InfoPath Dev

I'm using cascading dropdown list and I want to refresh each dropdown in the on change event For example: I have two dropdown list, in the first one I have the following options A,B,C in the second one I have D,E,F if I select in the first DropDown option A I want to refresh the second one and so on with each DropDown. For now I put an empty string like this "" on change event but that's not the proper form. On the other, hand how can I create an autopostback when I change values in one of the dropdown list, I mean a "refresh" when I upload the form to SharePoint. The idea is when the form is uploaded on the web I want to make another refresh when I change a value in one of the dropdownlist. Im' Coding with C# on Visual Studio For Applications

You are on the right track.  If the values in the second list are a resultant list based on the first dropdown list then you have to blank the field and requery the data to get the correct  values.  If you only requery the data you will still have one value in the list showing that is from the original query.

How can I requery the data. I'm using a DataBase from microsoft sql server and in the each dropdownlist on change event contain a paramater that I passed to another dropdownlist to charge the Data. How Can I Requery??

You need to reexecute the query in your code.   If you need additional help please post the code so we can take a look. 

So do we need to develop some code in order to achieve this? How can I blank the field when the query is different?