I'm trying to teach myself reflection and have been googling but I can't wrap my head around it entirely. I created a class called DataClass which contains a method called GetClassFromDB as you can see below, which will be inherited from multiple classes.
What I am attempting to do is have my dataclass read the TableName property that is defined within objResults. Once I pull in the tablename from objResults I would query the SQL database for a dataset. Once I have the dataset I would create a new object of the same TYPE inheriting this class (Which will be different types) and populate it from the dataset. Once I have the newly populated class I will return it for use.
I believe I have gotten most of the way there properly (Please correct me if there is a better way), but my real question is this. How can I create a new class of the type thats deriving that class from that string name that I getting in my code, or the type. I would want to have all the accessible properties from objResults available.
Namespace MyApp
Public Class DataClass
Private _TableName As String
Private _Name As String
Overridable ReadOnly Property TableName As String
Get
Return _TableName
End Get
End Property
Public Overloads Function GetClassFromDB() As Object
Try
Dim BaseObject As New Object
'Get the object name
Dim objName As String = MyBase.GetType().Name
'Gets the type thats calling this method
Dim objDerived As Type = MyBase.GetType()
'Get the property info to request the tablename from the derived class
Dim TableName As PropertyInfo = objDerived.GetProperty("TableName")
Dim TableNameString As String = TableName.GetValue(Me, Nothing).ToString
'Once I get the table name from objResults I can perform the SQL
Dim QueryResults as DataSet = SQLiteCLass.Query("Select * FROM TableNameString")
'Once I get data from the SQL I want to create a new object of the type deriving this method.
'In this example is objResults
Dim NewObject as objDerived
'Now I can fill my new object with the results and return it as an object
'THIS IS MY QUESTION - How can I create a new object of the TYPE that I receive from Reflection
Return False
Catch ex As Exception
Return False
End Try
End Function
End Class
End Namespace
and this is a sample class that would inherit my dataclass.
Public Class objResults
Inherits MyApp.DataClass
Private _GameID As Guid
Public Property GameID As Guid
Get
Return _GameID
End Get
Set(ByVal value As Guid)
_GameID = value
End Set
End Property
Public Overrides ReadOnly Property TableName As String
Get
Return "This is my tablename"
End Get
End Property
End Class
and this is how I would use this in code.
Dim objResult as New objResults
Dim TodaysResult as objResultsCollection
TodaysResult = objResult.GetClassFromDB()
Related
I have this class:
Public Class clsServCasam
Public ID As Long
Public CANT As Decimal
Public PRICE As Decimal
End Class
I create a variable of that type and get the object from an API result:
Dim myObj As clsServCasam()
Dim rsp As HttpWebResponse = CType(rq.GetResponse(), HttpWebResponse)
If rsp.StatusCode = HttpStatusCode.OK Then
Using sr = New StreamReader(rsp.GetResponseStream())
myObj = JsonConvert.DeserializeObject(Of clsServCasam())(sr.ReadToEnd())
End Using
Then I try to get the field names from the object:
For Each p As System.Reflection.PropertyInfo In myObj.GetType().GetProperties()
Debug.Print(p.Name, p.GetValue(myObj, Nothing))
Next
But, instead of class fields (ID, PRICE, ...) I got:
- Length
- LongLength
- Rank
Update
As Steven Doggart pointed out, the above loop won't work because it looks for properties rather than fields. So, I tried changing the loop to this:
For Each p As FieldInfo In myObj.GetType.GetFields()
Debug.Print(p.Name)
Next
But now I'm getting no results at all.
In your code, myObj is not declared as clsServCasam. Rather, it is declared as clsServCasam(), which means it's an array of clsServCasam objects. So, when you use reflection to iterate over its properties, you're getting the properties of the array rather than the actual clsServCasam type.
