how to connect my combox with textbox

Answer 1

You need to hook up the SelectedIndexChanged event. Depending on whether this is a WinForms or Web App will determine the exact code but ultimately they're pretty much the same.

Answer 2

You need to subscribe to SelectionChanged event of your ComboBox, then in eventhandler method use the following

if (comboBox.SelectedItem == null) 
{
   textBox.Text = string.Empty; //or any default value
   return;
}
textBox.Text = comboBox.SelectedValue.ToString();

Answer 3

I think you can use binding to do that.

txtbox.DataBindings.Add("Text", comboBox, "Text")