold Certified Partner. Bryant has designed and developed solutions for Fortune 500 companies in a variety of industries, including financial, accounting, and publishing.

    Comments (7)

    Share your thoughts on this story.

    Add your comment
    You must be signed in to add a comment. Sign-in | Register

    In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


    @ThingsExpo Stories
    SYS-CON Events has announced today that Roger Strukhoff has been named conference chair of Cloud Expo and @ThingsExpo 2016 Silicon Valley. The 19th Cloud Expo and 6th @ThingsExpo will take place on November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. "The Internet of Things brings trillions of dollars of opportunity to developers and enterprise IT, no matter how you measure it," stated Roger Strukhoff. "More importantly, it leverages the power of devices and the Interne...
    Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, discussed how research has demonstrated the value of Machine Learning in delivering next generation analytics to imp...
    In addition to all the benefits, IoT is also bringing new kind of customer experience challenges - cars that unlock themselves, thermostats turning houses into saunas and baby video monitors broadcasting over the internet. This list can only increase because while IoT services should be intuitive and simple to use, the delivery ecosystem is a myriad of potential problems as IoT explodes complexity. So finding a performance issue is like finding the proverbial needle in the haystack.
    The Internet of Things will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, demonstrated how to move beyond today's coding paradigm and shared the must-have mindsets for removing complexity from the develo...
    SYS-CON Events announced today that MangoApps will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. MangoApps provides modern company intranets and team collaboration software, allowing workers to stay connected and productive from anywhere in the world and from any device.
    The IETF draft standard for M2M certificates is a security solution specifically designed for the demanding needs of IoT/M2M applications. In his session at @ThingsExpo, Brian Romansky, VP of Strategic Technology at TrustPoint Innovation, explained how M2M certificates can efficiently enable confidentiality, integrity, and authenticity on highly constrained devices.
    SYS-CON Events announced today that LeaseWeb USA, a cloud Infrastructure-as-a-Service (IaaS) provider, will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. LeaseWeb is one of the world's largest hosting brands. The company helps customers define, develop and deploy IT infrastructure tailored to their exact business needs, by combining various kinds cloud solutions.
    Early adopters of IoT viewed it mainly as a different term for machine-to-machine connectivity or M2M. This is understandable since a prerequisite for any IoT solution is the ability to collect and aggregate device data, which is most often presented in a dashboard. The problem is that viewing data in a dashboard requires a human to interpret the results and take manual action, which doesn’t scale to the needs of IoT.
    Internet of @ThingsExpo has announced today that Chris Matthieu has been named tech chair of Internet of @ThingsExpo 2016 Silicon Valley. The 6thInternet of @ThingsExpo will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
    When people aren’t talking about VMs and containers, they’re talking about serverless architecture. Serverless is about no maintenance. It means you are not worried about low-level infrastructural and operational details. An event-driven serverless platform is a great use case for IoT. In his session at @ThingsExpo, Animesh Singh, an STSM and Lead for IBM Cloud Platform and Infrastructure, will detail how to build a distributed serverless, polyglot, microservices framework using open source tec...
    “delaPlex Software provides software outsourcing services. We have a hybrid model where we have onshore developers and project managers that we can place anywhere in the U.S. or in Europe,” explained Manish Sachdeva, CEO at delaPlex Software, in this SYS-CON.tv interview at @ThingsExpo, held June 7-9, 2016, at the Javits Center in New York City, NY.
    "My role is working with customers, helping them go through this digital transformation. I spend a lot of time talking to banks, big industries, manufacturers working through how they are integrating and transforming their IT platforms and moving them forward," explained William Morrish, General Manager Product Sales at Interoute, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
    The 19th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Digital Transformation, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportuni...
    A critical component of any IoT project is what to do with all the data being generated. This data needs to be captured, processed, structured, and stored in a way to facilitate different kinds of queries. Traditional data warehouse and analytical systems are mature technologies that can be used to handle certain kinds of queries, but they are not always well suited to many problems, particularly when there is a need for real-time insights.
    IoT is rapidly changing the way enterprises are using data to improve business decision-making. In order to derive business value, organizations must unlock insights from the data gathered and then act on these. In their session at @ThingsExpo, Eric Hoffman, Vice President at EastBanc Technologies, and Peter Shashkin, Head of Development Department at EastBanc Technologies, discussed how one organization leveraged IoT, cloud technology and data analysis to improve customer experiences and effi...
    The Internet of Things will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform and how we integrate our thinking to solve complicated problems. In his session at 19th Cloud Expo, Craig Sproule, CEO of Metavine, will demonstrate how to move beyond today's coding paradigm ...
    Big Data engines are powering a lot of service businesses right now. Data is collected from users from wearable technologies, web behaviors, purchase behavior as well as several arbitrary data points we’d never think of. The demand for faster and bigger engines to crunch and serve up the data to services is growing exponentially. You see a LOT of correlation between “Cloud” and “Big Data” but on Big Data and “Hybrid,” where hybrid hosting is the sanest approach to the Big Data Infrastructure pro...
    From wearable activity trackers to fantasy e-sports, data and technology are transforming the way athletes train for the game and fans engage with their teams. In his session at @ThingsExpo, will present key data findings from leading sports organizations San Francisco 49ers, Orlando Magic NBA team. By utilizing data analytics these sports orgs have recognized new revenue streams, doubled its fan base and streamlined costs at its stadiums. John Paul is the CEO and Founder of VenueNext. Prior ...
    The Quantified Economy represents the total global addressable market (TAM) for IoT that, according to a recent IDC report, will grow to an unprecedented $1.3 trillion by 2019. With this the third wave of the Internet-global proliferation of connected devices, appliances and sensors is poised to take off in 2016. In his session at @ThingsExpo, David McLauchlan, CEO and co-founder of Buddy Platform, discussed how the ability to access and analyze the massive volume of streaming data from millio...
    Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 19th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo Silicon Valley Call for Papers is now open.