Sharepoint - Get List items with a different user

Guid id = SPContext.Current.Site.ID;                
SPUser user = SPContext.Current.Web.EnsureUser("Loginname");              
  using (SPSite site = new SPSite(ID,user.UserToken))
   {
      using (SPWeb web = site.OpenWeb())
       { 
         //write code for further operation
       }
    }

You might want to check the SPSecurity.RunWithElevatedPrivileges method to give user an admin privileged to the list.

Impersonation in SharePoint