尝试从ListBox键值对中读取“值”时出现无效的强制转换错误

Question

I'm using Visual Studio 2010 express and working on a C# WinForms application.

My form has a ListBox object ( listData ) which has its DataSource set to use a Key-Value paired Dictionary object ( dictionary ).

This is the Dictionary and how it is assigned as a DataSource to listData -

Dictionary<string, uint> dictionary = new Dictionary<string, uint>();`
listData.DataSource = new BindingSource(dictionary, null);
listData.DisplayMember = "Key";
listData.ValueMember = "Value";

And when debugging I see that the "Value" is being assigned correctly and is clearly a number. Yet when I try to accept the same "Value" into a uint lastSelectedIndex , I get this cast error-

What am I doing wrong here?

---

This actually worked for me:

lastSelectedIndex = ((KeyValuePair<string, uint>)listData.SelectedItem).Value;

Answer 1

You should change this line.

listData.DataSource = new BindingSource(dictionary, null);

to

listData.DataSource = dictionary;

The BindingSource constructor requires two parameters. first one is for data source and second one for DataMember (ValueMember as we can say). You have specified null value into second parameter that's why BindingSource has taken whole KeyValuePair object as a DataMember.

I don't think that you need to create object of BindingSource class to bind the Dictionary class. but, if you still want to use then you should also specify the DataMember in second parameter.

listData.DataSource = new BindingSource(dictionary, "Value");

But, I don't know whether it will work or not. i haven't tried like this before.

The another approach is to convert the SelectedValue into KeyValuePair object and get the Value from it.

uint lastSelectedIndex = ((KeyValuePair<string, uint>)listData.SelectedValue).Value

You are trying to cast KeyValuePair object into uint. So, it cannot be convertable. You have to convert it into KeyValuePair type first then get the value from Value property of that object.

I would like to suggest you to create another class where the class is having two properties

public class MyDataClass
{
    public uint Value{ get; set;}
    public string Display{get;set;}
    public MyDataClass(string display, uint val)
    {
        Display = display;
        Value = val;
    }
    public override string ToString()
    {
         return this.Display;
    }
}

The create a List<MyDataClass> object and fill the data into it.

Now you can assig that List object directly into that List control.

List<MyDataClass> lstItems = new List<MyDataClass>();
lstItems.Add(new MyDataClass("Item 1", 1));
lstItems.Add(new MyDataClass("Item 2", 2));
lstItems.Add(new MyDataClass("Item 3", 3));


listData.DataSource = lstItems;
listData.DisplayMember = "Display";
listData.ValueMember = "Value";

Answer 2

The reason for this issue is the order you have used to assign the DataSource and the ListBox' ValueMember property. If you assign the DataSource as last step it works:

Dictionary<string, uint> dictionary = new Dictionary<string, uint>();
dictionary.Add("1", 1);
dictionary.Add("2", 2);
dictionary.Add("3", 3);

listData.DisplayMember = "Key";
listData.ValueMember = "Value";
var bs = new BindingSource();
bs.DataSource = dictionary;
listData.DataSource = bs;  // as last step

The ListBox' SelectedIndexChanged event will be triggered as soon as you assign the DataSource. Since you haven't specified the ValueMember at that time you get the InvalidCastException .