public class TestTextBox : TextBox
    {
        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register("Value", typeof(int?), typeof(TestTextBox),
                new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, ValueChangedCallback));

        public TestTextBox()
        {
            this.LostFocus += (sender, args) =>
            {
                int value;
                if (string.IsNullOrWhiteSpace(this.Text)) this.Value = null;
                else if (int.TryParse(this.Text, out value)) this.Value = value;
                else this.Value = null;
            };
        }

        private static void ValueChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var control = d as TestTextBox;

            if (control == null) return;

            control.Text = string.Format("{0:F0}", control.Value);
        }

        public int? Value
        {
            get { return (int)this.GetValue(ValueProperty); }
            set { this.SetValue(ValueProperty, value); }
        }
    }

    public class TestViewModel : INotifyPropertyChanged, IDataErrorInfo
    {
        private int? _testValue;

        public int? TestValue
        {
            get { return this._testValue; }
            set
            {
                if (this._testValue != value)
                {
                    this._testValue = value;
                    this.RaisePropertyChanged(nameof(this.TestValue));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void RaisePropertyChanged(string propertyName)
        {
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public IDictionary<string, string> ValidationErrors { get; } = new Dictionary<string, string>();

        public string this[string columnName]
        {
            get
            {
                this.ValidationErrors.Remove(columnName);

                string errorText = null;

                switch (columnName)
                {
                    case nameof(this.TestValue):
                        errorText = "TESTFEHLER";
                        break;
                }

                if (!string.IsNullOrWhiteSpace(errorText)) this.ValidationErrors.Add(columnName, errorText);

                return errorText;
            }
        }

        public string Error { get; } = null;
    }