How to Create a Multi-Select Dropdown List with Checkboxes in WinForms

When building Windows Forms applications, you may need a dropdown list control that allows users to select multiple items simultaneously using checkboxes, rather than limiting selection to a single item like a standard ComboBox.

Solution
You can implement multi-item selection with checkboxes in Windows Forms by using the C1MultiSelect control (available in the C1.Win.Input assembly).

Implementation Steps:

1. Add the Control: Place a C1MultiSelect control on your Form from the C1.Win.Input assembly.

2. Bind or Populate Data: Bind a data collection (such as a List<T> or BindingSource) to the control's data source. The control automatically renders checkboxes for each dropdown item and displays selected items as tags or text in the main edit area.

List<string> emails;

public NewMessageForm()
{
    InitializeComponent();

    // Populate data source
    emails = new List<string>
    {
        "abc@gmail.com",
        "test1@gmail.com",
        "test2@gmail.com",
        "xyz@gmail.com"
    };

    // Bind data source to control
    this.bindingSource1.DataSource = emails;
    this.c1MultiSelect1.DataSource = this.bindingSource1;
}