如何创建从CommonDialog类派生的自定义用户控件

我想扩展FolderBrowserDialog，以便有选项包括所选文件夹的子文件夹(添加复选框设置为包含或不包含)。我发现我不能扩展基本的FolderBrowserDialog，因为它是一个密封类。

所以我认为最简单的解决方案是创建一个用户控件，这是来自CommonDialog(相同的类是FolderBrowserDialog)，从标准FolderBrowserDialog复制代码，只是改变它一点点，所以它也会有"包括子文件夹"复选框。

但是当我从默认FolderBrowserDialog复制代码时，它给了我一个错误:

missing partial modifier on declaration of type [my_class_name] another partial declaration of this type exists c#

指向"[my_class_name]. designer .cs"文件。

namespace my_custom_folder_open
{
    // Summary:
    //     Prompts the user to select a folder. This class cannot be inherited.
    [DefaultEvent("HelpRequest")]
    [DefaultProperty("SelectedPath")]
    [Designer("System.Windows.Forms.Design.FolderBrowserDialogDesigner, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
    public sealed class UserControl2 : CommonDialog
    {
        // Summary:
        //     Initializes a new instance of the System.Windows.Forms.FolderBrowserDialog
        //     class.
        public UserControl2();
        // Summary:
        //     Gets or sets the descriptive text displayed above the tree view control in
        //     the dialog box.
        //
        // Returns:
        //     The description to display. The default is an empty string ("").
        [Browsable(true)]
        [DefaultValue("")]
        [Localizable(true)]
        public string Description { get; set; }
        //
        // Summary:
        //     Gets or sets the root folder where the browsing starts from.
        //
        // Returns:
        //     One of the System.Environment.SpecialFolder values. The default is Desktop.
        //
        // Exceptions:
        //   System.ComponentModel.InvalidEnumArgumentException:
        //     The value assigned is not one of the System.Environment.SpecialFolder values.
        [Browsable(true)]
        [Localizable(false)]
        public Environment.SpecialFolder RootFolder { get; set; }
        //
        // Summary:
        //     Gets or sets the path selected by the user.
        //
        // Returns:
        //     The path of the folder first selected in the dialog box or the last folder
        //     selected by the user. The default is an empty string ("").
        [Browsable(true)]
        [DefaultValue("")]
        [Localizable(true)]
        public string SelectedPath { get; set; }
        //
        // Summary:
        //     Gets or sets a value indicating whether the New Folder button appears in
        //     the folder browser dialog box.
        //
        // Returns:
        //     true if the New Folder button is shown in the dialog box; otherwise, false.
        //     The default is true.
        [Browsable(true)]
        [DefaultValue(true)]
        [Localizable(false)]
        public bool ShowNewFolderButton { get; set; }
        // Summary:
        //     Occurs when the user clicks the Help button on the dialog box.
        [Browsable(false)]
        [EditorBrowsable(EditorBrowsableState.Never)]
        public event EventHandler HelpRequest;
        // Summary:
        //     Resets properties to their default values.
        public override void Reset();
        protected override bool RunDialog(IntPtr hWndOwner);
    }
}

哪里可能有问题?

顺便说一句，我已经创建了项目作为Windows窗体控制库..