get json value in c#

I am returning a string

"{\"valid\":\"true\",\"number\":\"0721224548924\",\"itemname\":\"adidas\",\"alias\":\"\",\"description\":\"\",\"avg_price\":\"\",\"rate_up\":0,\"rate_down\":0}"

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 string sLine = "";
                int i = 0;

                while (sLine != null)
                {
                    i++;
                    sLine = objReader.ReadLine();
                    if (sLine != null)
                        Console.WriteLine("{0}:{1}", i, sLine);
                }

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Not sure how to get the value of each on the c# end.