This example shows how to control icon images drawn by Windows shell.

    The ExtractIcon handler is an ActiveX library like all other Shell Extensions. The first step to be done is to create a new ActiveX Library. Create it with the following sequence of operations: open the Repository dialog using File | New | Other... menu, then select the ActiveX tab and click the ActiveX Library icon.

Fig. 1. New ActiveX Library creation.

    Save the created project.

    The next step is to add the instance of TSxDataModule to the project. Every Shell+ component should be placed on SxDataModule ¹. This special descendant of TDataModule supplies several internal methods that allow Shell+ components to safely operates in multiple Shell threads and automates their registration and initialization. Note that you can place several Shell+ components onto the same SxDataModule as well as you can add multiple TSxDataModule instances to your project. It is easy to add new SxDataModule to the project - just click corresponding icon in the Repository:

Fig. 2. How to create new SxDataModule instance.

    You can save your project now.

The next step is to create the ExtractIcon itself. Shell+ Library contains a special component that takes care about all ExtractIcon functionality. The only thing you need to do is to place it on SxDataModule and set its properties. Select the Shell+ tab in the palette, click the TsxExtractIcon component and insert it into module.

    Take a look on ExtractIcon properties. First of all you will see that the new GUID was created and assigned to the component's CLSID property. It is this GUID will be used by system to identify your CopyHook handler. Other properties are described in the table below:

Table 1. TSxExtractIcon component properties.

    As you've just seen TSxExtractIcon supplies several ways to store and handle icon images. In this example we shell use the ImageList component to store icons. Place it on the form and add two icons into it. You can use any icons as you wish, the first one will be used to indicate the empty text document while the second will represent the document with a text.
    Now fill TSxExtractIcon properties as shown below:

Fig. TSxExtractIcon component properties

    As it was mentioned above the TSxExtractIcon component can ask the end user whether to override an existing icon handler. Let's create a special query form that will be used below to illustrate this opportunity. This form will be shown at Shell Extension registration to confirm overwriting of an existing icon handler. Create a new form and configure it as shown below:

Fig 4. Confirmation query window.

    Every time the Shell needs the icon to be displayed it calls the Shell extension's method to receive it. As a result the TSxExtractIcon.OnExtractIcon event of is fired in our example. This event gives you a lot of opportunities: you can return any icon as you wish depending on file size, contents or any other condition. In our example the size of file is used as a criteria what icon to be shown.
    Select the Events tab of the Object Inspector and double click the OnExtractIcon event. Add the following code to the event handler:

procedure TSxModule1.SxExtractIcon1ExtractIcon(Sender: TObject;
  var IconLarge, IconSmall: TIcon);
begin
  if FileGetSize((Sender as TSxExtractIcon).FileName)<>0 then
     // this icon will be shown for file with contents
     ImageList.GetIcon(1,IconLarge) 
  else
     // this one will be used for an empty file
     ImageList.GetIcon(0,IconLarge); 
end;

    If another icon-handler extension was registered for the file type and the OverwriteExistent property was set to True then the OnKeyAlreadyExists event will be fired. Let's implement its handler as follows:

procedure TSxModule1.SxExtractIcon1KeyAlreadyExist(RegisteredClass: TGUID;
  var Overwrite: Boolean);
begin
  Overwrite:=MessageDlg('Extension already registered. Owerwrite?', 
             mtWarning, [mbYes, mbNo], 0)=mrYes;
  if Overwrite then
  begin
    GUIDWindow:=TGUIDWindow.CreateParented(0);
    try
      GUIDWindow.Caption:='Store this GUID';
      GUIDWindow.ShowModal;
    finally
      GUIDWindow.Free;
    end;
  end;
end;

    Save the project and compile it - it is ready now! As well as any other ActiveX library the Shell+ project requires registration. You can register it in several ways:

• from command prompt using MS regsvr32 utility: regsvr32.exe Project1.dll
• from command prompt using Borland's tregsvr utility: tregsvr Project1.dll

To know more about shell extension installation please follow the link.

Now is a time to test the example. To do it create a new text document on your desktop: Right click | New | Text Document. Note that the icon of newly created file is exactly that you've specified for an empty text document:

To continue the test add a couple of words to the document and save it. As you will see the file will be displayed with a new icon. It is this icon was assigned to files with contents:

    Don't forget to uninstall the example when all tests are finished. Note that it is an example only and it will show its confirmation window until you will not unregister it. As well as installation the uninstallation can be done in many ways:

• from command prompt using MS regsvr32 utility: regsvr32.exe /u Project1.dll
• from command prompt using Borland's tregsvr utility: tregsvr -u Project1.dll

The detailed information about extension uninstallation is available here.

1. The only exception is the TSxTrayIcon component that does not need to be placed on SxDataModule instance.

Use links below to download source codes of this example as well as binary files:

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