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 = ax + b (Called a "first degree function" in Norwegian), and of course it would be possible to upgrade it to a second degree function y= ax^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