Question

I created a Converter to convert from double to integer.

But the line “return (int)value;” always gets a “specified cast is not valid.”

What do I have to do so that my Converter successfully converts a double and sends back an integer?

Converter:

namespace TestChangeAngle
{
    [ValueConversion(typeof(double), typeof(int))]
    class DoubleToIntegerConverter : IValueConverter
    {
        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return (int)value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }

        #endregion
    }
}

XAML:

<Page x:Class="TestChangeAngle.Page1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:TestChangeAngle"
    Title="Page1">
    <Page.Resources>
        <local:DoubleToIntegerConverter x:Key="DoubleToIntegerConverter"/>
    </Page.Resources>

    <StackPanel HorizontalAlignment="Left" Margin="20">
        <Image Source="images\logo2.png" 
               RenderTransformOrigin="0.5, 0.5"
               Width="100" 
               Margin="10">
            <Image.RenderTransform>
                <RotateTransform Angle="{Binding ElementName=TheSlider, Path=Value}"/>
            </Image.RenderTransform>
        </Image>
        <Slider x:Name="TheSlider"
                Width="200" 
                Minimum="0"
                Maximum="360"
                HorizontalAlignment="Center"
                Margin="10" 
                Cursor="Hand"/>
        <TextBox x:Name="TheAngle"
                 Margin="10"
                 Width="100">
            <TextBox.Text>
                <Binding ElementName="TheSlider"
                         Path="Value"
                         UpdateSourceTrigger="PropertyChanged"
                         Converter="{StaticResource DoubleToIntegerConverter}"
                         Mode="TwoWay">
                    <Binding.ValidationRules>
                        <local:MinMaxValidationRule Minimum="0" Maximum="360"/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>

    </StackPanel>
</Page>

Answer 1

You are attempting to cast (not convert) from double to int, which won’t work. You need to do an implicit conversion or use Convert.ToInt32() — since the argument is actually of type object I think you’ll need the latter to keep the compiler happy. It’s up to you whether you want to include the culture’s format provider or not.

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    return Convert.ToInt32(value);
}

You can use the cast operator when the object’s have the same shape, that is, when one object is an instance of the type to which you are casting. For example, if class Foo extends class Bar, then you can cast an object of type Foo to type Bar – because a Foo object has all the methods and properties that a Bar object would. You couldn’t, however, cast an object of type Bar to type Foo because Foo changes (or can change, as far as the compiler is concerned) the shape of Bar, by adding methods or properties that a Bar object doesn’t have.

In your case you are dealing with primitive types which only share the object interface – there isn’t an inheritance relationship between them except that they both derive from object. There is, however, an implicit conversion between the two. You can assign an object one type to a variable of the other, though you may lose some precision in the value.

double x = 1.1;
int y = 0;

y = x;  // implicit conversion, this works, y will be 1
x = y;  // implicit conversion, this works, x will be 1.0

You can’t, however, cast an object of one type to the other. Casting implies that you will be using the object as if it were of the other type. In this case the shapes differ and it can’t be done.