How to update query string and reload Page on DropDownList SelectedIndexChanged event?

Question

have Dropdownlist which have languages how to reload page each time change selected item from dropdown ?

  protected void DDLLanguages_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (Request.RawUrl.Contains("Language") == false )
            {
                Response.Redirect(Request.RawUrl + "?Language=" + DDLLanguages.SelectedValue);
            }
            else
            {
             //i need to update the quesry string here 

            }

        }

Answer 1

 protected void DDLLanguages_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (Request.RawUrl.Contains("Language") == false )
            {
                Response.Redirect(Request.RawUrl + "?Language=" + DDLLanguages.SelectedValue);
            }
            else
            {
             //i need to update the quesry string here 
   Response.Redirect(Request.Url.GetLeftPart(UriPartial.Path)+ "?Language=" + DDLLanguages.SelectedValue);

            }

    }

Answer 2

The accepted answer wipes out any existing query string values.

// parse url into UriBuilder //
var uri = new UriBuilder(Request.RawUrl);

// parse query part of url into a NameValueCollection //
var query = HttpUtility.ParseQueryString(uri.Query);

// update/create "Language" entry in the NameValueCollection //
query["Language"] = DDLLanguages.SelectedValue;

// put updated NameValueCollection back into uri querystring //
uri.Query = query.ToString();

// redirect page to updated uri //
Response.Redirect(uri.ToString());