How perform calculation on a XAML Binding value: reverse it, multiply it, subtract from it or add to it?

Question

First; the question is rhetorical, I have an answer! I have gotten so much help from looking here that I wanted to give this neat trick back.

Imagine that you have a value that you want to bind to, but it is somehow or somewhat wrong.

• I had a situation where I wanted to bind to a value, but when the value was 1, I needed 0, and vice-versa.
• There was a time when I wanted to bind the width of an element to the width of a parent - 68px.

Answer 1

Enter the FirstDegreeFunctionConverter:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Data;

namespace GenericWPF
{
    /// <summary>
    /// Will return a*value + b
    /// </summary>
    public class FirstDegreeFunctionConverter : IValueConverter
    {
        public double A { get; set; }
    public double B { get; set; }

    #region IValueConverter Members

    public object Convert( object value, Type targetType, object parameter, System.Globalization.CultureInfo culture )
    {
        double a = GetDoubleValue( parameter, A );

        double x = GetDoubleValue( value, 0.0 );

        return ( a * x ) + B;
    }

    public object ConvertBack( object value, Type targetType, object parameter, System.Globalization.CultureInfo culture )
    {
        double a = GetDoubleValue( parameter, A );

        double y = GetDoubleValue( value, 0.0 );

        return ( y - B ) / a;
    }

    #endregion


    private double GetDoubleValue( object parameter, double defaultValue )
    {
        double a;
        if( parameter != null )
            try
            {
                a = System.Convert.ToDouble( parameter );
            }
            catch
            {
                a = defaultValue;
            }
        else
            a = defaultValue;
        return a;
    }
}

How to use it?

You make a resource for each use in the resource section:

<GenericWPF:FirstDegreeFunctionConverter x:Key="ReverseOne"
                            A="-1"
                            B="1" />

<Border Opacity="{Binding Path=Opacity
    , ElementName=daOtherField
    , Converter={StaticResource ReverseOne}}" />

<GenericWPF:FirstDegreeFunctionConverter x:Key="ListboxItemWidthToErrorWidth"
     A="1"
     B="-68" />

<TextBox MaxWidth="{Binding Path=ActualWidth
   , Converter={StaticResource ListboxItemWidthToErrorWidth}
   , RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}" />

The name comes from the function y = a*x + b (Called a "first degree function" in Norwegian), and of course it would be possible to upgrade it to a second degree function y= a*x^2 + bx + c, but I haven't found a use for it yet.

I had a situation where I wanted to make columns based on width. Each time I got 200 pixels more width, I wanted the container to show me another column. At that time I hardcoded a converter, but I should have made a y=(a/x) + b converter instead.

Now, what should I have named this converter so that everybody understand what it is? Since I'm a Norwegian, I used the expression we learned in school, directly translated. Please, if you have a suggestion or an opinion, let me know. Any refinements or improvements you have thought of would also be appreciated...

---

Maybe "LinearTransformConverter" would better communicate what the converter does for you, I'll think about it. Any other proposals? Tor

Answer 2

What is even better is a PolynomialConverter, here's the one-way version:

public class PolynomialConverter : IValueConverter
{
    public DoubleCollection Coefficients { get; set; }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double x = (double)value;
        double output = 0;
        for (int i = Coefficients.Count - 1; i >= 0 ; i--)
            output += Coefficients[i] * Math.Pow(x, (Coefficients.Count - 1) - i);

        return output;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        //This one is a bit tricky, if anyone feels like implementing this...
        throw new NotSupportedException();
    }
}

Examples:

<!-- x^2 -->
<vc:PolynomialConverter Coefficients="1,0,0"/>
<!-- x + 5 -->
<vc:PolynomialConverter Coefficients="1,5"/>
<!-- 2x + 4 -->
<vc:PolynomialConverter Coefficients="2,4"/>

---

Alternatively one could use the ConverterParameter instead to not set the Coefficients in the converter itself.

DoubleCollection coefficients = DoubleCollection.Parse((string)parameter);
//...