Forcing html Input element to Numeric Only Input

While developing a shopping cart function for a client, I needed to restrict the user to entering numbers only in an Input field

I came across this handy post;

http://www.itjungles.com/javascript/how-to-use-javascript-to-force-numeric-value-in-textbox

Basically, we need to add the following function to the JavaScript section;

function isNumericKey(evt)

{

    var charCode = (evt.which) ? evt.which : event.keyCode

    if (charCode > 31 && (charCode < 48 || charCode > 57)) 

    {

        return false;

    }

    else

    {

        return true;

    }

}    

We then add the the following to your input element;

onkeypress="return isNumericKey(event)"

Hey Presto! You have a numeric only Input Element!

MVVM WPF Validating multiple items together

In an application I am developing, I required that two checkboxes be validated together, when either one of them were clicked. I also needed to only show the Validation error once in my Error Message Box.

To achieve this we first call the OnPropertyChanged routine, which in turn raises the PropertyChanged event, for each linked Property when any of the linked properties change.

For example;

Public Property Property1 As Boolean

    Get

        Return _Property1

    End Get

    Set(value As Boolean)

        If _Property1 <> value Then

            _Property1 = value

            OnPropertyChanged("Property1")

            OnPropertyChanged("Property2")

        End If

    End Set

End Property

For the Validation errors, I have employed a simple Key, Value Pair list to store all of the Validation Error Results. Thus, I simply check to see if the other Property has Registered an Error, and remove it if so;

        ''' <summary>

        ''' Validates the Current Item

        ''' </summary>

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

        ''' <returns></returns>

        ''' <remarks></remarks>

        Public Overrides Function ValidateItemValue(ColumnName As String) As String

            Dim strResult As String = ""

            Select Case ColumnName

                Case "Property1"

                    If Property1 = False And Property2 = False Then

                        strResult = "Please select either Property1, Propert2 or Both!"

                        If Me.lstErrors.ContainsKey("Property2") = True Then

                            Me.lstErrors.Remove("Property2")

                        End If

                    End If

                Case "Property2"

                    If Property1 = False And Property2 = False Then

                        strResult = "Please select either Property1, Property2 or Both!"

                        If Me.lstErrors.ContainsKey("Property1") = True Then

                            Me.lstErrors.Remove("Property1")

                        End If

                    End If

            End Select

            SortErrorList(ColumnName, strResult)        ' Add or Remove an Error Item

            Return strResult

        End Function

Instantiating a Generic Type of T

I found a requirement recently where I needed to Instantiate a new instance of a Generic Type.

Ordinarily we would declare a new instance of a class using a Generic Type using something like;

Public Class ViewModel_Class(Of T)

However, If we then try to create a new Object of Type T by performing the following…

Dim MyNewObject As New T

…an error will be produced, reporting “New cannot be used on a type parameter that does not have a new constraint”.

This error is telling us that the Compiler doesn’t know if the Generic Type has an implicit “New” Sub, which is can use to Instantiate a new Object.

So, we need to add a Generic Type Constraint for a New Sub by specifying this when we declare our Generic Type;

Public Class ViewModel_Class(Of T as {New})

This tells the compiler that whatever Type T is, it will have a New Sub which it can use to instantiate the New Object.

You can read more on this subject on the Microsoft Website at;

http://msdn.microsoft.com/en-us/library/w256ka79(v=vs.110).aspx