InfoPath’s custom control support allows users to create controls for three different types of binding:
Simple data type (string, numbers, etc)
Controls that are written to bind to simple data types would allow a control to do custom processing on data before it gets put into the XML. The control can be designed to have UI that combines multiple simple controls (buttons, textboxes, checkboxes, etc…) to encapsulate them into one control. For example, multiple buttons can be combined with a textbox to make an in-document calculator control. Combining these all into one control will provide a simple way to re-use the same control in multiple forms.
XML stream
InfoPath SP-1 allows XML nodes to follow any schema. This allows form developers to work with industry standard XML Schemas. One example is a MathML control that could be written for InfoPath to save XML compliant with the MathML schema and display it in InfoPath form in a meaningful manner.
IXMLDomNode
For greater flexibility, InfoPath custom controls can be bound to an IXMLDomNode and any msxml operations can be performed directly on the XML tree.
Deployment
Custom controls need not be installed on every form users’ computer before loading the form. Controls can be packaged with your form solution so that on first open, the control will be registered and installed. Controls should be stored in a CAB files and able to self-register.
Security
Custom controls in InfoPath have a few security constraints to keep users safe. Firstly, all custom controls used within InfoPath require the IObjectSafety interface to be implemented and the control to be marked as Safe for Scripting and Safe for Initialization. Without these two flags, InfoPath will not load the form and will notify the user.
In addition, for solution which have controls packaged in the form solution, it is necessary to sign the CAB files with a digital signature. When users open the form for the first time, they will be prompted on whether or not they will accept the control signed with that signature (similar to the ActiveX control dialogs you see in Windows 2k/XP).
For detailed information on how to write a custom control for InfoPath, see Lab 06 in the MSDN InfoPath 2003 Training. (http://msdn.microsoft.com/library/en-us/odc_ip2003_tr/html/odc_INF_Lab_06.asp)
