UserDataSource within a matrix

Hello everyone.

I'm using the following code to set a value within a matrix columns specific row:

oMatrix.SetLineData(RowNum);
oForm.DataSources.UserDataSources.Item("CheckBox").ValueEx = "Y";

the problem is that whenever that happens, and that new value is being set, all the rest of the columns (which are set to diffrent userdatasources) in that chosen row are being deleted. is there any way to prevent that happening?

Thanks & Regards

Moshe