Busy on C# programming

Now, my task was extracting dictionary entries from a sql server database. I was programming in c# language. After the programming practice on knowledge base import program and developing a simple website for knowledge base writing, I had been familiar with C# a lot. Its string process ability was very good.
This time, my key point was on arraylist. There was a bug I thought in C#. That was is you use:
-------------------------------------------
string[] array_test= ……
foreach(string i in array_test)
{
string str_temp = i.ToString();
int int_start = str_temp.IndexOf("test"); //there will be an error. It is said that int can not transform into string
}
-------------------------------------------
I did not know why it happened. My solution was using one more function importing i.ToString() and return the string processing result.

There was some difference between C# and C++ programming. I would be know more about their difference.