Selecting default item from Combobox C#

You can set using SelectedIndex

comboBox1.SelectedIndex= 1;

OR

SelectedItem

comboBox1.SelectedItem = "your value"; // 

The latter won't throw an exception if the value is not available in the combobox

EDIT

If the value to be selected is not specific then you would be better off with this

comboBox1.SelectedIndex = comboBox1.Items.Count - 1;

Remember that collections in C# are zero-based (in other words, the first item in a collection is at position zero). If you have two items in your list, and you want to select the last item, use SelectedIndex = 1.

This means that your selectedindex is out of the range of the array of items in the combobox. The array of items in your combo box is zero-based, so if you have 2 items, it's item 0 and item 1.