IValueConverter.Convert(Object, TypeName, Object, String) Method

Reference

Definition

Namespace:: Microsoft.UI.Xaml.Data

Important

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Edit

Modifies the source data before passing it to the target for display in the UI.

public:
 Platform::Object ^ Convert(Platform::Object ^ value, TypeName targetType, Platform::Object ^ parameter, Platform::String ^ language);

IInspectable Convert(IInspectable const& value, TypeName const& targetType, IInspectable const& parameter, winrt::hstring const& language);

public object Convert(object value, System.Type targetType, object parameter, string language);

function convert(value, targetType, parameter, language)

Public Function Convert (value As Object, targetType As Type, parameter As Object, language As String) As Object

Parameters

value: Object
Platform::Object

IInspectable

The source data being passed to the target.

targetType: TypeName Type

The type of the target property, as a type reference (System.Type for Microsoft .NET, a TypeName helper struct for Visual C++ component extensions (C++/CX)).

parameter: Object
Platform::Object

IInspectable

An optional parameter to be used in the converter logic.

language: String
Platform::String

winrt::hstring

The language of the conversion.

Returns

Object

Platform::Object

IInspectable

The value to be passed to the target dependency property.

Examples

The following example shows how to implement the Convert method using the parameter and language parameters.

//
// MainPage.xaml.h
// Declaration of the MainPage class.
// 

#pragma once

#include "MainPage.g.h"

namespace IValueConverterExample
{

    // Simple business object.
    [Windows::UI::Xaml::Data::Bindable]
    public ref class Recording sealed 
    {
    public: 
        Recording (Platform::String^ artistName, Platform::String^ cdName, Windows::Foundation::DateTime release)
        {
            Artist = artistName;
            Name = cdName;
            ReleaseDate = release;
        }
        property Platform::String^ Artist;
        property Platform::String^ Name;
        property Windows::Foundation::DateTime ReleaseDate;
    };

    public ref class DateFormatter  sealed : Windows::UI::Xaml::Data::IValueConverter 
    {
        // This converts the DateTime object to the Platform::String^ to display.
    public:
        virtual Platform::Object^ Convert(Platform::Object^ value, Windows::UI::Xaml::Interop::TypeName targetType, 
            Platform::Object^ parameter, Platform::String^ language)
        {
            Windows::Foundation::DateTime dt = safe_cast<Windows::Foundation::DateTime>(value); 
            Windows::Globalization::DateTimeFormatting::DateTimeFormatter^ dtf =
                Windows::Globalization::DateTimeFormatting::DateTimeFormatter::ShortDate;
            return dtf->Format(dt); 
        }

        // No need to implement converting back on a one-way binding 
        virtual Platform::Object^ ConvertBack(Platform::Object^ value, Windows::UI::Xaml::Interop::TypeName targetType, 
            Platform::Object^ parameter, Platform::String^ language)
        {
            throw ref new Platform::NotImplementedException();
        }
    };

    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public ref class MainPage sealed
    {
    public:
        MainPage()
        {	
            m_myMusic = ref new Platform::Collections::Vector<Recording^>();

            // Add items to the collection.

            // You can use a Calendar object to create a Windows::Foundation::DateTime
            auto c = ref new Windows::Globalization::Calendar();
            c->Year = 2008;
            c->Month = 2;
            c->Day = 5;
            m_myMusic->Append(ref new Recording("Chris Sells", "Chris Sells Live",
                c->GetDateTime()));

            c->Year = 2007;
            c->Month = 4;
            c->Day = 3;
            m_myMusic->Append(ref new Recording("Luka Abrus",
                "The Road to Redmond", c->GetDateTime()));
            
            c->Year = 2007;
            c->Month = 2;
            c->Day = 3;
            m_myMusic->Append(ref new Recording("Jim Hance",
                "The Best of Jim Hance", dt));
            InitializeComponent();

            // Set the data context for the combo box.
            MusicCombo->DataContext = m_myMusic;	
        }


    protected:
        virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;

    private:
        Windows::Foundation::Collections::IVector<Recording^>^ m_myMusic;
    };
}

using System;
using System.Collections.ObjectModel;
using System.Globalization;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;

namespace ConverterParameterEx
{
    public partial class Page : UserControl
    {

        public ObservableCollection<Recording> MyMusic =
            new ObservableCollection<Recording>();
        public Page()
        {
            InitializeComponent();

