I have a date picker in my view:

<DatePicker MinimumDate="{Binding Today}" Date="{Binding SelectedDate, Mode=TwoWay}"/>

And in my viewModel class i have these two properties:

private DateTime selectedDate;
    public DateTime SelectedDate
    {
        get { return selectedDate; }
        set
        {
            selectedDate = value;             
            OnPropertyChanged(nameof(SelectedDate));
            //ExecuteFunction();
        }
    }

    private DateTime today;
    public DateTime Today
    {
        get { return today; }
        set
        {
            today = value;
            OnPropertyChanged(nameof(Today));
            ExecuteFunction();
        }
    }

I want to execute a function when my SelectedDate changes (when the user chooses the date) but i get this error:

However, i can execute the same function when its only in the "Today" property (i did that for testing). Anyone know why this happens?

Thanks!

CodePudding user response：

This only happens when the page loads for the first time with this code the function is not be called when it loads for the first time.

private bool First { get; set; } = true;

    private DateTime selectedDate;
    public DateTime SelectedDate
    {
        get { return selectedDate; }
        set
        {
            selectedDate= value;             
            OnPropertyChanged(nameof(SelectedDate));

            if (!First)
            {
                ExecuteFunction();
            }
            else
            {
                First = false;
            }
        }
    }

CodePudding user response：

Now that you have isolated the problem, a better fix is to change ExecuteFunction so it copes with a bad date. OR don't call ExecuteFunction when the date is bad.

Several possible ways to fix:

1. Try-catch inside ExecuteFunction.

2. set a breakpoint, find out what value that bad date is. Add a test to skip ExecuteFunction when that value is seen.

3. Since it happens during InitializeComponent, that means xaml is setting it - almost certainly setting it to the value it already has - so you could change SelectedDate setter to:

set
{
    if (selectedDate != value)
    {
        selectedDate = value;             
        OnPropertyChanged(nameof(SelectedDate));
        ExecuteFunction();
    }
}