I'm trying to use a generic list without knowing the type when loading the page. I have a typePropertyCollection which inherits from List(Of PropertyData). The usercontrol that uses this collection doesn't know what type of data is used (which objects). So when the page is loaded, I pass along the type to the usercontrol using a dependencyproperty. This type ends up in this method:
Private Shared Sub OnObjectTypeChanged(ByVal obj As DependencyObject, ByVal args As DependencyPropertyChangedEventArgs)
Dim objectType As Type = TryCast(args.NewValue, Type)
Dim aList As List(Of PropertyData) = New TypePropertyCollection(Of objectType)
End Sub
I can succesfully retrieve the type from the EventArgs and put it in a variable. When I'm creating a new typePropertyCollection, I want to pass the type to the generic list, but it says the objectType isn't defined, although is is declared just the line above.
Any suggestions?
Edit
The class typepropertyCollection looks like this:
Public Sub New()
Dim properties = New List(Of PropertyInfo)(GetType(T).GetProperties(BindingFlags.Public Or BindingFlags.Instance))
For Each propertyToCheck In properties
Dim descriptionAttribute = propertyToCheck.GetCustomAttributes(GetType(DescriptionAttribute), True)
If Not descriptionAttribute Is Nothing AndAlso descriptionAttribute.Length > 0 Then
Add(New PropertyData() With {.Description = DirectCast(descriptionAttribute(0), DescriptionAttribute).Description, .PropertyName = propertyToCheck.Name})
Else
Add(New PropertyData() With {.Description = propertyToCheck.Name, .PropertyName = propertyToCheck.Name})
End If
Next
End Sub
To use this collection, I'm creating a new class which inherits from the typcollection:
Public Class CustomerTypePropertyCollection
Inherits TypePropertyCollection(Of Person)
End Class
I cannot do this because Person (I named it Person here to make it easier) is not known in that solution. It should also be possible to make collection of other types which or not known. That's why I wanted to pass the type of the object and use it that way.
Dim aList As List(Of PropertyData) = New TypePropertyCollection(Of Type)
This error is because you are attempting to create a TypePropertyCollection of Type 'objectType', objecttype is the variable name of a variable with a type of type .'. You would need a TypePropertyCollection of Type Type Type, not the variable name. Come back to me if there are other issues beyond this.
Related
When I try to use the .Where() method on a list, this does method does not seem to be defined if the list is of a generic type:
In my program, I have a class called Warning, and in another class, a list of warnings, defined as:
Dim warningList As List(Of Warning)
When I try to manipulate this list as:
Dim item = warningList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
This works completely fine, but when I try it like this:
Dim itemList
if(type = "Warning") Then 'Please note that this condition is true...
itemList = warningList
End If
Dim item = itemList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
I get an exception, stating that method .Where() is not defined for class Warning
Can anybody tell me why this is?
Thank you!
Now that you've edited your question it's clear.
You declare itemList without a type, so it's Object implicitly(in VB.NET with option strict set to Off which i strongly recommend against).
Now that you have declared a variable of type Object you can asssign any type to it. But you would have to cast it back to its real type List(Of Warning) to be able to use list or LINQ methods(which extend IEnumerable(Of T).
But instead declare it with the correct type:
Dim itemList As List(Of Warning)
if(type = "Warning") Then
itemList = warningList
End If
Dim item = itemList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
Including to comment to explain why Warning is not related to this problem:
That's not the real code. If warningList is really a List(Of Warning)
you should be able to use Enumerable.Where(if LINQ is
imported). The fact that you assign this instance to another variable
(on declaration) doesn't change anything because that variable's type
is also a List(Of Warning). So itemList.Where should work too. Warning
has nothing to do with it because the type which is extended by Where
is IEnumerable(Of T), T can be any type(even Object). Since List(Of T)
implements IEnumerable(Of T) you can use Enumerable.Where on any list
(or array).
