i have managed to get solution for this
public static void GetWebUrl()
{
string fileUrl ="https://demo.sharepoint.com/sites/Site1/SubSite1/subsiteDocumentLib/DEMO_Doc.docx";
Uri fileUri = new Uri(fileUrl);
ClientContext rootContext = new ClientContext(fileUri.Scheme + Uri.SchemeDelimiter + fileUri.Host);
Uri webUrl = Web.WebUrlFromPageUrlDirect(rootContext, fileUri);
ClientContext subContext = new ClientContext(webUrl.ToString());
}