The interface IDataErrorInfo does not work for the second window

Question

I use the interface IDataErrorInfo to visualize errors in View. It works in MainWindow but not in AddContactView, although the code is the same.
The code is in the attached text fileApp-Code

Answer 1

Hi Sarah,
in window "AddContactView" you use data object "Contact" (Type ContactModel).

    <Grid Margin="0,10,0,0">
        <StackPanel DataContext="{Binding Contact}">

            <StackPanel Orientation="Vertical" VerticalAlignment="Center" HorizontalAlignment="Center" >
                <Label Content="Firstname:"  FontWeight="SemiBold"/>
                <TextBox Height="25" Width="290" MaxLength="91" Margin="0,0,5,0" Text="{Binding Firstname, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding ErrorCollection[Firstname]}" HorizontalContentAlignment="left" VerticalContentAlignment="Center"/>
...

Type "ContactModel" doesn't implement Interface IDataErrorInfo

public class ContactModel
{
    public int Id { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
}

Please, use my early posted code with IDataErrorInfo in ContactModel.