WPF Per binding instance of a value converter?

How do I ensure that a new instan开发者_运维百科ce of a value converter is created for every binding that it is used in?

You need to specify x:Shared="False" on the converter resource. Here is an example:

<BooleanToVisibilityConverter x:Key="MyConverter" x:Shared="False"/>

You can make a separate resource in each control:

   <TextBox>
        <TextBox.Resources>
            <Converters:VisibilityConverter x:Key="conv"/>
        </TextBox.Resources>
        <TextBox.Text>
            <Binding Converter="{StaticResource conv}"/>
        </TextBox.Text>
    </TextBox>

Why? It should be deterministic, and unaware of any state except that which is passed in to it via its parameters. I have used value converters to keep a static list of images (a value is convertered to an image), and this works fine - even when the same converter is used across several columns in a datagrid with thousands of rows. (Note that the converter is still unaware of any external state).

In any case, see if this answers your question: Are value converters instantiated per-binding in WPF?

Inherit from MarkupExtension and then create your converter in the constructor.

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        _converter = new FlagsToBoolConverter();
        return _converter;
    }

Here's a full example. It's a flags converter that needed to be instanced to keep the source value for ConvertBack.

using System;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

namespace Something
{
    public class FlagsToBoolConverter : MarkupExtension, IValueConverter
    {
        private FlagsToBoolConverter _converter;

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            _converter = new FlagsToBoolConverter();
            return _converter;
        }

        private ulong _sourceValue;

        public virtual object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            try
            {
                var type = value.GetType();
                if (type.IsEnum)
                {
                    ulong mask = (ulong)System.Convert.ChangeType(Enum.Parse(type, (string)parameter), typeof(ulong));
                    _sourceValue = (ulong)System.Convert.ChangeType(Enum.Parse(type, value.ToString()), typeof(ulong));
                    return ((mask & _sourceValue) != 0);
                }
                return value;
            }
            catch (Exception ex)
            {
                Console.WriteLine("FlagsEnumValueConverter: Invalid Cast(to) Value={0} Type={1} Param={2} Exception{3}", value, targetType, parameter, ex);
            }
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            try
            {
                if (targetType.IsEnum)
                {
                    var original = this._sourceValue;
                    var originalEnum = Enum.Parse(targetType, original.ToString());
                    var maskEnum = Enum.Parse(targetType, (string)parameter);
                    var mask = (ulong)System.Convert.ChangeType(maskEnum, typeof(ulong));
                    _sourceValue ^= mask;
                    var sourceEnum = Enum.Parse(targetType, _sourceValue.ToString());
                    Console.WriteLine($"Modified Value: {original} ({originalEnum}) by Mask {mask} ({maskEnum}) Result = {_sourceValue} ({sourceEnum})");
                    return sourceEnum;
                }
                return value;
            }
            catch (Exception ex)
            {
                Console.WriteLine("FlagsEnumValueConverter: Invalid Cast(from) Value={0} Type={1} Param={2} Exception{3}", value, targetType, parameter, ex);
            }
            return value;
        }
    }
}

Your prefix needs to be declared xmlns:src="clr-namespace:Something". And then in your binding replace the normal converter with Converter={src:FlagsToBoolConverter}

  <CheckBox IsChecked="{Binding SomeFlagsEnum, ConverterParameter=FlagA, Converter={src:FlagsToBoolConverter}}">FlagA</CheckBox>
  <CheckBox IsChecked="{Binding SomeFlagsEnum, ConverterParameter=FlagB, Converter={src:FlagsToBoolConverter}}">FlagB</CheckBox>
  <CheckBox IsChecked="{Binding SomeFlagsEnum, ConverterParameter=FlagC, Converter={src:FlagsToBoolConverter}}">FlagC</CheckBox>

If you put your converters into resources and reference them through a {StaticResource ConverterName} lookup, then they are instanciated only once per instance of resource dictionary.

But a better approach is to inherit your converters from MarkupExtension and use them directly instead of adding them to the resources and reference them.

Here is an example of such a base class:

http://www.snippetsource.net/Snippet/18/base-class-for-valueconverters-in-wpf

Greetings Christian