Question

我在下面写了代码，但它无法正常工作。如您所见，我定义了ValueMember和DisplayMember。但是，对于所有项目，它都不显示SelectedItem或SelectedValue，要么我选择Selectable项目。

private class ComboBoxItem
{
    public int Value { get; set; }
    public string Text { get; set; }
    public bool Selectable { get; set; }
}

private void Form1_Load(object sender, EventArgs e)
{
    this._cmb.ValueMember = "Value";
    this._cmb.DisplayMember = "Text";
    this._cmb.Items.AddRange(new[] {
        new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=0},
        new ComboBoxItem() { Selectable = true, Text="Selectable1", Value=1},
        new ComboBoxItem() { Selectable = true, Text="Selectable2", Value=2},
    });

    this._cmb.SelectedIndexChanged += (cbSender, cbe) =>
    {
        var cb = cbSender as ComboBox;

        if (cb.SelectedItem != null && cb.SelectedItem is ComboBoxItem && ((ComboBoxItem)cb.SelectedItem).Selectable == false)
        {
            // deselect item
            cb.SelectedIndex = -1;
        }
    };

    private void button1_Click(object sender, EventArgs e)
    {
        label.Text = _cmb.SelectedItem.ToString();  // it says Object reference not set to an instance of an object.
        label.Text = _cmb.SelectedText.ToString();    // it returns ""
        label.Text = _cmb.SelectedValue.ToString();  // it says Object reference not set to an instance of an object.
    }
}

Answer 1

如果您在使用NullRefernceException时获得SelectedItem，则表示未选择任何项目。确保选中了一个项目，SelectedItem和SelectedValue都可以使用。 SelectedText不是你想的那样。它的作用类似于TextBox上具有相同名称的属性。您只需要Text，而不是SelectedText。

Answer 2

不完全确定为NullReferenceException发生了selectedItem为什么，但以下代码可以正常运行。由于某种原因，SelectedValue将为null，因此我只是将SelectedItem转换回ComboBoxItem类型，然后可以访问其所有属性

namespace WindowsFormsApplication1
{
  public partial class Form1 : Form
  {

    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        //label.Text = _cmb.SelectedItem.ToString();  // it says Object reference not set to an instance of an object.
        //label.Text += _cmb.SelectedText.ToString();   // it returns ""
        //label.Text += _cmb.SelectedValue.ToString();  // it says Object reference not set to an instance of an object.
        ComboBoxItem item = (ComboBoxItem) this._cmb.SelectedItem;
        if (item != null)
        {
            label.Text = item.Value.ToString();
            label.Text += item.Text;
        }

    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this._cmb.ValueMember = "Value";
        this._cmb.DisplayMember = "Text";
        this._cmb.Items.AddRange(new[] {
            new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=0},
            new ComboBoxItem() { Selectable = true, Text="Selectable1", Value=1},
            new ComboBoxItem() { Selectable = true, Text="Selectable2", Value=2},
        });
        this._cmb.SelectedIndexChanged += (cbSender, cbe) =>
        {
            var cb = cbSender as ComboBox;

            if (cb.SelectedItem != null && cb.SelectedItem is ComboBoxItem && ((ComboBoxItem)cb.SelectedItem).Selectable == false)
            {
                // deselect item
                cb.SelectedIndex = -1;
            }
        };
    }
}

public class ComboBoxItem
{
    public int Value { get; set; }
    public string Text { get; set; }
    public bool Selectable { get; set; }
}

}

为什么选择组合框的值不起作用

2 个答案: