WPF数据转换

在基本绑定中，信息从源到目标的传递过程中没有任何变化。这看起来是符合逻辑的，但我们并不总是希望出现这种行为。通常，数据源使用的是低级表达方式，我们可能不希望直接在用户界面使用这种低级表达方式。WPF提供了两个工具，来进行数据转换：

字符串格式化

通过设置 Binding.StringFormat 属性对文本形式的数据进行转换——例如包含日期和数字的字符串。

值转换器

该功能更强大，使用该功能可以将任意类型的源数据转换为任意类型的对象表示，然后可以传递到关联的控件。

使用StringFormat属性

可以看到后面两个StringFormat属性以花括号 {} 开头，完整值是 {}{0:C}，而不是 {0:C}，第一个则只有 {0:C}，这是因为在StringFormat 值以花括号开头时需要 {} 转义序列。

使用值转换器

为创建子转换器需要执行以下四个步骤：

1、创建一个实现IValueConverter接口的类

2、为该类声明添加ValueConversion特性，并指定目标数据类型

3、实现Convert()方法，该方法将数据从原来的格式转换为显示的格式

4、实现ConvertBack()方法，该方法执行反向变换，将值从显示格式转换为原格式

[ValueConversion(typeof(decimal), typeof(string))]
public class PriceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        decimal price = (decimal)value;
        return price.ToString("C", culture);
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        string price = (string)value;
        if (decimal.TryParse(price, System.Globalization.NumberStyles.Any, culture, out decimal result))
        {
            return result;
        }
        return value;
    }
}

要使用这个转换器，可以将其添加到页面的资源下，然后使用 Binding.Converter 指定。

多重绑定

可以将多个字段绑定到同一个输出控件，可以通过 StringFormat 或 MultiBinding.Converter 来格式化数据。多重绑定的值转换器需要实现的接口是 IMultiValueConverter，与 IValueConverter 接口比较类似，只是转换函数的第一个参数改成了数组形式。

public class MultiValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        decimal price = (decimal)values[0];
        int volume = (int)values[1];
        return (price * volume).ToString("C");
    }
    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

完整代码如下：

MainWindow.xaml

    
        
        
        
        
    
    
        
            
            
        
        
            
            
            
            
            
            
        

        
        

        ConvertWithConverter:
        

        ConvertDateTime:
        

        ConvertImagePath:
        

        
            
                
                    
                    
                    
                
            
        
        
            
                
                    
                    
                
            
        
    

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Globalization;
using System.IO;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace TestDataConverter;

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    protected virtual bool SetProperty(ref T member, T value, [CallerMemberName] string? propertyName = null)
    {
        if (EqualityComparer.Default.Equals(member, value))
        {
            return false;
        }
        member = value;
        OnPropertyChanged(propertyName);
        return true;
    }
}

public class Order : ViewModelBase
{
    public decimal price = 0;
    public decimal Price { get => price; set => SetProperty(ref price, value); }
    public int volume = 0;
    public int Volume { get => volume; set => SetProperty(ref volume, value); }

    public DateTime orderDate = DateTime.Now;
    public DateTime OrderDate { get => orderDate; set => SetProperty(ref orderDate, value); }

    public string image = string.Empty;
    public string Image { get => image; set => SetProperty(ref image, value); }
}
[ValueConversion(typeof(decimal), typeof(string))]
public class PriceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        decimal price = (decimal)value;
        return price.ToString("C", culture);
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        string price = (string)value;
        if (decimal.TryParse(price, System.Globalization.NumberStyles.Any, culture, out decimal result))
        {
            return result;
        }
        return value;
    }
}
[ValueConversion(typeof(decimal), typeof(Brush))]
public class PriceToBackgroundConverter : IValueConverter
{
    public decimal MinimumPriceToHighlight { get; set; }
    public Brush HighlightBrush { get; set; }
    public Brush DefaultBrush { get; set; }

    public object Convert(object value, Type targetType, object parameter,
      System.Globalization.CultureInfo culture)
    {
        decimal price = (decimal)value;
        if (price >= MinimumPriceToHighlight)
            return HighlightBrush;
        else
            return DefaultBrush;
    }
    public object ConvertBack(object value, Type targetType, object parameter,
      System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}
public class ImagePathConverter : IValueConverter
{
    private string imageDirectory = Directory.GetCurrentDirectory();
    public string ImageDirectory
    {
        get { return imageDirectory; }
        set { imageDirectory = value; }
    }
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        string imagePath = Path.Combine(ImageDirectory, (string)value);
        return new BitmapImage(new Uri(imagePath));
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException("The method or operation is not implemented.");
    }
}
public class MultiValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        decimal price = (decimal)values[0];
        int volume = (int)values[1];
        return (price * volume).ToString("C");
    }
    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        myGrid.DataContext = Order;

        Order.Price = 100;
        Order.Volume = 10;
        Order.OrderDate = DateTime.Now;
        Order.Image = "image1.gif";
    }
    public Order Order = new Order();

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Console.WriteLine("");
    }
}