完整的异常消息：

System.InvalidCastException
  HResult=0x80004002
  Message=Unable to cast object of type 'System.String' to type 'MyListBoxItem'.
  Source=Image Crop
  StackTrace:
   at Image_Crop.Form1.listBox1_SelectedIndexChanged(Object sender, EventArgs e) in D:\Csharp Projects\Image Crop\Form1.cs:line 387
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Windows.Forms.ListBox.OnSelectedIndexChanged(EventArgs e)
   at System.Windows.Forms.ListBox.set_SelectedIndex(Int32 value)
   at System.Windows.Forms.ListBox.RefreshItems()
   at System.Windows.Forms.ListBox.OnDisplayMemberChanged(EventArgs e)
   at System.Windows.Forms.ListControl.SetDataConnection(Object newDataSource, BindingMemberInfo newDisplayMember, Boolean force)
   at System.Windows.Forms.ListControl.set_DisplayMember(String value)

字符串形式的项目内容屏幕截图：
exception message
这是一个项目对象的屏幕截图。我怎样才能访问消息？
item object
代码：

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            var item = ((ListBox)sender).SelectedItem;
            MyListBoxItem mm = (MyListBoxItem)item;
            var val = mm.Message;
            int index = val.LastIndexOf(",");
            string you = val.Substring(index + 1);
            if (File.Exists(you))
            {
                pictureBox1.Image = System.Drawing.Image.FromFile(you);
            }
        }

异常消息错误位于以下行：

MyListBoxItem mm = (MyListBoxItem)item;

我试着把它转换成字符串，但没有用。
这是表单1中的MyListBoxItem类：

public class MyListBoxItem
    {
        public Color ItemColor { get; set; }
        public string Message { get; set; }
    }