WPF Bind to a Method

In this part we will bind to a method. As we type a number into a textbox the results will be displayed in label below it. For this example we use a class which figures out the area of a circle.


Let's start off with a class which calculates the area of a circle. This class has one function named CalculateArea.

Public Class Area

Public Function CalculateArea(ByVal Radius As Double) As Double
Return Math.PI * Radius ^ 2
End Function
End Class


Now that we have a class set up to calculate the area we need 2 additonal classes. The first needs to inherit from ValidationRule to alert the user if they enter an incorrect value (textbox's border turns red for invalid values). The other need to inherit from IValueConveter to convert the user input from string to double and back again.


Imports System.Windows.Controls
Imports System.Windows.Data

Public Class ValidDouble
Inherits ValidationRule


Public Overrides Function Validate(ByVal value As Object, ByVal cultureInfo As System.Globalization.CultureInfo) As System.Windows.Controls.ValidationResult
Dim num As Double

If Double.TryParse(value.ToString, num) Then
Return New ValidationResult(True, Nothing)
Else
Return New ValidationResult(False, "Invalid Number")
End If
End Function
End Class


Public Class DoubleToString
Implements IValueConverter

Public Function Convert(ByVal value As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.Convert
If value IsNot Nothing Then
Return value.ToString
Else
Return Nothing
End If
End Function

Public Function ConvertBack(ByVal value As Object, ByVal targetType As System.Type, ByVal parameter As Object, ByVal culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.ConvertBack
Dim num As Double
If (value IsNot Nothing) AndAlso Double.TryParse(value.ToString, num) Then
Return num
Else
Return Nothing
End If
End Function
End Class

Now that we have the classes we need its time to register the namespaces with the form.


xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WPFBindToMethod"
xmlns:system="clr-namespace:System;assembly=mscorlib"
Title="WPFBindToMethod" Height="300" Width="300"
>
Finally we will use an ObjectDataProvider to bind to the method and register the ValidationRule with the form.



MethodName="CalculateArea" x:Key="CircleArea">

0







The Form's complete XAML



xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WPFBindToMethod"
xmlns:system="clr-namespace:System;assembly=mscorlib"
Title="WPFBindToMethod" Height="300" Width="300"
>

MethodName="CalculateArea" x:Key="CircleArea">

0















Radius of the Circle:


BindsDirectlyToSource="true" UpdateSourceTrigger="PropertyChanged"
Converter="{StaticResource doubleToString}">






Area of the Circle: