Create Your Own Custom File Type


This article describes a simple approach to creating a custom file type.  In the example provided, a custom file type is created around a serializable class that is used as a data container holding all of the elements necessary to support an application designed to interact with the defined file type.  A separate module is used to serialize and deserialize files of this user defined file type.

It is possible to include more than a single data container class as the basis for the file type; placing a collection of objects into a hash table will allow for greater complexity but will still operate in the same manner.  The approach is useful if you are in need of a way to create a file type that is specific to a particular application.

Getting Started:

In order to get started, unzip the downloaded files and open the project provided.  Within the project you will find three main files: frmMain.cs, ProjectSerializer.cs, and PersonalData.cs.

frmMain.cs:  This is the application and it is used to represent an application that interacts exclusively with the custom file type.  This form contains a collection of text boxes used to capture or display values associated with instances of the PersonalData class.  It only contains three primary functions, one to save a file as the custom type, one to open a file on startup, and one to open a file of the custom type through a file open dialog.  To open a file, the application captures the path to the file by means of an OpenFileDialog, the path of the file along with a new instance of the PersonalData class is subsequently passed to the ProjectSerializer's deserialize function which in turn deserializes the content of the stored file and passes it back to the Open File function.  Once the deserialized values are applied back to the PersonalData object, the Open File function populates the form textboxes from the values contained in that object.  To save a file, the values currently displayed in the form's text boxes are gathered and used to populate the properties contained in a new instance of the PersonalData class.  The class along with the file path captured from a SaveFileDialog are passed to the ProjectSerializer's serialize method where a binary formatter is used to serialize the object's content into a file created at the specified file path.

ProjectSerializer.cs:  This is a module that contains two functions; one to serialize the data contained on the form and one to deserialize the data from a stored file saved as the custom file type.  Both methods use the binary formatter although it would work equally well with soap formatter.

PersonalData.cs:  A serializable class used contain the data associated with an instance of a custom file type as generated from the fields on in the frmMain form.  One important point to make here is that this class is defined as serializable; if it were not the operation would fail.

Each of the files mentioned herein are contained in the sample application; the code is fairly well annotated and should be straight forward and easy to follow.  Sufficient annotation is contained within these files so they should be easy enough to understand.  At this point, you may wish to open the project, run it to see how it works, and then examine the code files to see how the project was constructed.

Creating a File Type Association.

The process of creating a file type association through the use of the Visual Studio 2005 setup and deployment project is far easier to manage than it once was; you can manually code the necessary information into the application, and depending upon the installation package you are using, you may need to do that.  If you are interested in that approach, take a look at this link,, [see File Associations the hard way].

First off, in the main form of the application, you will need to modify the form load event handler to respond to the receipt of a command line argument (the file name), this is now a trivial bit of code and it should look something like this:

Private void frmMain_Load(System.Object sender, System.EventArgs e)


    'check each parameter to get the file name (there is only one though)

    for (String In My.Application.CommandLineArgs param)




            ' pass the file path if it exists





            'do nothing, just open the application with no file




In this example, when the form loads, it will examine the contents of My.Application.CommandLineArgs to see if it contains a file name; since that is all we are going pass in, it will either be empty or will contain a file path.   If a file path is present, it will passed to a subroutine called, "OpenFromPath" which captures the data from the file and populates the form's text boxes.

In order to pass the file path to the command line arguments, you need to set up a couple of things in the Setup and Deployment project.  To begin, add a setup and deployment project to the existing solution and configure it to suit your requirements (the sample code include a setup project).  Once the project has been added, click on the setup project's name in the solution explorer, click on the "View" and then click on the "File Types" option.  This will bring up a File Types Designer in the main window of Visual Studio.

Once the file type designer has been displayed, right click on "File Types on Target Machine" and the click on the "Add File Type" option.  This will add an empty file type to the tree view, select the new file type's node and look in the property editor:


Figure 1:  File Type Property Editor

In the property editor, set the name to match the name of the custom type, set the command to point to the application (as the Primary output from the application), key in a description, set the extension to the custom file type's extension, and set an icon for the file type.  Having set those values, click on the Open node.

Figure 2:  The Open node under the custom file type

Once you have clicked on the open node, the property editor will show these properties:

Figure 3:  Property Editor Set to the Open Node

These default properties are correct in that the default process is set to "open" and the "%1" is set to pass the file name of a selected file to the application's command line arguments on startup.  After a user installs the application, when they double click on a file of the custom file type, it will pass the file path to the application and open it.  Also, if the user right clicks on the custom file type icon, they will be able to select the "open with" option and be presented with a link to your application as indicated in the following figure:

Figure 4: Open With option in Windows Explorer context menu


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