.NET Winforms Combobox databinding with ComboBoxStyle.DropDownList

Question

I'm running into a weird issue with databinding the Text property of a combobox to a string property of a custom object. I was able to successfully bind the data when the DropDownStyle was ComboBoxStyle.DropDown. However, I need a read only combobox so I needed to change the style to DropDownList. When I did this, the Text property no longer updates the custom object. (The setter for the property is never executed). I've checked the BindingComplete event args to see if there were any binding errors and the binding was successful.

I've also tried to bind to SelectedValue, SelectedItem and neither work.

Looking at the documentation 'Text' is listed as a BindableAttribute and the description of the DropDownList style mentions nothing of disabling data binding.

// This class has a 'StringValue' property for the current string representation of the value of the parameter
// It also has a method 'DisplayNames' which returns a list of the valid string values this parameter can have
// The class has implemented INotifyPropertyChanged
Parameter p = new Parameter();

ComboBox cobx = new ComboBox();
cobx.Name = p.Name + "_ComboBox";
cobx.Items.AddRange(ep.DisplayNames().ToArray());
cobx.Text = p.StringValue;
cobx.AutoSize = true;
cobx.DropDownStyle = ComboBoxStyle.DropDownList;

Binding b = new Binding("Text", ep, "StringValue", true, DataSourceUpdateMode.OnPropertyChanged);
b.BindingComplete += new BindingCompleteEventHandler(control_BindingComplete);
cobx.DataBindings.Add(b);

//cobx.DataBindings.Add("Text", ep, "StringValue", false, DataSourceUpdateMode.OnPropertyChanged);

void control_BindingComplete(object sender, BindingCompleteEventArgs e)
{
if (e.BindingCompleteState != BindingCompleteState.Success)
int break_pt_here = 0;
}

I'm running Windows 10
Project is being compiled against .NET 4.7.2
I'm using WinForms

Answer 1

Hi NeilHamiltonJr-4091,
When using the DropDownList style, you can only set "Text" to the value that actually appears in the list.
Here is my test code example:

private void Form1_Load(object sender, EventArgs e)  
{    
    MyData data = new MyData() { Name = "Red" };  
    this.bindingSource1.DataSource = data;  
    comboBox1.DataBindings.Add(new Binding("SelectedValue", this.bindingSource1, "MyEnum", true, DataSourceUpdateMode.OnPropertyChanged));  
    comboBox1.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;  
    comboBox1.FormattingEnabled = true;  
  
    comboBox1.Items.AddRange(new object[] { "", "Green", "Red", "Yellow", "Blue", "Gold", "Purple" });  
    comboBox1.Text = data.Name;  
    comboBox1.Location = new System.Drawing.Point(98, 87);  
    comboBox1.Name = "shiftColourComboBox";  
    comboBox1.Size = new System.Drawing.Size(121, 21);  
}  

The result:

Best Regards,
Daniel Zhang

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Answer 2

I eventually got it to work. My guess is that when the DropDownList style is selected the combo box drifts to more of the ListControl. Looking at that class I saw the DataSource, DisplayMember, ValueMember. I ended up changing the DisplayNames() method to return a List<EnumString>

class EnumString{ public string Value {get; set;}}

I passed that List to the DataSource with DisplayMember as 'Value' and ValueMember as 'Value'. SelectedValue was then populated on selection of a combobox item.

Then using this binding
cobx.Databindings.Add("SelectedValue", p, "StringValue", true, DataSourceUpdateMode.OnPropertyChanged);

Seems to be working. I feel like there is a better way. I'll have to refactor some classes with some of this in mind.

Anyway, thank you, Daniel.