For instance, this would work more like you're expecting:
For Each item As clsServCasam in myObj
For Each p As PropertyInfo In item.GetType().GetProperties()
Debug.Print(p.Name, p.GetValue(item, Nothing))
Next
Next
However, I think you'll find that that still won't work because it iterates over the properties rather than the fields. In the definition of the clsServCasam class, all of the members are fields rather than properties, so the only properties that it have would be ones that are inherited from Object. You will need to either iterate over the fields using GetFields, like this:
For Each item As clsServCasam in myObj
For Each f As FieldInfo In item.GetType().GetFields()
Debug.Print(f.Name, f.GetValue(item))
Next
Next
Or you'll need to change them to properties:
Public Class clsServCasam
Public Property ID As Long
Public Property CANT As Decimal
Public Property PRICE As Decimal
End Class
Or, if you are using an older version of the VB compiler which doesn't support auto-properties:
Public Class clsServCasam
Public Property ID As Long
Get
Return _id
End Get
Set(value As Long)
_id = value
End Set
End Property
Public Property CANT As Decimal
Get
Return _cant
End Get
Set(value As Decimal)
_cant = value
End Set
End Property
Public Property PRICE As Decimal
Get
Return _price
End Get
Set(value As Decimal)
_price = value
End Set
End Property
Private _id As Long
Private _cant As Decimal
Private _price As Decimal
End Class
I have a situation where I'm taking advantage of events raised by a class that inherits Bindinglist(Of t). I'm trying to figure out if there is a way to create and return a new item (Of t) when the list is empty or that specific Key string is not found when using the "FIND" function shown below? My code looks as follows:
BINDINGLIST(OF T) CLASS:
Imports System.ComponentModel
Public Class mylist(Of t)
Inherits BindingList(Of t)
'find an item by keystring
Public Function Find(ByVal KeyString) As t
Dim myItem As t
'Check if any objects of t exist in the list
If Me.Items.Count > 0 Then
Dim properties As PropertyDescriptorCollection = _
TypeDescriptor.GetProperties(Me.Items(0).GetType)
'find the key property
Dim myProperty As PropertyDescriptor = properties.Find("Key", False)
'search for string match
For Each myItem In Me.Items
If myProperty.GetValue(myItem) = KeyString Then
Return myItem
Exit Function
End If
Next
'THIS IS WHERE I NEED HELP:
'need to assign default property values to myitem of t and return
End If
'THIS IS WHERE I NEED HELP:
'need to assign default property values to myitem of t and return
End Function
End Class
You would need to use a constraint on your generic type parameter to require that it has a parameterless constructor.
Public Class MyList(Of T As New)
You can then create an instance of that type by invoking that constructor as you would for any other type, e.g.
Return New T
Note that your class will then only be able to store items of a type that has a parameterless constructor. If that's a limitation that you can't accept then you will simply not be able to create an instance of T because there is no single way to create an instance of an arbitrary type.
I am trying to convert
Public Class TestClass
Public FirstName As String
End Class
To
Public Class AnotherClass
Public Property FirstName As String
End Class
I wrote a function that will convert the member of one class to member of another class, so if I pass some class type that has Public Property LastName AS String it will convert it to (for instance) AnotherClass Type variable and I will be able to get the value so I am happy here.
Public Shared Function ConvertModelToValidationDataModel(Of T)(ByVal oSourceObject As Object) As T
Dim oSourceObjectType As Type
Dim oSourceObjectProperties() As PropertyInfo
Dim oDestinationObjectProperties() As PropertyInfo
Dim oDestinationObject As Object
Dim oDestinationObjectType As Type
oDestinationObject = Activator.CreateInstance(Of T)()
oDestinationObjectType = GetType(T)
oDestinationObjectProperties = oDestinationObjectType.GetProperties
oSourceObjectType = oSourceObject.GetType()
oSourceObjectProperties = oSourceObjectType.GetProperties()
If Not oSourceObjectProperties Is Nothing Then
If oSourceObjectProperties.Count > 0 Then
For Each oDestinationObjectPropertyInfo As PropertyInfo In oDestinationObjectProperties
For Each oSourceObjectPropertyInfo As PropertyInfo In oSourceObjectProperties
If oDestinationObjectPropertyInfo.Name = oSourceObjectPropertyInfo.Name Then
oDestinationObjectPropertyInfo.SetValue(oDestinationObject, oSourceObjectPropertyInfo.GetValue(oSourceObject, Nothing))
End If
Next
Next
End If
End If
Return oDestinationObject
End Function
The problem is I want to pass TestClass (the variable FirstName is not a property but I want it to be converted to a property variable) and be able to convert it and get the value but for some reason it does not pass the value and obviously it looks like the function converts it to a non-property variable of another class - not the property variable like I want it to.
**
Short version:
**
When I pass in a class type that has property variables (Public Property FirstName As String) - I get back a class of another type, all the values are passed and converted to property variables.
When I pass in class type that has variables (Public FirstName As String) I am not able to get the value and it does not convert it to property variable.
Question: Why I am not able to get the value and convert it to a property variable when passing in a class type that has a non-property variable?
Solution
Thanks the guys in the comment section below for helping me visualize the fact that I was asking an object for the properties while object only had fields.
