Validating Combobox.SelectedItem without the use of Binding
===

This question is based on [this solution][1], but I somehow cannot get this to work using a ComboBox.  I would like to validate that upon clicking a submit button, the combobox has a selected item and is not null.  

**Please note that I am not binding to anything on purpose, and not because I don't know how to.  But if the answer is that there is no way I can use the validation rules without binding (to ComboBoxes specifically, I've done it to textboxes via the linked solution), please let me know.  I can't accept answers with Binding Solutions in it.**

Taking the solution from the link, here is how I use validation without binding in a textbox:

    <TextBox.Text>
        <Binding RelativeSource="{RelativeSource Self}" Path="Text" UpdateSourceTrigger="LostFocus">       
            <Binding.ValidationRules>
                <Filters:IntegersOnlyValidator/>
            </Binding.ValidationRules>    
        </Binding>  
    </TextBox.Text> 


  [1]: http://stackoverflow.com/questions/6236047/validationrules-without-binding


Here is what I have so far:

XAML:

    <ComboBox DataContext="{StaticResource ChargeAssigneeViewSource}" Name="ChargeAssigneeBox" ItemsSource="{Binding}" Width="85">
        <ComboBox.SelectedItem>
            <Binding RelativeSource="{RelativeSource Self}" Path="GetType" Mode="TwoWay">
                <Binding.ValidationRules>
                    <my:ComboBoxValidationRule ErrorMessage="Please select an Assignee" />
                </Binding.ValidationRules>
            </Binding>
        </ComboBox.SelectedItem>
    </ComboBox>

Validation Rule:
    
    class ComboBoxValidationRule : ValidationRule
    {
        private string errorMessage;

        public string ErrorMessage
        {
            get { return errorMessage; }
            set { errorMessage = value; }
        }
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            if (value == null)

                return new ValidationResult(false, ErrorMessage);

            return new ValidationResult(true, null);

        }
    }

Button Click:

    private void AddAnHours()
        {
            Employee currEmployee;

            if (!ValidateElement.HasError(ChargeAssigneeBox))
            {
                if (!ValidateElement.HasError(analystTimeTxtBox))
                {
                    currEmployee = ChargeAssigneeBox.SelectedItem as Employee;
                    item.AddTime(currEmployee, DateTime.Now, double.Parse(analystTimeTxtBox.Text));
                    analystTimeTxtBox.Clear();
                    ChargeAssigneeBox.SelectedItem = null;
                }
            }
  
            UpdateTotals();

        }

The error that I get is in this line:

    currEmployee = ChargeAssigneeBox.SelectedItem as Employee;

but my ItemsSource is binding properly so even though I have selected an item, the selecteditem is not converting it to an employee object.  I suspect it has something to do with what I am binding it to:
    
    <Binding RelativeSoruce="{RelativeSource Self}" Path="GetType"....>

Help would be greatly appreciated, thanks.
                    