How can I validate multiple properties when any of them changes?

Question

I have two date fields: StartDate and EndDate. StartDate must be earlier than EndDate.

If the user changes the StartDate to something greater than the EndDate, a red

Answer 1

The simplest way is to raise a PropertyChanged notification for in the setter for both properties that need to be validated like bathineni suggests

private DateTime StartDate
{
    get { return _startDate; }
    set
    {
        if (_startDate != value)
        {
            _startDate = value;
            RaisePropertyChanged("StartDate");
            RaisePropertyChanged("EndDate");
        }
    }
}

private DateTime EndDate
{
    get { return _endDate; }
    set
    {
        if (_endDate!= value)
        {
            _endDate= value;
            RaisePropertyChanged("StartDate");
            RaisePropertyChanged("EndDate");
        }
    }
}

However if that doesn't work for you, I figured out one way to validate a group of properties together, although your classes have to implement INotifyPropertyChanging in addition to INotifyPropertyChanged (I'm using EntityFramework and by default their classes implement both interfaces)

Extension Method

public static class ValidationGroup
{
    public delegate string ValidationDelegate(string propertyName);
    public delegate void PropertyChangedDelegate(string propertyName);

    public static void AddValidationGroup(this T obj, 
        List validationGroup, bool validationFlag,
        ValidationDelegate validationDelegate, 
        PropertyChangedDelegate propertyChangedDelegate)
        where T : INotifyPropertyChanged, INotifyPropertyChanging
    {

        // This delegate runs before a PropertyChanged event. If the property
        // being changed exists within the Validation Group, check for validation
        // errors on the other fields in the group. If there is an error with one
        // of them, set a flag to true.
        obj.PropertyChanging += delegate(object sender, PropertyChangingEventArgs e)
        {
            if (validationGroup.Contains(e.PropertyName))
            {
                foreach(var property in validationGroup)
                {
                    if (validationDelegate(property) != null)
                    {
                        validationFlag = true;
                        break;
                    }
                }
            }
        };

        // After the Property gets changed, if another field in this group was
        // invalid prior to the change, then raise the PropertyChanged event for 
        // all other fields in the Validation Group to update them.
        // Also turn flag off so it doesn't get stuck in an infinite loop
        obj.PropertyChanged += delegate(object sender, PropertyChangedEventArgs e)
        {
            if (validationGroup.Contains(e.PropertyName))
            {
                if (validationFlag && validationDelegate(e.PropertyName) == null)
                {
                    validationFlag = false;
                    foreach(var property in validationGroup)
                    {
                        propertyChangedDelegate(property);
                    }
                }
            }
        };
    }
}

To use it, add the following call to the constructor of any class that should validate a group of properties together.

this.AddValidationGroup(
    new List { "StartDate", "EndDate" },
    GetValidationError, OnPropertyChanged);

I've tested this with up to 3 properties in a Validation Group and it seems to work OK.