如何在c＃winforms应用程序中获取列表框项目的“密钥”？

Question

I am writing a winforms app in which a user selects an item from a listbox and edits some data that forms part of an associated object. The edits are then applied from the object list to an underlying file.

In ASP.Net assigning a different system value to a list item than the display text the user sees is trivial. In a winforms app you have to set the "Displaymember" and the "Valuemember" of each item in a slightly more complicated (and not oft related on the internet) process.

This I have done. In debug mode I have confirmed that every item now has a value which is the display member (a "friendly" string that the user sees) and a key, the valuemember, which holds the key to a hashtable object where the data to be updated exists.

So when a user picks a string to edit the program should pass the "key" to the hashtable, yank out the object and allow editing to take place upon it.

The catch?

I can't see any obvious way of telling the program to look at the item's valuemember. I naively expected it to populate the list box's "SelectedValue" property, but that would be too simple by far. So how the hell do I get to the list item value?

Answer 1

Using both the SelectedIndexChanged and SelectedValueChanged didn't work for me - the ListBox's SelectedValue property was always null. This surprised me, too.

As a lame workaround, you could just pull the object out of the ListBox directly, using the SelectedIndex :

public Form1()
{
    InitializeComponent();

    this.listBox1.DisplayMember = "Name";
    this.listBox1.ValueMember = "ID";

    this.listBox1.Items.Add(new Test(1, "A"));
    this.listBox1.Items.Add(new Test(2, "B"));
    this.listBox1.Items.Add(new Test(3, "C"));
    this.listBox1.Items.Add(new Test(4, "D"));
    this.listBox1.Items.Add(new Test(5, "E"));
}

private void OnSelectedIndexChanged(object sender, EventArgs e)
{
    if(-1 != this.listBox1.SelectedIndex)
    {
        Test t = this.listBox1.Items[this.listBox1.SelectedIndex] as Test;
        if(null != t)
        {
            this.textBox1.Text = t.Name;
        }
    }
}

( Test is just a simple class with two properties - ID and Name ).

It seems like there should be a better way, but if nothing else this should work.

Answer 2

Okay so the answer came as a result of Andy's answer, hence my upvoting that answer.

But when I created a little class and tried to cast the listitem into that class the program threw an exception.

Revealingly the exception told me that the program could not cast a DictionaryEntry into a class of the type I had defined.

So I deleted the proxy class and reframed the request thus:

DictionaryEntry de = (DictionaryEntry)listbox.SelectedItem;

string htKey = de.Key.ToString();

And it's all good.

Bizarrely simple answer in the end. Thanks for the hint Andy.

Answer 3

I know this is a very old post, but I was unable to cast my list box items into a Dictionary item. This solution worked for me in .NET 3.5 for windows forms.

KeyValuePair<int, string> kvp = (KeyValuePair<int, string>)listBoxSystems.SelectedItem;
string szValue = kvp.Value;

Answer 4

Try grabbing the "ValueMember" from within the ListBox1_SelectedValueChanged event.

private void ListBox1_SelectedValueChanged(object sender, EventArgs e)
{
    if (ListBox1.SelectedIndex != -1)
    {
        string orbit = ListBox1.SelectedValue.ToString();
    }
}

ArrayList planets = new ArrayList();
planets.Add(new Planet("Mercury", "1"));
planets.Add(new Planet("Venus", "2"));

//Remember to set the Datasource
ListBox1.DataSource = planets;
//Name and Orbit are properties of the 'Planet' class
ListBox1.DisplayMember = "Name";
ListBox1.ValueMember = "Orbit";

Answer 5

A simple and clean way:

Add Item (include Key and Value) to ListBox:

lsbListBoxName.Items.Insert(0, New ListItem("Item 1", 1))
lsbListBoxName.Items.Insert(0, New ListItem("Item 2", 2))
...

Get Item on user selection:

Private Sub lsbListBoxName_SelectedIndexChanged(sender As Object, e As EventArgs) Handles lsbListBoxName.SelectedIndexChanged
    Console.WriteLine(TryCast(lsbListBoxName.SelectedItem, ListItem).Text)
    Console.WriteLine(TryCast(lsbListBoxName.SelectedItem, ListItem).Value)
End Sub

the lsbListBoxName is name of ListBox and the code is VB.NET you can use This Online Tool to convent to C# .

Answer 6

I ended up here when I was searching how to get the value of an Item in ListBox, then the obvious came to my mind.
The secret is that Item method in C# , VB and others is an array, so to get a value of any Item you just have to write this:

//Get value of the #1 Item in the ListBox;
ListBox1.Items[1].toString();

To get all the Items and put into a document or to a string, just do a for like this:

string Value;
for (int c = 0; c < ListBox1.Items.Count; c++)  {
    Value = Value + ListBox1.Items[c].toString();
}

I hope I helped you guys out. This is the most simple answer to your post.