            // Add items to the collection.
            MyMusic.Add(new Recording("Chris Sells", "Chris Sells Live",
                new DateTime(2008, 2, 5)));
            MyMusic.Add(new Recording("Luka Abrus",
                "The Road to Redmond", new DateTime(2007, 4, 3)));
            MyMusic.Add(new Recording("Jim Hance",
                "The Best of Jim Hance", new DateTime(2007, 2, 6)));

            // Set the data context for the combo box.
            MusicCombo.DataContext = MyMusic;
        }
    }

    // Simple business object.
    public class Recording
    {
        public Recording() { }
        public Recording(string artistName, string cdName, DateTime release)
        {
            Artist = artistName;
            Name = cdName;
            ReleaseDate = release;
        }
        public string Artist { get; set; }
        public string Name { get; set; }
        public DateTime ReleaseDate { get; set; }
    }

    public class DateFormatter : IValueConverter
    {
        // This converts the DateTime object to the string to display.
        public object Convert(object value, Type targetType, 
            object parameter, string language)
        {
            // Retrieve the format string and use it to format the value.
            string formatString = parameter as string;
            if (!string.IsNullOrEmpty(formatString))
            {
                return string.Format(
                    new CultureInfo(language), formatString, value);
            }
            // If the format string is null or empty, simply call ToString()
            // on the value.
            return value.ToString();
        }

        // No need to implement converting back on a one-way binding 
        public object ConvertBack(object value, Type targetType, 
            object parameter, string language)
        {
            throw new NotImplementedException();
        }
    }
}

Remarks

The targetType parameter of the Convert method uses different techniques of reporting the type system info, depending on whether you're programming with Microsoft .NET or Visual C++ component extensions (C++/CX).

For Microsoft .NET, this parameter passes an instance of the System.Type type.
For Visual C++ component extensions (C++/CX), this parameter passes a TypeName structure value. TypeName::Kind contains the simple string name of the type, similar to Microsoft .NET's Type.Name. When the converter is invoked by the binding engine, the targetType value is passed by looking up the property type of the target dependency property. You might use this value in your Convert implementation for one of two reasons:
Your converter has the expectation that it's always going to return objects of a specific type, and you want to verify that the binding that the converter is called for is using the converter correctly. If not, you might return a fallback value, or throw an exception (but see "Exceptions from converters" below).
Your converter can return more than one type, and you want the usage to inform your converter which type it should return. For example, you could implement an object-to-object conversion and an object-to-string conversion within the same converter code.

language comes from the ConverterLanguage value of a specific binding, not system values, so you should expect that it might be an empty string.

parameter comes from the ConverterParameter value of a specific binding, and is null by default. If your converter uses parameters to modify what it returns, this usually requires some convention for validating what is passed by the binding and handled by the converter. A common convention is to pass strings that name modes for your converter that result in different return values. For example you might have "Simple" and "Verbose" modes that return different length strings that are each appropriate for display in different UI control types and layouts.

Exceptions from converters

The data binding engine does not catch exceptions that are thrown by a user-supplied converter. Any exception that is thrown by the Convert method, or any uncaught exceptions that are thrown by methods that the Convert method calls, are treated as run-time errors. If you are using the converter in situations where the binding can use fallbacks or otherwise show reasonable results even if a conversion failure occurs, consider having your converter return DependencyProperty.UnsetValue and not throw exceptions. DependencyProperty.UnsetValue is a sentinel value that has special meaning in the dependency property system, and bindings that are passed this value will use FallbackValue.

Another alternative to throwing exceptions is to return the original value unchanged, and let the binding instance handle what it might do with that value. In most cases UI bindings that fail won't be error cases. They just won't use the source value and will instead use DependencyProperty.UnsetValue to show nothing, or use fallbacks.

try/catch based on doing something to value is a common implementation pattern for the Convert method, but you shouldn't rethrow, for the reasons mentioned above.

For an example that shows how to implement the Convert method using the parameter and language parameters, see the IValueConverter interface.

Applies to

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