how to convert an integer string to an integer

Try to avoid Parse(...) or Convert.ToInt(...).
Use TryParse(...) instead.
Reason: only TryParse(...) allows to react on parsing mismatches, the other two functions throw exceptions in that case.
 
E.g. If you try to parse and set to a default in case of a parse failure occurs, you may use the following:

static readonly int DEFAULT_FOR_V = 123;
...
string s = "...";
...
int v = int.TryParse(s, out v) ? v : DEFAULT_FOR_V;
...

 
Cheers
Andi

Use int.Parse():

string str = "123456";
int i = int.Parse(str);