Asp.net how to solve the error

Im designing my web page, in that i have typed in the book id and typed Nameofthebook. and then click the button the stack of book are available is go to another page and then not available is display the error message is book is not available My project (Library management system)

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

public partial class Bookcheck : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string constr = null;
        constr = ConfigurationManager.ConnectionStrings["librarymanagementconnetionstring"].ConnectionString;
        SqlConnection cnn = new SqlConnection(constr);
 开发者_运维知识库       cnn.Open();
        DataSet ds = new DataSet();
        DataTable dt = new DataTable();
        SqlDataAdapter da = new SqlDataAdapter("SELECT*FROM BOOKREGISTRATIONDETAILS WHERE bookId='" + txtid.Text.Trim() + "'AND Nameofbook='" + txtnb.Text.Trim() + "'", constr);
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count > 0)
        {
            Response.Redirect("Issueofbook.aspx");
        }
        else
        {
            Msg.Text = "NO book available";
            Msg.Visible = true;
        }
    }       
}      

Error:

Object reference not set to an instance of an object.

I noticed that in librarymanagementconnetionstring, connection is spelled incorrectly. Is this the correct entry in your web.config for the connection string?

I'd say that it's the line

if (ds.Tables[0].Rows.Count > 0)

that giving the error.

If your DataSet has no tables then ds.Tables could be null.

If that's a possibility then you need to code for that.

If there are no rows ds.Tables[0].Rows won't be null and your test should work.

Change

if (ds.Tables[0].Rows.Count > 0)

to

if(ds.HasRows)