How do I select database at runtime.

Login.aspx.cs page
-------------------------
protected void btnSubmit_Click(object sender, EventArgs e)
{
ConnectionInnovator.innovatorSelected_DB = ddlinnovatorDB.SelectedItem.Value.ToString();
Session["DB"] = ddlinnovatorDB.SelectedItem.Value.ToString();
// Session["ddlValue"] = innovatorSelected_DB;
string user = txtUsername.Value;
string pass = txtPassword.Value;

Item login_result = null;
HttpServerConnection conn;

try
{

}
catch (Exception)
{
lblStatus.Text = "Connection failed at all, Check Username/Password.";
lblStatus.ForeColor = System.Drawing.Color.Red;
}

if (login_result.isError())
{
lblStatus.Text = "Connection failed, Check Settings and Login data.";
lblStatus.ForeColor = System.Drawing.Color.Red;
}
else
{

// Response.Cookies["ConnectionCookies"].Value = ConnectionInnovator.conStr;
// Response.Cookies["ConnectionCookies"].Expires = DateTime.Now.AddDays(1);
// Session["connectionStr"]=AddUpdateConnectionString("con");
Response.Redirect("~/Default.aspx");
}

}

below is second page
Default.aspx.cs
---------------------
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
// Session["DB"] = innovatorDBCaught;


// lblResult.Text = "You have choosed \"" + Session["DB"].ToString() + "\" database.";
// lblResult.Text = "You have choosed \"" + ConnectionInnovator.innovatorSelected_DB.ToString() + "\" database.";
lblResult.BackColor = System.Drawing.Color.Cyan;
lblResult.ForeColor = System.Drawing.Color.DarkBlue;
lblSearch.Text = "Input Searching Keyword : ";
ViewState["conDB"] = Session["DB"].ToString(); // Session["DB"] has null value when click search button second time
// i tried in outside from condition also.
// conStr = Session["connectionStr"].ToString();
// ConnectionInnovator.conStr = AddUpdateConnectionString("con");
}

// conStr = Request.Cookies["ConnectionCookies"].Value.ToString();
}


protected void btnSearch_Click(object sender, EventArgs e)
{


ConnectionInnovator.conStr = AddUpdateConnectionString("con");
BindInnovator();
}



protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
GridView1.PageIndex = e.NewPageIndex;
//GridView1.DataSource = dt;
//GridView1.DataBind();

BindInnovator();
}

private void BindInnovator()
{

using (SqlConnection scon = new SqlConnection(ConnectionInnovator.conStr))
{
if (scon.State == ConnectionState.Closed)
scon.Open();
using (SqlCommand scmd = new SqlCommand("waitfor delay '00:00:30';select * from Shippers",scon))
{
scmd.CommandTimeout = 300;
scmd.CommandType = CommandType.StoredProcedure;
scmd.CommandText = "SearchAllTables";
scmd.Parameters.Add("@SearchStr", SqlDbType.NVarChar).Value = txtSearchBox.Text.Trim();
scmd.Connection = scon;
using (SqlDataAdapter sda = new SqlDataAdapter(scmd))
{
DataTable dt = new DataTable();
sda.Fill(dt);
scon.Close();
GridView1.DataSource = dt;
GridView1.DataBind();
}
}
}

}
public string AddUpdateConnectionString(string name)
{
bool isNew = false;
string path = Server.MapPath("~/Web.Config");
XmlDocument doc = new XmlDocument();
doc.Load(path);

I want to search a column record in whole database because user don't know this column belongs to which column of table.
user will choose database from dropdownlist.
to do this i have created a login form called loginfrm.aspx where dropdownlist for database, userid and password is available.
after successful login redirects to the next page(default.aspx) where one textbox, one button for search and one gridview is available.
when default.aspx loads then we entered any values in textbox. it searched and gives output. but when we entered another values related to database. and clicks the button that time database name (selected from dropdownlist) goes missing. that session becomes null values (object doesnot support any value......) exception comes