Thank you for writing and thank you for sharing your feedback with us.
Indeed, it happens to introduce breaking changes in our releases. We do so, however, when it is really necessary. Simply, we may not be able to implement a feature or improve a control as a whole if we do not change the architecture of this control. Hence come the breaking changes. We also strive to keep our API clean and easy-to-understand as much as possible which also sometimes cause breaking changes. However, if we decide that a method or a property should be removed, we do not remove it right away. Usually, we keep it in our code base for an year, giving our customers time to update their applications before the real breaking changes occur. That said, it is recommended to update your Telerik version at least once a year so that you can still run your project thanks to the obsolete properties and methods.
In addition, we provide a list of Release Notes which describe the breaking changes which are introduced in each release. Usually, a breaking change is accompanied by the new API that you should use, if such is available.
Last, but not least, we are here to help you. Whenever you experience a difficulty with upgrading a newer release, you should not hesitate to contact us. I checked your support history and did not find much about your issues with the newer releases. Contacting us will save you time and efforts and will make the process of updating to the latest release smoother.
Now allow me to answer each of your specific questions in the order they have been given:
1. RadComboBox did not support checked items out-of-the-box. This was a matter of customization. The same customization is available to RadDropDownList and it is even demonstrates in our Demo application, section DropDown & List >> Creating New Items. If you want your RadDropDownList to stay opened after you press a checkbox, you should add a RadCheckBox control instead of RadCheckBoxElement. This is demonstrated in the attached sample project.
2. We will be grateful if you provide us with a list of the deprecated methods for which you can't find the necessary information. This will allow us to provide you with adequate assistance and address any documentation issue on our side.
3. Generally, it seems that something in your layout is changed and as a result a blank page appears. For more information about the appearance of blank pages, please refer to this knowledge base article. If this still does not help, please contact the Reporting support guys so that they can assist you further.
4. I would suggest that you take a look at the Release Notes of the Telerik ASP.NET Ajax. If they still do not help, please describe your cases in details in the appropriate support threads.
5. As I mentioned, we are trying to make our API more clean and easy-to-understand. The term 'easy-to-understand' implies being similar to the API of the Microsoft controls. This is why IsChecked was hidden, and instead Checked was introduced.
6. I am not sure what exactly are your concerns here, however sometimes such changes are needed,
because of changes in architecture or changes in the functionality where you need to use another event for a certain operation, for example.
To sum up, I would recommend updating on regular basis. This will allow you to avoid possible breaking changes. In addition, updating regularly will allow you to meet a small number of breaking changes for a release which is better than meeting a big number of breaking changes at once. Last, but not least, please do not hesitate to contact us whenever you have doubts about the API or control you should use instead of the currently obsoleted API. We will be glad to assist you.
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.