Question

I have such class:

public class Foo
{
  public string Regn{get;set;}

  public string DocName{get;set;}

  ...
}

In the my application this class uses with IEnumerable:

IEnumerable<Foo> items;

How to get new IEnumerable, where for all items with the same Regn and DocName property DocName sets like this(only if objects with same DocName >1):

item.DocName=item.DocName+".1";//+"2",etc.

[UPDATE]
Input sample:

Regn DocName
1    1
1    2
1    2
2    5
2    5
2    6

Output:

Regn DocName
1    1
1    2.1
1    2.2
2    5.1
2    5.2
2    6

Answer 1

If you have a default constructor for Foo try to use this:

var newItems = items.
            GroupBy(f => Tuple.Create(f.Regn, f.DocName)).
            SelectMany(gr => gr.Count()<=1 ? gr : gr.Select((f, i) => new Foo
            {
                Regn = f.Regn,
                DocName = f.DocName + "." + (i + 1)
            }));