[RESOLVED] Nullable Property question

[Nitesh]

Hi All,

I have Nullable properties in my class:

Public Property FileStatusID() As Integer?
        Get
            Return m_FileStatusID
        End Get
        Set(ByVal value As Integer?)
            m_FileStatusID = value
        End Set
    End Property

I assumed I would be able to store null values in the property. But when I try to assign a value from a datatable row like this:

File.FileStatusID = filetable.Rows(0)("FileStatusID")

I get an Invalid Cast error when a null value is encountered. Do I have to do an ISDBNull check?? what's the best way?

[NickThissen]

A database null value is 'DBNull.Value', which is not equal to a VB null value, which is 'Nothing'.
You'd have to check if your database returns DBNull.Value, and in that case assign Nothing. I usually use something like this:

Private Function ToInt(ByVal obj As Object) As Integer?
   If obj Is Nothing OrElse obj Is DBNull.Value Then Return Nothing
   Return CInt(obj)
End Function

[Nitesh]

thanks NickThissen will try it out now

[jmcilhinney]

You can store Nothing is a nullable value type but database nulls are not Nothing. They are instances of the DBNull class, so they are definitely something. NT's code will work but, assuming .NET 3.5 or later, it's much simpler than that. In fact, you already know how to do it because I showed you in your last thread. I guess it didn't sink in.

File.FileStatusID = filetable.Rows(0).Field(Of Integer?)("FileStatusID")

[Nitesh]

thanks JM. it has sunk in now

One more question. say my property has a value of nothing.

If I try to assign the property value to a textbox I get an error saying:

Nullable object must have a value.

what's the correct way to handle this. I've seen some solutions but I'm not so happy with them

[jmcilhinney]

If you want a String then you just call ToString. If there's a value then you'll get that value converted to a String and if there isn't a value then you'll get an empty String. If you actually want to use the value if there is one then you can use the HasValue and Value properties, e.g.

Dim myInteger As Integer
 
If myNullableInteger.HasValue Then
    myInteger = myNullableInteger.Value
Else
    myInteger = 0
End If

There are situations where you would need to use HasValue and Value but, if you want to do what I have above, then it can be done more simply like so:

Dim myInteger As Integer = myNullableInteger.GetValueOrDefault()

With no argument, GetValueOrDefault will return the default value for the type if HasValue is False. If you want to specify a different default value, pass that to the method as an argument.

[Nitesh]

thanks again JM. works great

[Nitesh]

hey guys,

JUst posting this here to help someone else incase they ever come across it. I always used to assign date.MinValue to my Date Variables if the db value was null.

Now with nullable types it just added a new dimension in handling null values. thanks to JM i've gained even more knowledge and I am so happy that I'm using the best possible methods.

So the thing with nullable datetime properties is that you can't format them using the ToString method Like I used to:

Datevar.ToString("yyyy/MM/dd")

so after some research I stumbled upon Extension methods. What you have to do is:

Since I'm using asp.net: Add a class to your project. Replace the word class with Module.

add the Extension Attribute to your functions and subs:

Imports Microsoft.VisualBasic
Imports System.Runtime.CompilerServices
Public Module Extensions
    <Extension()> _
Public Function ToString(ByVal dateTime As System.Nullable(Of DateTime), ByVal format As String) As String

        Return dateTime.ToString(format, "")

    End Function

    <Extension()> _
    Public Function ToString(ByVal dateTime As System.Nullable(Of DateTime), ByVal format As String, ByVal returnIfNull As String) As String
        If dateTime.HasValue Then
            Return dateTime.Value.ToString(format)
        Else
            Return returnIfNull
        End If

    End Function

    <Extension()> _
    Public Function ToShortDateString(ByVal dateTime As System.Nullable(Of DateTime)) As String

        Return dateTime.ToShortDateString("")

    End Function
    <Extension()> _
    Public Function ToShortDateString(ByVal dateTime As System.Nullable(Of DateTime), ByVal returnIfNull As String) As String

        If dateTime.HasValue Then
            Return dateTime.Value.ToShortDateString()
        Else

            Return returnIfNull
        End If

    End Function

End Module

Then Just Add an Import.Extensions to your page. With this Extension method defined above, you will now be able to call

.ToShortDateString(string.empty)

directly on your Nullable DateTime types and it will either return the actual DateTime formatted according to the format string, or it will return a blank string.

example usage: I have a nullable DateTime Property ReOpenedDate in my File Class. To assign it's value to a textbox I do this:

Me.txtReopenedDate.Text = File.ReOpenedDate.ToShortDateString(String.Empty)

so much more elegant than checking for Date.Minvalue