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Posted 29 Apr 2002


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Extending Explorer with Band Objects using .NET and Windows Forms

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29 Apr 20029 min read
Walks you through the implementation of an Explorer bar with the help of BandObject base class. Describes implementation details of the BandObject class.

Sample Image - dotnetBandObjects.jpg


A lot has been already said about extending Windows and Internet Explorer with Band Objects, Browser Bands, Toolbar Bands and Communication Bands. So if you are familiar with COM and ATL you even might have implemented one yourself. Well, in case your were waiting for an easy way to impress your friends with something like Google Toolbar here it is - the .NET way (or should I say Windows Forms and COM Interop way?). In this tutorial I am going to show how to create any of the mentioned above Band Object types with the help of the BandObject control. Later I will also talk about some implementation details of the BandObject.

Hello World Bar step by step


Build a Release version of BandObjectLib and register it in the Global Assembly Cache. The easiest way to do this is to open BandObjectLib.sln in Visual Studio, set the active configuration to Release and select 'Rebuild Solution' from the 'Build' menu. The second project in the solution - RegisterLib - is a C++ utility project that performs the 'gacutil /if BandObjectLib.dll' command that puts assembly into GAC.

As you probably already know, Band Objects are COM components. And for the .NET framework to find an assembly that implements a COM component it must be either be registered in the GAC or located in the directory of the client application. There are two possible client applications for Band Objects - explorer.exe and iexplorer.exe. Explorer is located in the windows directory and IE somewhere inside 'Program Files'. So GAC is actually the only one option in this case. Thus .NET assemblies that implement Band Objects should be registered in GAC and all libraries they depend on - like BandObjectLib.dll - should also be there.

Assemblies in the GAC must have strong names and thus key pairs are required. I have provided the BandObjects.snk file with a key pair but I encourage you to replace it with your own. See the sn.exe tool for more details.


Create a new Windows Control Library project and call it SampleBars. We are going to rely on the base functionality of BandObjectLib so we have to add a reference to BandObjectLib\Relase\bin\BandObjectLib.dll. As we are developing a 'Hello World Bar', rename UserControl1.cs and the UserControl1 class inside it appropriately - into HelloWolrdBar.cs and HelloWorldBar. Also put the following lines at the beginning of HelloWorldBar.cs:

using BandObjectLib;
using System.Runtime.InteropServices;


Make HelloWorldBar class inherit BandObject instead of System.Windows.Forms.UserControl. As I mentioned earlier, Band Objects are COM components so we should use the Guid attribute. Use guidgen.exe to generate your unique GUID or you can use the one I have generated for you:


We also have to sign our assembly with a strong name. You can do this by putting the following line into AssemblyInfo.cs file:

[assembly: AssemblyKeyFile(@"..\..\..\BandObjects.snk")]


Now its time to decide what kind of Band Object we want to develop. Lets make it an Explorer Toolbar as well as a Horizontal Explorer Bar (also known as a Browser Communication Band). All we need to do to implement this decision is to add custom BandObject attribute to our HelloWorldBar class:

[BandObject("Hello World Bar",
BandObjectStyle.Horizontal | BandObjectStyle.ExplorerToolbar,
HelpText = "Shows bar that says hello.")]
public class HelloWorldBar : BandObject
{ ...

That's enough to make our control available through 'View->Explorer Bars' and 'View->Toolbars' explorer menus. It also takes care of menu item text - "Hello World Bar", and hen the menu item is highlighted status bar displays "Shows bar that says hello.". Don't you like declarative programming and custom attributes?


Now it is time to open HelloWorldBar.cs in the Visual Studio Designer and put some controls on it. Although in my version of HelloWorldBar I decided to put a single button with 'Say Hello' caption on it you are free to do something more personalized. I made the size of the button equal to the size of the control's client area and also set its Anchor property to the combination of all possible styles - 'Top, Bottom, Left, Right'. The background color is 'HotTrack' and ForeColor is 'Info'.

