Question

There is

Dictionary<string,List<string>> dictionary=new Dictionary<string,List<string>>();

and

var c;

var c returns me values : 
100, "somestring"
100,"someotherstring"
200,"two"

foreach(var d in c)
{
 dict.Add(d.key,d.value);
// Need to add key value pairs here for dictionary. if key is same then values should get concatenated.
}

foreach(keyvaluepair<string,List<string>> pair in dictionary)
{
    // This loop should output something like below...
       100,"somestring,someotherstring"
       200,"two"

}

Answer 1

Make a dictionary with List<string> as value, and then just add values :

foreach(var d in c)
{
  if (!dict.ContainsKey(d.Key))
    dict.Add(d.Key, new List<string>());
  dict[d.Key].Add(d.Value);
}

and later you can get comma delimited string from list with string.Join

string commaDelimitedList = string.Join(",", valueList.ToArray());