Проверка данных в MVVM

Вот простой пример, в котором используется IDataErrorInfo. Это должно помочь вам начать работу.

XAML:

<Grid>
    <Grid.Resources>
        <ControlTemplate x:Key="LeftErrorTemplate">
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding AdornedElement.(Validation.Errors).[0].ErrorContent, ElementName=ErrorAdorner}" Background="Red" Foreground="White" FontWeight="Bold" VerticalAlignment="Center"/>
                <AdornedElementPlaceholder x:Name="ErrorAdorner">
                    <Border BorderBrush="Red" BorderThickness="1" />
                </AdornedElementPlaceholder>
            </StackPanel>
        </ControlTemplate>
    </Grid.Resources>
    <TextBlock Height="23" HorizontalAlignment="Left" Margin="158,66,0,0" Name="textBlock1" Text="Name" VerticalAlignment="Top" Width="44" />
    <TextBox Height="23" HorizontalAlignment="Left" Margin="217,65,0,0" Name="textBox1" VerticalAlignment="Top" Width="120" 
             Text="{Binding Name, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" 
             Validation.ErrorTemplate="{StaticResource LeftErrorTemplate}"/>
</Grid>

Код позади:

using System;
using System.Windows;
using System.ComponentModel;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var vm = new ViewModel();

            this.DataContext = vm;
        }
    }

    public class ViewModel : ObservableBase, IDataErrorInfo
    {
        private string _Name;

        public string Name
        {
            get { return _Name; }
            set
            {
                _Name = value;
                OnPropertyChanged("Name");
            }
        }

        public string Error
        {
            get { throw new NotImplementedException(); }
        }

        public string this[string columnName]
        {
            get
            {
                string errorMessage = string.Empty;

                switch (columnName)
                {
                    case "Name":
                        if (string.IsNullOrEmpty(Name))
                            errorMessage = "Enter name";
                        else if (Name.Trim() == string.Empty)
                            errorMessage = "Enter valid name";
                        break;
                }
                return errorMessage;
            }
        }

    }

    public class ObservableBase : INotifyPropertyChanged
    {

        public event PropertyChangedEventHandler PropertyChanged;

        public void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Привет, вам нужно будет создать какой-то метод, который проверяет аннотации, такие как ValidateMethod в моем коде

public class ViewModel:INotifyPropertyChanged,IDataErrorInfo
    {
        int _password;

        [Required(ErrorMessage = "Field 'Range' is required.")]
        [Range(1, 10, ErrorMessage = "Field 'Range' is out of range.")]
        public int Password
        {
            get
            {
                return _password;
            }
            set
            {
                if (_password != value)
                {
                    _password = value;
                    Validate("Password", value);
                    Notify("Password");
                }
            }
        }

        private void Validate(string propertyName, object value)
        {
            if (string.IsNullOrEmpty(propertyName))
                throw new ArgumentNullException("propertyName");

            string error = string.Empty;

            var results = new List<System.ComponentModel.DataAnnotations.ValidationResult>(2);

            bool result = Validator.TryValidateProperty(
                value,
                new ValidationContext(this, null, null)
                {
                    MemberName = propertyName
                },
                results);

            if (!result && (value == null || ((value is int || value is long) && (int)value == 0) || (value is decimal && (decimal)value == 0)))
                return;

            if (!result)
            {
                System.ComponentModel.DataAnnotations.ValidationResult validationResult = results.First();
                if (!errorMessages.ContainsKey(propertyName))
                    errorMessages.Add(propertyName, validationResult.ErrorMessage);
            }

            else if (errorMessages.ContainsKey(propertyName))
                errorMessages.Remove(propertyName);
        }

        #region INotifyPropertyChanged 

        public void Notify(string propName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propName));

        }
        public event PropertyChangedEventHandler PropertyChanged;

        #endregion

        #region IDataErrorInfo

        public string Error
        {
            get { throw new NotImplementedException(); }
        }

        private Dictionary<string, string> errorMessages = new Dictionary<string, string>();

        public string this[string columnName]
        {
            get 
            { 
                if(errorMessages.ContainsKey(columnName))
                    return errorMessages[columnName];
                return null;

            }
        }

        #endregion
    }

> xaml

<TextBox Text="{Binding Password, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" Height="70" Width="200" />

xaml.cs

public MainWindow()
    {
        InitializeComponent();
        DataContext = new ViewModel();
    }

Вам необходимо вызвать метод Validate из установщика свойства, которое применяет DataAnnotations, и обязательно вызвать его перед уведомлением PropertyChanged.