You can use System.Convert library to convert this.
private static string myString = "123";
private int id = Convert.ToInt32(myString);
Or, you can use System.Int32
private string myString = "123";
private int id = Int32.Parse(myString);
If your string is not numeric then it will throw an exception. In that case you can use the following.
private static string myString = "123";
private static string myString = "123";
private static int convertedValue;
private bool success = Int32.TryParse(myString, out convertedValue);
Here success will determine if your string was converted successfully or not. The convertedValue will contain the converted string value if its successful, otherwise contain its initialized value.