The BandObject control has several properties specific to the Band Objects (and so classes derived from it) - Title , MinSize, MaxSize and IntegralSize. I set Title for HelloWorldBar to "Hello Bar" and both MinSize and Size to '150, 24'. Oh, and in button's On Click event handler I put code that displays a message box. This is what my final code looks like (and most of it was generated by VS.Net):

using System;
using System.ComponentModel;
using System.Windows.Forms;

using BandObjectLib;
using System.Runtime.InteropServices;

namespace SampleBars
    [BandObject("Hello World Bar", BandObjectStyle.Horizontal 
         | BandObjectStyle.ExplorerToolbar, HelpText = "Shows bar that says hello.")]
    public class HelloWorldBar : BandObject
        private System.Windows.Forms.Button button1;
        private System.ComponentModel.Container components = null;

        public HelloWorldBar()

        protected override void Dispose( bool disposing )
            if( disposing )
                if( components != null )
            base.Dispose( disposing );

        #region Component Designer generated code
        private void InitializeComponent()
            this.button1 = new System.Windows.Forms.Button();
            // button1
            this.button1.Anchor = (((System.Windows.Forms.AnchorStyles.Top 
                | System.Windows.Forms.AnchorStyles.Bottom) 
                | System.Windows.Forms.AnchorStyles.Left) 
                | System.Windows.Forms.AnchorStyles.Right);
            this.button1.BackColor = System.Drawing.SystemColors.HotTrack;
            this.button1.ForeColor = System.Drawing.SystemColors.Info;
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(150, 24);
            this.button1.TabIndex = 0;
            this.button1.Text = "Say Hello";
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // HelloWorldBar
            this.Controls.AddRange(new System.Windows.Forms.Control[] { this.button1 });
            this.MinSize = new System.Drawing.Size(150, 24);
            this.Name = "HelloWorldBar";
            this.Size = new System.Drawing.Size(150, 24);
            this.Title = "Hello Bar";


        private void button1_Click(object sender, System.EventArgs e)
            MessageBox.Show("Hello, World!");


Ok, now we are ready to build SampleBars.dll but its not enough to see it in explorer yet. We have to put our assembly into the GAC as well as register it as a COM server. There are tools - gacutil.exe and regasm.exe that do just this. The C++ utility project named Register in my version of the SampleBars solution liberates me from using these tools manually. It has no files in it, just the following post-build command (debug version):

cd $(ProjectDir)..\bin\Debug
gacutil /if SampleBars.dll
regasm SampleBars.dll

Of cause you have to make sure that Register project is the last one to be built in the solution using Project Dependencies / Build Order.

After building the solution, and executing the gacutil and regasm commands, we are finally ready to start Explorer and see our toolbar and explorer bar. And if you did everything right you should be able to see something like the picture at the top of the article. On this picture you can also see how HelloWorldBar looks in the Windows Taskbar. To achieve this all you need to do is to modify BandObject attribute adding the BandObjectStyle.TaskbarToolBar flag.


There are several issues that you might face developing a band object. First of all, every time you rebuild your project Visual Studio tends to generate new version of the assembly. It does this because of the following line in AssemblyInfo.cs:

[assembly: AssemblyVersion("1.0.*")]

I recommend using something like "" or you'll end up with multiple versions of your assembly in GAC. As a matter of fact, according to Jeffrey Richter, the auto increment feature of assembly version is a bug; the original intent was to increment the version of a file, not the assembly.

Another issue, more specific to BandObjects, is that explorer caches COM components. This means that even after you deployed a new version of your assembly into GAC, Explorer will not use it until restarted. What may help is setting the 'Launch folder windows in separate process' Explorer setting on. Also if you are getting "Unexpected error creating debug information file..." it is also due to the fact that previous version of your assembly is loaded into Explorer's process space.

Inside the BandObject

Ok, now we can look inside the BandObject class implementation. Lets start with ComComInterop.cs file. Explorer requires band objects to implement a set of COM interfaces - IObjectWithSite, IDeskBand and others. Unfortunately these interfaces are not available in the required form of type library so you cannot start using them just by adding a new reference to your project. These interfaces declarations are available in the form of C++ classes and MIDL interfaces. So before using them in .NET you have to convert these declarations into one of the .Net languages. This file also contains several structs and enums used by these interfaces. The whole process of converting is quite straightforward - you see HWND make it IntPtr, IUnknown - Object etc. Probably the most difficult for me was dealing with the DESKBANDINFO structure. You see, in the C++ version one of its parameters is declared as

WCHAR wszTitle[256];

It took quite a lot of debugging until I figured out what the C# version should look like. Note the use of CharSet.Unicode and that SizeConst is set to 255 not to 256.

public struct DESKBANDINFO
    public UInt32        dwMask;
    public Point        ptMinSize;
    public Point        ptMaxSize;
    public Point        ptIntegral;
    public Point        ptActual;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst=255)]
    public String        wszTitle;
    public DBIM         dwModeFlags;
    public Int32        crBkgnd;

And so the BandObject class implements required interfaces. The entry point is the IDeskBand.GetBandInfo method. Its implementation simply gets the values of the Size, MinSize, IntegralSize and Title properties, and populates the DESKBANDINFO structure. That gives Explorer hints how to display and resize band object.

The implementation of other methods of IDeskBand interface was easy: ShowDW() delegates to Control.Show() or Hide(), CloseDW() calls Dispose() and GetWindow() simply returns the Handle property of the control.

IObjectWithSite is equired to establish communication with the hosting Explorer process. Inside the SetSite() method of the BandObject control tries to get a reference to an IWebBrowser interface - an interface implemented by the top-level object of Explorer. It will be available through the BandObject.Explorer property. In the case of a taskbar toolbar there is no IWebBrowser interface so it handles this situation gracefully. As soon as a pointer to IWebBrowser is retrived, BandObject fires the ExplorerAttached event. Handling this event is useful when you want to add extra initialization code - subscribing to Web Browser events etc.

The IInputObject implementation is more interesting. This interface is required if you want your band object to participate in processing of keyboard input. The user can navigate from one explorer interface object (address bar, folder view) to another by pressing 'Tab' or 'Shift+Tab'. When it is your band object's turn to be activated or deactivated, Explorer calls the UIActivateIO() method. BandObject's implementation of it simply calls Select() on one of its child controls that acquires focus. Which control to select depends on the tab order of controls and whether user navigates forward or backward (with Shift key). BandObject also makes sure that focus can leave it (in case last control is selected and user presses Tab). This logic is implemented inside the TranslateAcceleratorIO() method. It first checks if the 'Tab' or 'F6' keys were pressed. Then depending on the state of 'Shift' key it tries to move focus from one child control to another using the SelectNextControl() method. The last parameter of SelectNextControl is false as we don't want to cycle through controls forever (from last to first and vice versa). If there is no next control we return zero which signals to Explorer that we don't know how to process this command. So Explorer itself processes the command, moving focus to an appropriate user interface object.

And finally my favorite part: the Register() and Unregister() methods. These methods are adorned with Com{Un}registerFunction attributes so the regasm.exe tool knows to call them when assembly is registered as a COM server. Register function checks its input parameter for presence of BandObjectAttribute. If it is present then its Name, Style and HelpText properties are used to create apropriate registry settings. For instance, to make your COM component to be considered as a 'Browser Communication Band' you have to mark it as implementing the 'Browser Communication Band' COM category. To make it an Explorer toolbar you have to register its CLSID under SOFTWARE\Microsoft\Internet Explorer\Toolbar. But you don't have to worry about these details. All you need to know is what BandObjectStyle flag to use and Register() takes care of the rest. Similarly, the Unregister() method takes care of what have to be removed from the registry when assembly is unregistered.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Pavel Zolnikov
Web Developer
United States United States
No Biography provided

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QuestionGlobal shortcut? Pin
SirWindfield28-Jun-19 23:26
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GeneralWorking perfectly for IE 11 on windows 7 + SP1 Pin
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QuestionWorking perfectly on Win10 with VS 2015 Pin
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AnswerRe: Working perfectly on Win10 with VS 2015 Pin
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AnswerRe: Working perfectly on Win10 with VS 2015 Pin
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QuestionI hope this is not a stupid question - windows 10 toolbar not showing Pin
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QuestionRe: I hope this is not a stupid question - windows 10 toolbar not showing Pin
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QuestionIs it a bug? Pin
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QuestionProject Builded, nothing happend Pin
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GeneralRe: Wonderful!!! Pin
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QuestionIt doesn't show up on IE9 Pin
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QuestionHow to make it work on Win 7 64 bit Pin
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