Hi Marco,

You didn't really answer my questions so I'm still not sure what kind of
setup you got.  In any case, I have included a demonstration sample with
databinding against a business object (Person) and an ErrorProvider.  I have
not used the validating event, but person.ValidateName can be used for
validation checks before saving/closing.  Create a blank form called Form1
and replace the code with the following:

   public partial class Form1 : Form
   {
       private ErrorProvider errorProvider = new ErrorProvider();
       private BindingList<Person> list = new BindingList<Person>();
       private TextBox textBox1 = new TextBox();
       private ListBox listBox1 = new ListBox();

       public Form1()
       {
           InitializeComponent();
       }

       protected override void OnLoad(EventArgs e)
       {
           // Create some dummy data to display
           list.Add(new Person("Alfa"));
           list.Add(new Person("Beta"));
           list.Add(new Person("Gamma"));
           list.Add(new Person("All"));

           // Databindings
           listBox1.DataSource = list;
           listBox1.DisplayMember = "Name";
           textBox1.DataBindings.Add("Text", list, "Name");

           // We'll update the errorprovider when the text is changed
           textBox1.TextChanged += new EventHandler(textBox1_TextChanged);

           // Add the controls
           Controls.Add(textBox1);
           listBox1.Location = new Point(0, 30);
           Controls.Add(listBox1);
       }

       void textBox1_TextChanged(object sender, EventArgs e)
       {
           // Get the selected person and display any error text it contains
           Person person = listBox1.SelectedItem as Person;
           if(person != null)
               errorProvider.SetError(sender as Control, person.NameError);
       }

       // Our business object
       class Person
       {
           private string _name;
           private string _nameError;

           public string Name
           {
               get { return _name; }
               set
               {
                   if (_name != value)
                   {
                       _name = value;
                       // Any changes to the Name property
                       // will call the ValidateName method
                       ValidateName();
                   }
               }
           }

           // This property contains any error text for this business object
           public string NameError
           {
               get { return _nameError; }
               set
               {
                   if (_nameError != value)
                   {
                       _nameError = value;
                   }
               }
           }

           public Person(string name)
           {
               this.Name = name;
           }

           // This method has a double function
           // When called it will update NameError with the proper error text
           // but it will also let the caller know if the business object
is valid
           // 'if(person.ValidateName()){ SavePerson(person); }'
           public bool ValidateName()
           {
               if (!Name.StartsWith("A"))
               {
                   NameError = "Name must start with A";
                   return false;
               }
               else
               {
                   NameError = "";
                   return true;
               }
           }
       }
   }