NullReferenceException: Object reference not set to an instance of an object #2

问题

Error shown on the website. Im using asp net visual C# webform, access data source (MS access) When I click on Add to Cart button on productdetails.aspx

Line 41:         int intOrderNo = (int)Session["sOrderNo"];
Line 42:         string strUnitPrice = (string)Session["sUnitPrice"];
Line 43:         decimal decUnitPrice = decimal.Parse(strUnitPrice);

For myOrder table in Ms Access There is oOrderNo, oDate, oUserName, oPaymentMode, oStatus,

 protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
    // test to remind customer to login first
    if ((string)Session["sFlag"]!="T")
    {
        Type csType = this.GetType();
        ClientScript.RegisterStartupScript(csType, "Error", scriptErrorLogin);
    }

    // Connect to database  
    OleDbConnection mDB = new OleDbConnection();
    mDB.ConnectionString = "Provider = Microsoft.ACE.OLEDB.12.0;Data source=" + Server.MapPath("~/App_Data/webBase.accdb");
    mDB.Open();
    OleDbCommand cmd;
    DetailsViewRow row0 = DetailsView1.Rows[0];
    string strProductID = row0.Cells[1].Text;
    mDB.Close();

    // save as session variables
    Session["sProductID"] = strProductID;
    DetailsViewRow row4 = DetailsView1.Rows[4];
    Session["sUnitPrice"] = row4.Cells[1].Text;


    int intOrderNo = (int)Session["sOrderNo"];
    string strUnitPrice = (string)Session["sUnitPrice"];
    decimal decUnitPrice = decimal.Parse(strUnitPrice);
    string strSQL = "INSERT INTO orderItems(uOrderNo, uProductID, uUnitPrice)" + "VALUES(@eOrderNo, @eProductID, @eUnitPrice)";
    cmd = new OleDbCommand(strSQL, mDB);
    cmd.Parameters.AddWithValue("@eOrderNo", intOrderNo);
    cmd.Parameters.AddWithValue("@eProductID", strProductID);
    cmd.Parameters.AddWithValue("@eUnitPrice", decUnitPrice);

    mDB.Open();
    cmd.ExecuteNonQuery();
    mDB.Close();

    Response.Redirect("ShoppingCart.aspx");

回答1:

Try below code :

Line 41:         int intOrderNo = Session["sOrderNo"] == null ? 0 : (int)Session["sOrderNo"];
Line 42:         string strUnitPrice = Session["sUnitPrice"] == null ? string.Empty : (string)Session["sUnitPrice"];
Line 43:         decimal decUnitPrice = string.IsNullOrWhiteSpace(strUnitPrice) ? 0 : decimal.Parse(strUnitPrice);

回答2:

Try this

Line 41:         int intOrderNo = Session["sOrderNo"] == DBNull.Value ? 0 : (int)Session["sOrderNo"];
Line 42:         string strUnitPrice = Session["sUnitPrice"] == DBNull.Value ? string.Empty : (string)Session["sUnitPrice"];

null Vs DBNull.Value

回答3:

Just check that if Session variable is Null-

 if( Session["sOrderNo"] != null && all the session variables )
{
     //Now check your condition here
}
else {
       //Perform any operation
    }

来源：https://stackoverflow.com/questions/21226065/nullreferenceexception-object-reference-not-set-to-an-instance-of-an-object-2