what is mean System.format exception

In Database companyid is taken as Numeric,

autoincrement property is false.because for that i write code.

so what can i do next?

The only thing I can think of is that you've defined the 'companyid' column to be an integer, or perhaps set its AutoIncrement property to true and so the system thinks it's an integer. The expression dr["companyid"] would then be expected to have a runtime type of integer.

However, there are some companyids already in the database which are not integers but strings (say "C01") and so the  Int32.ToString() method, which is getting called at runtime, is having a problem because it's trying to parse an integer to a string - hence the format exception.  

I am work one form a.aspx in that, i take one dropdown list name as 'companygroup'.  ,that list item is bound from database.

when 1st list item selected ,then in another dropdownlist  company name as items which belongs inthat company group must arrive.for that i write query in selected indexchanged of 1st  dropdown list.and kept Autopostback=true .it work properly.

but my problem is when i select 2nd list i.e company list item than i want a companyid in one textbox

so that i write query in 2nd dropdownlist selected index change

 please see code

As other guys said, you are trying to convert a data type that is not a valid conversion such as converting a string to an integer.

can i see ur code where u r getting error?

As a frequent sufferer from this exception, I can tell you that it usually occurs when passing strings to methods which convert them to numbers or dates. For example, these (rather silly) statements will throw a FormatException:

      string s = "Hello";
      int i = Convert.ToInt32(s); 

      // or

      string s = "1000000";
      DateTime dt = DateTime.Parse(s); 

However, it's not confined to these situations and can be thrown more generally when a method argument is simply not in the format which the method expects. 

To resolve it, make sure that the argument is in an appropriate format (read the docs if unsure) before you pass it. 

With out knowing the code what you have written it is hard to tell that post the code and mention where you are getting the error