string ctypeChar = string.Empty;
if (Case_type.SelectedItem != null && !string.IsNullOrEmpty(Case_type.SelectedItem.ToString())
cTypeChar = Case_type.SelectedItem.ToString()[0];
Then replace Case_type in your append with cTypeChar.
But...
You are really making your query wrong, you should be
using parameters[
^]. Read up on how to use parameterized queries so you don't subject yourself to SQL injection attacks.
[Edit]
Just read a little closer, you are passing Case_type as a string into the function. You can use this code to calculate the Case_type value to pass to the function. I wrote the code as if Case_type was the actual combobox.