WPF Restricting Text Entry to Numeric Characters, the MVVM Way

I needed to Limit several TextBoxes in my WPF MVVM application to only allow Numeric Characters.

I stumbles upon a very useful post by Jermey Likeness which described a method using Attached Behaviours, which worked well for me;

http://csharperimage.jeremylikness.com/2009/10/silverlight-behaviors-and-triggers_07.html

I modified this code slightly to allow for Decimal point control also.

The VB.net Code is as follows;

Imports System.Collections.ObjectModel

Namespace Behaviours

    Public Class clsTextBoxFilters

#Region "Variables"

        Private Shared ReadOnly _controlKeys As List(Of Key) = New List(Of Key) From {Key.Back,

                                                                                           Key.CapsLock,

                                                                                            Key.LeftCtrl,

                                                                                            Key.RightCtrl,

                                                                                            Key.Down,

                                                                                            Key.[End],

                                                                                            Key.Enter,

                                                                                            Key.Escape,

                                                                                            Key.Home,

                                                                                            Key.Insert,

                                                                                            Key.Left,

                                                                                            Key.PageDown,

                                                                                            Key.PageUp,

                                                                                            Key.Right,

                                                                                            Key.LeftShift,

                                                                                            Key.RightShift,

                                                                                            Key.Tab,

                                                                                            Key.Up,

                                                                                            Key.Decimal,

                                                                                            Key.OemPeriod}

#End Region

#Region "Helper Functions"

        ''' <summary>

        ''' Check if the key is a Numeric

        ''' </summary>

        ''' <param name="key__1"></param>

        ''' <returns></returns>

        ''' <remarks></remarks>

        Private Shared Function _IsDigit(key__1 As Key) As Boolean

            Dim shiftKey As Boolean = (Keyboard.Modifiers And ModifierKeys.Shift) <> 0

            Dim retVal As Boolean

            If key__1 >= Key.D0 AndAlso key__1 <= Key.D9 AndAlso Not shiftKey Then

                retVal = True

            Else

                retVal = key__1 >= Key.NumPad0 AndAlso key__1 <= Key.NumPad9

            End If

            Return retVal

        End Function

#End Region

#Region "Getters and Setters"

        ''' <summary>

        ''' Get the value of the Numeric Filter Dependancy Property

        ''' </summary>

        ''' <param name="src"></param>

        ''' <returns></returns>

        ''' <remarks></remarks>

        Public Shared Function GetIsPositiveNumericFilter(src As DependencyObject) As Boolean

            Return CBool(src.GetValue(IsPositiveNumericFilterProperty))

        End Function

        ''' <summary>

        ''' Set the Value of the Numeric Filter Dependancy Property

        ''' </summary>

        ''' <param name="src"></param>

        ''' <param name="value"></param>

        ''' <remarks></remarks>

        Public Shared Sub SetIsPositiveNumericFilter(src As DependencyObject, value As Boolean)

            src.SetValue(IsPositiveNumericFilterProperty, value)

        End Sub

        ''' <summary>

        ''' Get the Value of the IsIntegerOnly Dependancy Property

        ''' </summary>

        ''' <param name="src"></param>

        ''' <returns></returns>

        ''' <remarks></remarks>

        Public Shared Function GetIsIntegerOnly(src As DependencyObject) As Boolean

            Return CBool(src.GetValue(IsIntegerOnlyProperty))

        End Function

        ''' <summary>

        ''' Sets the Value of the IsIntegerOnly Dependancy Property

        ''' </summary>

        ''' <param name="src"></param>

        ''' <param name="value"></param>

        ''' <remarks></remarks>

        Public Shared Sub SetIsIntegerOnly(src As DependencyObject, value As Boolean)

            src.SetValue(IsIntegerOnlyProperty, value)

        End Sub

#End Region

#Region "Dependancy Properties"

        Public Shared IsPositiveNumericFilterProperty As DependencyProperty = DependencyProperty.RegisterAttached("IsPositiveNumericFilter", GetType(Boolean), GetType(clsTextBoxFilters), New PropertyMetadata(False, AddressOf IsPositiveNumericFilterChanged))

        Public Shared IsIntegerOnlyProperty As DependencyProperty = DependencyProperty.Register("IsIntegerOnly", GetType(Boolean), GetType(clsTextBoxFilters), Nothing)

#End Region

#Region "Dependancy Property Change Handlers"

        ''' <summary>

        ''' Determines if the Behaviour is active, adds the KeyDown event Handler

        ''' </summary>

        ''' <param name="src"></param>

        ''' <param name="args"></param>

        ''' <remarks></remarks>

        Public Shared Sub IsPositiveNumericFilterChanged(src As DependencyObject, args As DependencyPropertyChangedEventArgs)

            If src IsNot Nothing AndAlso TypeOf src Is TextBox Then

                Dim textBox As TextBox = TryCast(src, TextBox)

                If CBool(args.NewValue) Then

                    AddHandler textBox.KeyDown, AddressOf _TextBoxPositiveNumericKeyDown

                End If

            End If

        End Sub

#End Region

#Region "Event Handlers"

        ''' <summary>

        ''' KeyDown event... Check if the Key is valid... If so allow, if not then ignore the key

        ''' </summary>

        ''' <param name="sender"></param>

        ''' <param name="e"></param>

        ''' <remarks></remarks>

        Private Shared Sub _TextBoxPositiveNumericKeyDown(sender As Object, e As KeyEventArgs)

            Dim handled As Boolean = True

            Dim txtSender As TextBox = DirectCast(sender, TextBox)

            If e.Key = Key.OemPeriod Then                   'Is the current key a Decimal Point?

                If txtSender.GetValue(IsIntegerOnlyProperty) = True Then                    'Is this an Integer Only TextBox?

                    handled = True

                ElseIf InStr(txtSender.Text, ".") = 0 And Len(txtSender.Text) > 0 Then      'We only want 1 decimal point, and it can't be the first digit!

                    handled = False

                Else                                                                        'Otherwise ignore the keypress

                    handled = True

                End If

            ElseIf _controlKeys.Contains(e.Key) OrElse _IsDigit(e.Key) Then                 'Is the Key a numeric or an allowed key?

                handled = False

            End If

            e.Handled = handled

        End Sub

#End Region

    End Class

End Namespace

The XAML to add to the UserControl Header Namespaces is;

xmlns:Behaviors="clr-namespace:CriticalPath_WPF_MVVM_EF.Behaviours"

The XAML to add to the TextBox Is;

Behaviors:clsTextBoxFilters.IsPositiveNumericFilter="True" Behaviors:clsTextBoxFilters.IsIntegerOnly="True"

Very handy!