This is the code which I used. And I have added Microsoft.SharePoint.dll .
using Microsoft.SharePoint.Client;
using System.Web.Script.Serialization;
using Microsoft.SharePoint;
    class
    {          
      SPSite site = new SPSite(siteUrl);
    }
The Error shows like this :
 Could not load file or assembly 'Microsoft.SharePoint, Version=15.0.0.0,
 Culture=neutral, PublicKeyToken=71e9bce111e9429c' or one of its dependencies. 
 The system cannot find the file specified.
 
     
    