Now here is the ans using a simple LINQ query.
MyClass item = MyDataGrid.ItemsSource.Cast().Where((_, pos) => pos == row_number).SingleOrDefault();
Now we have the data grid named MyDataGrid and we have already populated it with a collection of MyClass. We are using extension methods of C# 3.0 to help us select a row where row number is known.
In the lambda expression
(_, pos) => pos == row_number
we do not care about the first parameter so I have replaced it with '_'. (Remember '_' is a valid C# identifier).
LINQ has a great power to make your code compact and less error prone. Still I see many people using lengthy (Wrong?) way of doing simple things like selecting from a list where the list value satisfy a certain condition. Many people still things LINQ as an Syntactic Sugar of SQL queries. You can obviously use LINQ to SQL for interacting with database but LINQ is not only that.
LINQ comes in handy while parsing XML too.
Other benefits are like Sorting, advance selection and searching.
