How to interact with 2 combo boxes in c#.net 4.0 in blow case study

Question

Here i created an application consists of 2 combo boxes 1 combobox contains a list of devices & the other contains its operations.
When I select a device & its operation the equivalent unique code must be printed in Textbox1,but its givong following errors

"Index was outside the bounds of Array".

&

If i want to change the unique code for each operation as printed in textbox on clicking the button located below it what is the logic behind it to save the new ones