Mvvm validation - model o viewmodel?

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C#

WPF

validation

MVVM

Hello,
i'm trying to implement the validation in MVVM WPF C# application.

In the MVVM Pattern, is it better to implement the validation (with IDataErrorInfo) in the Model o in the ViewModel?

What I have tried:

At the moment i've implement the validation inside the Model part.


ViewModel:

private Order selectedOrder;
public Order SelectedOrder
{
    get
    {
        return selectedOrder;
    }
    set
    {
        selectedOrder= value;
        OnPropertyChanged("SelectedOrder");
    }
}

as part of the model i have the definition of the class Order:

public class Order : Notifier, IDataErrorInfo
{        
    private string _NumOrder = "";
    public string NumOrder 
    {
        get
        {
              return _NumOrder;
        }
        set
        {
              _NumOrder = value;
              OnPropertyChanged("NumOrder");
        }
    }

        #region IDataErrorInfo
        string IDataErrorInfo.Error
        {
            get
            {
                return null;
            }
        }
        string IDataErrorInfo.this[string propertyName]
        {
            get
            {
                return GetValidationError(propertyName);
            }
        }

        #endregion

        #region Validation

        static readonly string[] ValidateProperty =
        {
            "NumOrder"
        };

        public bool IsValid
        {
            get
            {
                foreach(string property in ValidateProperty)                
                    if (GetValidationError(property) != null)
                        return false;
                return true;
            }
        }

        public string ValidateOrder()
        {
            if (String.IsNullOrWhiteSpace(numOrder))
            {
                return "Error, i need a value";
            }
            return null;
        }

        string GetValidationError(String propertyName)
        {
            string error = null;

            switch (propertyName)
            {
                case "NumOrder":
                    error = ValidateOrder();
                    break;
            }

            return error;
        }
        #endregion

}

In the View i'm trying to display the error, but it doesn't work.

<TextBox x:Name="txtNumOrder" FontSize="14" IsEnabled="{Binding ActionInRun,ConverterParameter=NumOrder, Converter={StaticResource EnumToBoolean}}" Text="{Binding SelectedOrder.numOrder, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}"/>

<Label FontSize="12" x:Name="lblNote" Content="{Binding ElementName=numOrder, Path=(Validation.Errors).CurrentItem.ErrorContent}" />

Because in the textbox field the validation works correctly, but the label don't show the error message.
How can i display the error correctly?
Thanks in advance