Question

我正在尝试使用C＃从下拉列表中删除所选项目，但是我无法让它工作。有谁能告诉我哪里出错了？我已经尝试了RemoveAll和RemoveAt标签（我认为我更正），但我可能会把它们放在错误的地方？感谢。

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

我不确定问题是什么，为什么你正在试图检索你要删除的书，但每次你对数据源进行更改时，你需要重新绑定它控制。

尝试在btnDelete_Click

中重复以下块

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

另一个可能的问题是，由于您在!Page.IsPostback()时未执行初始绑定，因此实际上是在删除事件之前重新绑定数据源。请注意，Page_Load 始终是回发后的第一个事件。

修改

您正在尝试将Catalogue catalogueInstance用作全局变量，然后在页面加载时反复读取同一文件。

asp.net上的全局变量不能像在win表单上使用的那样使用。您需要查看Viewstate，以便全局变量的状态持续将发生的各种页面刷新。

每次刷新页面时，都会不断从文件中重新加载目录，从而替换已删除的行。请记住，您只是删除了目录中的项目，但它们仍然存在于文件中。

尝试以下更简单的解决方案，我相信您会看到问题：

//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

您应该在IsPostBack中使用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();
    }
}

从列表C＃中删除项目

2 个答案: