WPF MultiValueConverter problem

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C#

WPF

C#4.0

.NET4

MVVM

Hi,

I have encountered a major problem with MultiBinding as Button.CommandParameter. In my application I have four textBoxes which contents should be used as a parameter for the Button.Command. For this purpose I've done something like this inn the XAML file:

<Button Grid.Row="0" Grid.Column="2" x:Name="btnUpdate" HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="5,40,5,0" Content="Update product" Command="{Binding Path=Update_Command}">
    <Button.CommandParameter>
        <MultiBinding Converter="{StaticResource ResourceKey=PVMObj}">
            <Binding ElementName="txtID" Path="Text" /> <!-- TextBox with the ID -->
            <Binding ElementName="txtName" Path="Text" /> <!-- TextBox with the Name -->
            <Binding ElementName="txtPrice" Path="Text" /> <!-- TextBox with the Price -->
            <Binding ElementName="txtReseller" Path="Text" /> <!-- TextBox with the reseller name -->
        </MultiBinding>
    <Button.CommandParameter>
</Button>

As you can see I have the necessary MultiValueConverter Object (PVMObj) bound to MultiBinding. And the PVM's implementation looks like this:

class PVM : IMultiValueConverter
{
    #region IMultiValueConverter Members
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if(values != null)
        {
            return new Product(ToInt32(values[0]), values[1].ToString(), ToDouble(values[2]), values[3].ToString());
            // Exception text is not in correct format!
        }
        return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
    #endregion
}

For debugging purposes I've modified the PMV as follows:

class PVM : IMultiValueConverter
{
    #region IMultiValueConverter Members
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if(values != null && values[3].ToString().Length > 0)
        {
            string s = values[0]+"-"+values[1]+"-"+values[2]+"-"+values[3];
            System.Windows.MessageBox.Show(s);

            // Designer Crashes with Unreported Exception in the following line.
            return new Product(s);
            // Application works but the teamLead says the designer should work I must resolve this issue...
        }
        return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
    #endregion
}

In this case the following scenario occurs: First a Messagebox appears with the following contents:
"1- - - "
Then I close this MessageBox and another one pops up:
"1-Sample- - "
then again
"1-Sample-19.95- "
And for last time:
"1-Sample-19.95-MediaTower"

How does this IMultiValueConverter works? From many examples as I saw people are simply using the object[] values array, yet I can not. Any thoughts what I am doing wrong?