设置枚举 ComboBox 的起始值

Question

been searching for a solution for this problem, found plenty solutions, but nothing changed my code's behaviour. This is in WinForms.

I am loading a form with a ComboBox, that contains the values and names of this enum, that is inside a class named "Node".

class Node
{
    public enum NodeType { Yield, Home, Parking, Light, None, Inbound, Outbound }
    public NodeType Type;
}

Then in my form, I have a ComboBox named "Type", which is set up like so (from the constructor):

Node node = new Node();
node.Type = Node.NodeType.Home;

Type = new ComboBox();
Type.Location = new Point(77, 41);
Type.Size = new Size(121, 24);
Type.DropDownStyle = ComboBoxStyle.DropDownList;
Type.DisplayMember = "Name";
Type.ValueMember = "Value";
Type.DataSource = Enum.GetValues(typeof(Node.NodeType));
Type.SelectedValue = node.Type;
Controls.Add(Type);

When the program runs, the list shows all the names, and on closing the form I am able to retrieve the selected value via. Type.SelectedValue. My problem is that the ComboBox doesn't start at the value that the Node is already set at. Essentially the line

Type.SelectedValue = node.Type;

doesn't do anything. I've tried using SelectedItem which didn't change anything, and

Type.SelectedIndex = (int)node.Type;

Which caused an ArgumentOutOfRangeException.

So, my question is: how do I set the start value of the ComboBox?

Answer 1

There are several mistakes in that code.

First, enum does not have Name and Value properties (in fact it does not have any property), so DisplayMember and ValueMember cannot be used and should be left blank (default). Which in turn means SelectedValue cannot be used and you need to use SelectedItem instead.

Second, you are using list data bound mode for the list portion of your ComboBox by setting DataSource property instead of populating Items , which is fine, but data binding occurs later in the process, so inside the constructor the Items property is empty and SelectedItem has no effect. In order to fix that, you need to move the data initialization part to your form Load event.

So, in your form constructor you'll have this:

Type = new ComboBox();
Type.Location = new Point(77, 41);
Type.Size = new Size(121, 24);
Type.DropDownStyle = ComboBoxStyle.DropDownList;
Controls.Add(Type);

and in your form Load event - this:

Node node = new Node();
node.Type = Node.NodeType.Home;

Type.DataSource = Enum.GetValues(typeof(Node.NodeType));
Type.SelectedItem = node.Type;

Answer 2

Type.SelectedIndex = 枚举中的索引，例如枚举是 {"apple", "pear", "pineapple"}，并且您想要默认为 pear，所以 Type.SelectedIndex = 1