如何使用mvvm在wpf中为依赖对象编写自定义设置程序

Question

how to write a custom setter for a dependency object in wpf using mvvm ?

In my ViewModel I have a dependency object called Seasonalprop which I use to bind to a TextBox in XAML. I would like to write a custom setter, so that it notifies the user when the provided string input cannot be converted to double. The error that I am getting is that value is a string and cannot be converted to double.

public double Seasonalprop
        {
            get { return (double)GetValue(SeasonalProperty); }
            set
            {
                try
                  {
                          Double.TryParse(value, out parsedouble);
                          SetValue(SeasonalProperty, value);

                  }
                  catch(Exception ex)
                  {
                          MessageBox.Show(" String Input cannot be converted to 
                          type double");
                  }

            }
        }

Answer 1

I think you want to write a custom settor so that it notifies the user if the value in text box is invalid.

Have a look at the docs for validation in WPF

The text content of the TextBox in the following example is bound to the Age property (of type int) of a binding source object named ods. The binding is set up to use a validation rule named AgeRangeRule so that if the user enters non-numeric characters or a value that is smaller than 21 or greater than 130, a red exclamation mark appears next to the text box and a tool tip with the error message appears when the user moves the mouse over the text box.

Answer 2

I think your whole concept was gone to a wrong direction. First of all, when a binding expression updates a dependency property, it will call its onwer's SetValue method but not its clr property wrapper. So a custom setter would do nothing for you in this situation. And as @peeyushsingh's answer, wpf has binding validation for this. So, something you need should be like:

        <TextBox Text="{Binding Seasonalprop, ValidatesOnExceptions=True}">
            <Validation.ErrorTemplate>
                <ControlTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Border BorderThickness="1" BorderBrush="Red" >
                            <AdornedElementPlaceholder/>
                        </Border>
                        <TextBlock Foreground="Red" Margin="2" Name="cc"
                                   Text="!  Not a double."/>
                    </StackPanel>
                </ControlTemplate>
            </Validation.ErrorTemplate>
        </TextBox>