During form load...
string[] items = { "item1", "Student", "Teacher", "Professor", "Teaching Assistent" };
AutoCompleteStringCollection allowedTypes = new AutoCompleteStringCollection();
allowedTypes.AddRange(items);
comboBox1.AutoCompleteCustomSource = allowedTypes;
comboBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
comboBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;