what is diffrence between convert.toint32 and int.parse

If you use Lutz Roeder's Reflector to inspect the implementation of Convert.ToInt32(string), you'll find that it calls int.Parse(string, CurrentCulture).

So it doesen't really matter which one you call.

Use Convert class when you convert object type that contains type in which you want to convert. For example when you execute ExecuteScalar of SqlDataReader it will return object type result but you know which type of object you expect and you use Convert.ToInt32, Convert.ToString or some other, to get the value. Same is when you want to get SelectedItem of the combo which is of type object but contains string and you need to use Convert.ToString to get the selected value.
'type'.Parse or 'type'.TryParse can be used only when converting string which contain targeted value type.