Deleting item from list C#

Question

I am trying to delete a selected item from the drop down list using C#, however I just can not get it to work. Can anyone show me where I am going wrong? I have tried the RemoveAll and RemoveAt tags (which I think i correct) but I may be placing them in the wrong place? Thanks.

public partial class DeleteBook : System.Web.UI.Page
{
Catalogue catalogueInstance;

protected void btnDelete_Click(object sender, EventArgs e)
{
    int id = Int32.Parse(txtID.Text);
    Book book = catalogueInstance.books.RemoveAt(b => b.id == id);
    if (book != null)
    {
        book.title = txtTitle.Text;
        book.year = Int32.Parse(txtYear.Text);
        book.author = txtAuthor.Text;
        book.publisher = txtPublisher.Text;
        book.isbn = txtISBN.Text;

        string jsonText = JsonConvert.SerializeObject(catalogueInstance);
        File.WriteAllText(FILENAME, jsonText);
    }
    txtSummary.Text = "Book ID of " + id + " has Been deleted from the 
    Catalogue" + Environment.NewLine;
}
}

Answer 1

I'm not sure what the problem is, and why exactly you are trying to retreive the book that you are deleting, BUT every time you make changes to the datasource, you need to rebind it to the control.

Try repeating the following block in btnDelete_Click

ddlDelete.DataSource = catalogueInstance.books;
ddlDelete.DataTextField = "title";
ddlDelete.DataValueField = "id";

Another possible problem is that since you are not performing the initial binding when !Page.IsPostback() , you are actually rebinding the datasource before the delete event. Remember, Page_Load is always the first event after a postback.

EDIT

You are trying to use Catalogue catalogueInstance as a global variable and then on page load read the same file over and over again.

1. Global variables on asp.net cannot be used the same way you would use on win forms. You need to look into Viewstate so that the state of the global variable persists the various page refreshes that will occur.
2. Every time the page refreshes, you keep re-loading your catalogue from a file, thus, replacing the deleted rows. Remember, you are only deleting the items from the catalogue but they are still there on the file.

Try the following, simpler solution and I'm sure you'll see the problem:

//Filepath for json file
// this can stay here
const string FILENAME = 
@"C:\Users\tstra\Desktop\19456932_CSE2ICX_Assessment_3\Bin\Books.json";

protected void Page_Load(object sender, EventArgs e)
{
    // this needs stay in here, enclosed within `!Page.IsPostback()`
    if (!Page.IsPostback()) 
    {
        Catalogue catalogueInstance;
        // reading data contained in the json filepath
        string jsonText = File.ReadAllText(FILENAME);

        //convert objects in json file to lists
        catalogueInstance = JsonConvert.DeserializeObject<Catalogue>(jsonText);

        // save the current state into ViewState
        ViewState["catalogueInstance"] = catalogueInstance;

        ddlDelete.DataSource = catalogueInstance.books;
        ddlDelete.DataTextField = "title";
        ddlDelete.DataValueField = "id";

        //binding the data to Drop Down List
        ddlDelete.DataBind();
   }
}

protected void btnDelete_Click(object sender, EventArgs e)
{
    // get catalogueInstance back from ViewState before trying to use it
    Catalogue catalogueInstance = (Catalogue)ViewState["catalogueInstance"];

    int id = Int32.Parse(txtID.Text);
    Book book = catalogueInstance.books.RemoveAt(b => b.id == id);
    if (book != null)
    {
        book.title = txtTitle.Text;
        book.year = Int32.Parse(txtYear.Text);
        book.author = txtAuthor.Text;
        book.publisher = txtPublisher.Text;
        book.isbn = txtISBN.Text;    

        string jsonText = JsonConvert.SerializeObject(catalogueInstance);
        File.WriteAllText(FILENAME, jsonText);

        // you need to reset, and rebind the datasource again
        ddlDelete.DataSource = catalogueInstance.books;
        ddlDelete.DataTextField = "title";
        ddlDelete.DataValueField = "id";

        //binding the data to Drop Down List
        ddlDelete.DataBind();
    }
    txtSummary.Text = "Book ID of " + id + " has Been deleted from the 
    Catalogue" + Environment.NewLine;
}

Answer 2

You should use IsPostBack in Page_Load :

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        // reading data contained in the json filepath
        string jsonText = File.ReadAllText(FILENAME);

        //convert objects in json file to lists
        catalogueInstance = JsonConvert.DeserializeObject<Catalogue>(jsonText);

        ddlDelete.DataSource = catalogueInstance.books;
        ddlDelete.DataTextField = "title";
        ddlDelete.DataValueField = "id";

        //binding the data to Drop Down List
        ddlDelete.DataBind();
    }
}