<Window x:Class="WpfApplication.EmployeeDetailWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Карточка сотрудника" Height="249" Width="338" WindowStartupLocation="CenterOwner" ResizeMode="NoResize">
    <Window.Resources>
        <Style TargetType="TextBox">
            <Setter Property="Validation.ErrorTemplate">
                <Setter.Value>
                    <ControlTemplate>
                        <StackPanel Orientation="Horizontal">
                            <AdornedElementPlaceholder/>
                            <TextBlock Foreground="Red" Padding="1">*</TextBlock>
                        </StackPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    <Grid FocusManager.FocusedElement="{Binding ElementName=LastName}">

        <TextBox KeyboardNavigation.TabIndex="0" x:Name="LastName" Text="{Binding LastName, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Height="23" Margin="10,10,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="302"/>

        <TextBox KeyboardNavigation.TabIndex="1" x:Name="FirstName" Text="{Binding FirstName, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Height="23" Margin="10,38,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="302"/>

        <Button KeyboardNavigation.TabIndex="3" IsCancel="True" Content="Отмена" Margin="0,0,10,10" VerticalAlignment="Bottom" HorizontalAlignment="Right" Width="75" />

        <Button KeyboardNavigation.TabIndex="2" Command="{Binding SaveCommand}" CommandParameter="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}, Path=DataContext}" Content="Сохранить" HorizontalAlignment="Right" Margin="0,0,90,10" VerticalAlignment="Bottom" Width="75">
            <Button.Style>
                <Style TargetType="{x:Type Button}">
                    <Setter Property="IsEnabled" Value="False" />
                    <Style.Triggers>
                        <MultiDataTrigger>
                            <MultiDataTrigger.Conditions>
                                <Condition Binding="{Binding ElementName=LastName, Path=(Validation.HasError)}" Value="false" />
                                <Condition Binding="{Binding ElementName=FirstName, Path=(Validation.HasError)}" Value="false" />
                            </MultiDataTrigger.Conditions>
                            <Setter Property="IsEnabled" Value="True" />
                        </MultiDataTrigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
        </Button>
    </Grid>
</Window>

public class EmployeeViewModel : IDataErrorInfo
{
    private IDataContext Context;
    private IWindowService Service;

    public EmployeeViewModel()
    {
        Context = new DataContext(); // todo IoC resolve
        Service = new WindowService(); // todo IoC resolve
    }

    public int EmployeeId { get; set; }

    public string FirstName { get; set; }
        
    public string LastName { get; set; }

    public ICommand SaveCommand
    {
        get
        {
            return new RelayCommand((action) =>
            {
                if (Service.IsValid)
                {
                    var vm = action as EmployeeViewModel;
                    Context.UpdateEmployee(vm.EmployeeId, vm.FirstName, vm.LastName);
                    Service.CloseActiveWindow(true);
                }
            });
        }
    }

    public string Error
    {
        get { return null; }
    }

    public string this[string columnName]
    {
        get
        {
            if (columnName == "FirstName")
            {
                if (string.IsNullOrEmpty(this.FirstName))
                {
                    return "Значение не должно быть пустым";
                }
            }

            if ((columnName == "LastName"))
            {
                if (string.IsNullOrEmpty(this.LastName))
                {
                    return "Значение не должно быть пустым";
                }
            }

            return null;
        }
    }
}