Here is the Updated Version of the function for those who interested
Public Shared Function ConvertModelToValidationDataModel(Of T)(ByVal oSourceObject As Object) As T
Dim oSourceObjectType As Type
Dim oSourceObjectFields() As FieldInfo
Dim oDestinationObjectProperties() As PropertyInfo
Dim oDestinationObject As Object
Dim oDestinationObjectType As Type
oSourceObjectType = oSourceObject.GetType()
oSourceObjectFields = oSourceObjectType.GetFields()
oDestinationObject = Activator.CreateInstance(Of T)()
oDestinationObjectType = GetType(T)
oDestinationObjectProperties = oDestinationObjectType.GetProperties
If Not oSourceObjectFields Is Nothing Then
If oSourceObjectFields.Count > 0 Then
For Each oSourceObjectFieldInfo As FieldInfo In oSourceObjectFields
For Each oDestinationObjectPropertyInfo As PropertyInfo In oDestinationObjectProperties
If oSourceObjectFieldInfo.Name = oDestinationObjectPropertyInfo.Name Then
oDestinationObjectPropertyInfo.SetValue(oDestinationObject, oSourceObjectFieldInfo.GetValue(oSourceObject))
End If
Next
Next
End If
End If
Return oDestinationObject
End Function
Dim objects As New List(Of Object)
With New Object
.prop1 = "Property 1"
.prop2 = "Property 2"
objects.add(.instance) 'i mean instance of New Object
End With
is it possible.
I ask new question because last question has mislead information and I don't give right answer. so here code.
No it is not possible. The With statement basically creates an implicit variable. All you can do with that variable is access members and there is no member that returns a reference to the object itself.
If you want succinct code to create, populate and add an object to a list then do this:
myList.Add(New SomeType With {.SomeProperty = someValue,
.SomeOtherProperty = someOtherValue})
Interestingly, you can make it work the way you wanted if you create your own extension method. I was under the impression that you could not extend the Object class but either I was wrong or that has changed because I just tried in VB 2013 and it worked. You can write a method like this:
Imports System.Runtime.CompilerServices
Public Module ObjectExtensions
<Extension>
Public Function Self(Of T)(source As T) As T
Return source
End Function
End Module
and then do something like this:
With New SomeType
.SomeProperty = someValue
.SomeOtherProperty = someOtherValue
myList.Add(.Self())
End With
I'm not sure that that really provides any benefit though, given the availability of the object initialiser syntax that I demonstrated first.
Hmmm... I just realised that that's not actually extending the Object class. It was my original intention to try to do so but then I realised that a generic method was better because it would then return the same type as you call it on. I did just test it with a non-generic method extending type Object and it did still worked though.
You should to create your own class By example :
Public Class Car
Private _NumberCar As Integer
Public Property NumberCar() As Integer
Get
Return _NumberCar
End Get
Set(ByVal value As Integer)
_NumberCar = value
End Set
End Property
Private _ColorCar As Color
Public Property ColorCar() As Color
Get
Return _ColorCar
End Get
Set(ByVal value As Color)
_ColorCar = value
End Set
End Property
Private _OwnerName As String
Public Property OwnerName() As String
Get
Return _OwnerName
End Get
Set(ByVal value As String)
_OwnerName = value
End Set
End Property
End Class
and in the Class where you want to add the cars object do this :
Dim CarList As New List(Of Car)
Dim item As New Car
With item
.NumberCar = 1243
.ColorCar = Color.Red
.OwnerName = "Ibra"
End With
CarList.Add(item)
strong text
I am trying to add rows to a public class that has public fields and am getting an error: Object reference not set to an instance of an object
Public Class EmailRecipient
EmailAddress As String = ""
FullName As String = ""
End Class
Public Class EmailDetails
Public FromEmail As String = ""
Public ToEmails As List(Of Emails) = nothing
End Class
Public Sub SetEmailDetails
'Populate EmailRecipient Class
Dim er As New EmailRecipient
er.EmailAddress = "rodney#norespect.com"
er.FullName = "Rodney Dangerfield"
'Populate EmailDetails Class
Dim ed As New EmailDetails
ed.FromEmail = "sender#danger.com" 'This works fine
ed.ToEmails.Add(er) 'Here error happens
End Sub
I'm guessing I need to create an instance of the EmailRecipient class before I can add an item to it.
Not sure how to do that with a Public Field in a Public Class??
It's been a rough day. I got up this morning, put a shirt on and a button fell off. I picked up my briefcase and the handle came off. I'm afraid to go to the bathroom.
Thanks for the help :-)
I'm guessing I need to create an instance of the EmailRecipient class
You already have an instance of the EmailRecipient class. That's your er variable. You actually have two errors here. First, you explicitly set ToEmails to Nothing:
Public ToEmails As List(Of Emails) = nothing
This means that your ToEmails variable is a Null Reference. It doesn't have an actual object yet.
The second issue is that you shouldn't get that excpetion, because this shouldn't even compile. You define ToEmails as a List(Of Emails), but tried to add an object of type "EmailRecepient" to it. That should be a compiler error. If it's not, you need to turn Option Strict or Option Infer back on.
So what you really need is an instance of a the List(Of EmailRecipient) type. Fix the bad line of code like this:
Public ToEmails As New List(Of EmailRecipient)