Property set when leave page

I have a CheckBox in a TabControl.
I bound the property Checked like this :

MyCheckBox.DataBindings.Add("Checked", MyBindingSource, "IsOn", true, DataSourceUpdateMode.OnPropertyChanged);

Property isOn :

public bool IsOn
{
    get
    {
        return _isOn;
    }
    set
    {
        bool someCondition;
        // a test is done
        if (value != _isOn && someCondition)
        {
            _isOn = value;
        }
        else
        {
            System.Windows.Forms.MessageBox.Show("Condition not OK");
        }
    }
}

When user click on the CheckBox, when someCondition is false, a message box is displayed and the CheckBox does not change.

My problem is when user changes page in TabControl, the message is displayed. I saw with debugger that when page changes in TabControl, property is set to the last value tried.

What can I do to not display a MessageBox when changing page?