Combobox.Text 始终返回 Null

本文关键字：返回 Null Text Combobox | 更新日期: 2023-09-27 17:57:14

我正在尝试从组合框中获取当前选定的选项，并且我尝试通过其文本

ComboBox.Text
Combobox.SelectedItem()

但是.Text返回一个空字符串，SelectedItem()返回 null

这是我如何填充组合框的代码。组合框的值取决于另一个组合框的值。

private void cboSite_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
        BackgroundWorker bw = new BackgroundWorker();
        cboPlan.Items.Clear();
        bw.DoWork += new DoWorkEventHandler(bw_cboPlan);
        bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_cboPlanComplete);
        site = cboSite.SelectedItem.ToString();
       Busy.IsBusy = true;
        Busy.BusyContent = "Loading Products";
        bw.RunWorkerAsync();
    }
    void bw_cboPlan(object sender, DoWorkEventArgs e)
    {
        SqlConnection con = new SqlConnection(Class.GetConnectionString());
        SqlCommand scProduct = new SqlCommand("spSelectProduct", con);
        scProduct.Parameters.Add(new SqlParameter("@Site",site));
        scProduct.CommandType = CommandType.StoredProcedure;
        SqlDataReader readerPortal;
        con.Open();
        readerPortal = scProduct.ExecuteReader();
        while (readerPortal.Read())
        {
            this.Dispatcher.Invoke((Action)delegate(){cboPlan.Items.Add(readerPortal[0]);});
        }
        con.Close();
    }
    void bw_cboPlanComplete(object sender, RunWorkerCompletedEventArgs e)
    {
        cboPlan.SelectedIndex = 0;
        Busy.IsBusy = false;
    }

虽然我可以在组合框中看到.Text值，但我无法在代码中使用它们。

编辑：空值由cboPlan组合框返回。

这是它在SelectedItem()的情况下返回 null 和在 .Text的情况下返回空字符串 

if (IsValid())
        {
            BackgroundWorker bw = new BackgroundWorker();
            cboPlan.Items.Clear();
            bw.DoWork += new DoWorkEventHandler(bw_Add);
            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_AddComplete);
            plan = cboPlan.Text;
            Busy.IsBusy = true;
            Busy.BusyContent = "Sending Request.";
            bw.RunWorkerAsync();
        }

XAML 用于组合框。

<ComboBox x:Name="cboSite" HorizontalAlignment="Left" Margin="461,52,0,0" VerticalAlignment="Top" Width="174" SelectionChanged="cboSite_SelectionChanged"/>
<ComboBox x:Name="cboPlan" HorizontalAlignment="Left" Margin="395,106,0,0" VerticalAlignment="Top" Width="240" />

Combobox.Text 始终返回 Null

Edit

好吧，你的代码中有这个：

  BackgroundWorker bw = new BackgroundWorker();
  // You will delete all items here!
  cboPlan.Items.Clear();
  bw.DoWork += new DoWorkEventHandler(bw_Add);
  bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_AddComplete);
  // plan is String.Empty because there are no items in the combobox!
  plan = cboPlan.Text;
  Busy.IsBusy = true;
  Busy.BusyContent = "Sending Request.";
  // You will start populating combobox asynchronously here 
  bw.RunWorkerAsync();

我不确定您要做什么，但是如果您想保存控件的值，请在调用Items.Clear()函数之前执行此操作。

原答案

我建议您在bw_cboPlan中填写数据库中的列表，并在回调函数中填充RunWorkerCompletedEventHandler组合框：

List<String> combovalues = new List<String>();
void bw_cboPlan(object sender, DoWorkEventArgs e)
    {
        SqlConnection con = new SqlConnection(Class.GetConnectionString());
        SqlCommand scProduct = new SqlCommand("spSelectProduct", con);
        scProduct.Parameters.Add(new SqlParameter("@Site",site));
        scProduct.CommandType = CommandType.StoredProcedure;
        SqlDataReader readerPortal;
        con.Open();
        readerPortal = scProduct.ExecuteReader();
        combovalues.Clear();
        while (readerPortal.Read())
        {
             combovalues.Add(readerPortal[0]); // untested
            //this.Dispatcher.Invoke((Action)delegate(){cboPlan.Items.Add(readerPortal[0]);});
        }
        con.Close();
    }
    void bw_cboPlanComplete(object sender, RunWorkerCompletedEventArgs e)
    {
        foreach(var item in combovalues)
            cboPlan.Items.Add(item);
        cboPlan.SelectedIndex = 0;
        Busy.IsBusy = false;
    }
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