Home > Articles > Programming > Windows Programming

  • Print
  • + Share This
Like this article? We recommend

Like this article? We recommend

Implementing shell extensions require us to delve into the world of COM.

The entire shell extension system is based on registration of a shell extension into the shell and implementing specific COM interfaces for different types of extensions.

For the shell to access those interfaces, we will need to expose a class as a COM object.

We can create a class that is exposed as a COM interface using COM Interop:

using System.Runtime.InteropServices; 

[Guid("2AA8DDCB-0540-4cd3-BD31-D91DADD81ED3"), ComVisible(true)]
public class CleanVS : IContextMenu, IShellExtInit 

There are several things here to pay attention to. First, we are adding two attributes to our new class; Guid and ComVisible. These attributes are used to expose a managed type as a COM object through Interop. The value in the Guid attribute is a randomly generated Guid value as can be created with GuidGen.exe. ComVisible simply annotates this as a COM visible type. By default, all types in an assembly will be exposed via COM. In this case, we only want this one type to be exposed so we add the [assembly:ComVisible(false)] to the assembly level attributes to hide all types unless expressly exposed via the ComVisible(true) attribute.

Secondly, our class implements two interesting interfaces: IContextMenu and IShellExtInit. These two interfaces are well-known interfaces. The easiest way to implement these interfaces is to simply include the interface definitions like so:

public interface IShellExtInit
 int Initialize (IntPtr pidlFolder, 
         IntPtr lpdobj, 
         uint /*HKEY*/ hKeyProgID);

public interface IContextMenu
 // IContextMenu methods
 int QueryContextMenu(uint hmenu, 
            uint iMenu, 
            int idCmdFirst, 
            int idCmdLast, 
            uint uFlags);
 void InvokeCommand (IntPtr pici);
 void GetCommandString(int idcmd, 
            uint uflags, 
            int reserved, 
            StringBuilder commandstring, 
            int cch);

In both of these interfaces, we show that they are well-known with the ComImport attribute. Because both of these interfaces are derived from IUnknown (in the COM nomenclature), we can annotate the interfaces with the InterfaceType attribute. Next, we need to add the GuidAttribute attribute. The Guids are the actual CLSIDs for the interfaces. Finally, the PreserveSig attributes on the methods ensures that the signature is maintained when exposed to COM Interop. The interface definition requires this because the interfaces are actually defined elsewhere. The important thing to know is that you need to include these in order to write our extension, these interfaces are critical.

Now that we have our class and the required interfaces to implement, we can start to put the guts of our shell extension together. The first method to focus on is the IShellExtInit.Initialize method:

IDataObject m_dataObject = null;
uint    m_hDrop = 0;

int IShellExtInit.Initialize(IntPtr pidlFolder, 
               IntPtr lpdobj, 
               uint hKeyProgID)
  m_dataObject = null;
  if (lpdobj != (IntPtr)0)
   // Store the COM Interface for the selected item
   m_dataObject = 
   FORMATETC fmt  = new FORMATETC();
   fmt.cfFormat   = CLIPFORMAT.CF_HDROP;
   fmt.ptd     = 0;
   fmt.lindex    = -1;
   fmt.tymed    = TYMED.TYMED_HGLOBAL;
   STGMEDIUM medium = new STGMEDIUM();
   m_dataObject.GetData(ref fmt, ref medium);
   m_hDrop     = medium.hGlobal;

 return 0;

The purpose of the Initialize method is to get information about the selected items in the shell and save that information for later calls. The m_dataObject and m_hDrop fields are used to store this information in the COM object between calls.


For more information about what's actually going on here, see the "Creating Shell Extension Handlers" article on the Microsoft MSDN site.

Understanding this code is not necessary to create a shell extension handler, but in the big picture it might be helpful. Note that we are ignoring real exceptions in the gathering of this code because alerting the user of a problem here would be of little value.

Next, we need to implement the IContextMenu methods. This interface is specific to the type of shell extension we are creating. If you were to create another type of shell extension, you may need to implement an entirely different interface.

The first method to implement is the QueryContextMenu:

int IContextMenu.QueryContextMenu(uint hmenu, 
                 uint iMenu, 
                 int idCmdFirst, 
                 int idCmdLast, 
                 uint uFlags)
 // The first id to use (should be 1)
 int id = 1;

 // If its a directory we can show the menu item
 if (IsDirectorySelected(uFlags))
  // Create a new Menu Item to add to the popup menu
  mii.cbSize    = 48;
  mii.fMask    = (uint)MIIM.ID | (uint)MIIM.TYPE | (uint)MIIM.STATE;
  mii.wID     = idCmdFirst + id;
  mii.fType    = (uint)MF.STRING;
  mii.dwTypeData  = "Clean VS.NET Temp Files";
  mii.fState    = (uint)MF.ENABLED;

  // Add it to the item
  DllImports.InsertMenuItem(hmenu, (uint)2, 1, ref mii);

  // Since we only added a single item, 
  // just increment the id by 1

 // Return the new id number if we actually added an item
 return id;

This method is called by the shell before it shows the popup menu, but after it has created it. The purpose of the call is to allow you to add a new item into the menu. In this case, we only want to add the menu item if the selected item is a directory. So we have a small subroutine (IsDirectorySelected) that determines this. If it is a directory, we create a new menu item. We are adding the menu item with the Win32 menu item structure. Again, this article is too short to explain the ins and outs of Win32 Menus, but it is explained very clearly on MSDN. The id variable is used to keep a count of how many menu items we added to the menu. Curiously, this needs to start with 1 and be incremented for every menu item you add. The id is returned whether you add the menu or not.

The next method to implement is the GetCommandString method:

void IContextMenu.GetCommandString(int idcmd, 
                  uint uflags, 
                  int reserved, 
                  StringBuilder commandstring, 
                  int cch)
  case (uint)GCS.VERB:
   commandstring = new StringBuilder("CleanVS".Substring(1, cch-1));
  case (uint)GCS.HELPTEXT:
   commandstring = new StringBuilder("Removes all temporary files " + 
    "from a directory (and sub directories) from Visual " + 
    "Studio Builds".Substring(1, cch-1));
  case (uint)GCS.VALIDATE:

This method is used to get the strings to populate the shell as needed to alert the user. For example, the GCS.VERB flag means to return a very simple and small name for the operation. You will notice that we truncate all strings to the length of the cch parameter. The GetCommandString method dictates the maximum length of returned strings and this allows us to make sure we never violate this contract.

Last to implement is the InvokeCommand method:

void IContextMenu.InvokeCommand(IntPtr pici)
  StringBuilder sb = new StringBuilder(1024);

  // Get the Directory Information
  DllImports.DragQueryFile(m_hDrop, 0, sb, sb.Capacity + 1);
  string directory = sb.ToString();

  // Show Status Form


 catch(Exception e)
  System.Windows.Forms.MessageBox.Show("Error : " + e.ToString(), 
   "Error in CleanVS");
  // Hide the status window

This method is used to actually perform the operation in response to a user selecting our menu item. We use a Win32 API call (DragQueryFile) to get the information about the selected items (as gathered by the Initialize method). This allows us to get the full name of the directory. We then show a status dialog and call the CleanDirectory method to actually do the cleaning.


The work that the shell extension performs is not the focus of this article. If you are interested, you can download the source code here for more information.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020