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

Use this trick, it prevents they call OnPropertyChanged each other :

private bool RPCfromStartDate = false;
private bool RPCfromEndDate = false;

public string this[string columnName]
{
    get 
    {
                string result = null;
                switch (columnName)
                {
                    case "StartDate":
                        if (StartDate.Date >= EndDate.Date)
                        {
                            result = "Start Date cannot be later than End Date";
                        }
                        if (!RPCfromEndDate)
                        {
                            RPCfromStartDate = true;                            
                            OnPropertyChanged("EndDate");
                            RPCfromStartDate = false;
                        } 
                    case "EndDate":
                        if (StartDate.Date >= EndDate.Date)
                        {
                            result = "End Date cannot be earlier than Start Date";
                        }
                        if (!RPCfromStartDate)
                        {
                            RPCfromEndDate = true;                            
                            OnPropertyChanged("StartDate");
                            RPCfromEndDate = false;
                        } 
                        break;
                }
...