HELP! Block selectedindexchanged when ComboBox is full

Everyone, hi, I try to explain my problem.

I am using a 3-tier architecture in my project. In DataBase I have a tablion that holds the Region and Cities, and I register them parametrically. In my main form I also take recorded data. When the user selects the city, the area automatically comes up.

I defined the method in the DAL layer,

At the BLL layer, I triggered the method,

On the Windows Form section,

The process does not give an error, but when I debug, I get the selectedindex value 4 times before entering more value. So, filling the comboBoxes in the main form below seems to be related to the codes I use. Because I clicked to open the main form screen in debug, it starts directly from the debug value does not enter me.

I would like your help. Everyone has good day