If you actually have multiple types and Warning is just one of it, you should implement a common interface. Here's an example:
Public Enum NotificationType
Warning
Info
[Error]
End Enum
Public Interface INamedNotification
ReadOnly Property Type As NotificationType
Property Name As string
End Interface
Public Class Warning
Implements INamedNotification
Public Sub New( name As String )
Me.Name = name
End Sub
Public Property Name As String Implements INamedNotification.Name
Public ReadOnly Property Type As NotificationType Implements INamedNotification.Type
Get
Return NotificationType.Warning
End Get
End Property
End Class
Now you can declare a List(Of INamedNotification) and fill it with whatever implements this interface, like the Warning class:
Dim notificationList As List(Of INamedNotification)
if type = "Warning" Then
itemList = warningList
Else If type = "Info"
itemList = infoList
End If
Dim item = notificationList.Where(Function(x) x.Name = "Foo").FirstOrDefault()
I'm using Entity Framework 6.0, DbContext. I'm using this method to copy an object and some related children:
Imports System.Data.Objects
Imports System.Data.Objects.DataClasses
Imports System.Runtime.CompilerServices
Public Module Entities
<Extension()>
Public Function CloneEntity(Of T As Class)(entity As T, context As ObjectContext, Optional include As List(Of IncludeEntity) = Nothing, Optional copyKeys As Boolean = False) As T
Return CloneEntityHelper(entity, context, include, copyKeys)
End Function
Private Function CloneEntityHelper(Of T As Class)(entity As T, context As ObjectContext, Optional include As List(Of IncludeEntity) = Nothing, Optional copyKeys As Boolean = False) As T
If include Is Nothing Then include = New List(Of IncludeEntity)()
Dim myType = entity.GetType()
Dim methodInfo = context.GetType().GetMethod("CreateObject").MakeGenericMethod(myType)
Dim result = methodInfo.Invoke(context, Nothing)
Dim propertyInfo = entity.GetType().GetProperties()
For Each info In propertyInfo
Dim attributes = info.GetCustomAttributes(GetType(EdmScalarPropertyAttribute), False).ToList()
For Each attr As EdmScalarPropertyAttribute In attributes
If (Not copyKeys) AndAlso attr.EntityKeyProperty
Continue For
End If
info.SetValue(result, info.GetValue(entity, Nothing), Nothing)
Next
If info.PropertyType.Name.Equals("EntityCollection`1", StringComparison.OrdinalIgnoreCase) Then
Dim shouldInclude = include.SingleOrDefault(Function(i) i.Name.Equals(info.Name, StringComparison.OrdinalIgnoreCase))
If shouldInclude Is Nothing Then Continue For
Dim relatedChildren = info.GetValue(entity, Nothing)
Dim propertyType As Type = relatedChildren.GetType().GetGenericArguments().First()
Dim genericType As Type = GetType(EntityCollection(Of ))
Dim boundType = genericType.MakeGenericType(propertyType)
Dim children = Activator.CreateInstance(boundType)
For Each child In relatedChildren
Dim cloneChild = CloneEntityHelper(child, context, shouldInclude.Children, shouldInclude.CopyKeys)
children.Add(cloneChild)
Next
info.SetValue(result, children, Nothing)
End If
Next
Return result
End Function
Public Class IncludeEntity
Public Property Name As String
Public Property Children As New List(Of IncludeEntity)
Public Property CopyKeys As Boolean
Public Sub New(propertyName As String, ParamArray childNodes() As String)
Name = propertyName
Children = childNodes.Select(Function(n) new IncludeEntity(n)).ToList()
End Sub
End Class
End Module
Now I'm using the code like below :
Dim litm, newitm As New MyObject
Dim inc = New List(Of IncludeEntity)()
inc.Add(New IncludeEntity("Child_list"))
litm=context.MyObjects.FirstOrDefault
newitm = litm.CloneEntity(CType(context, Entity.Infrastructure.IObjectContextAdapter).ObjectContext,include:=inc)
The code is executed without errors, but nothing is copied, so newitm is empty.
I have inspected the code and found that this line on the CloneEntity function :
Dim myType = entity.GetType()
Is producing a strange type.
I'm expecting that the type will be of MyObject type, but instead this return :
MyObject_F2FFE64DA472EB2B2BDF7E143DE887D3845AD9D1731FD3107937062AC0C2E4BB
This line too :
Dim result = methodInfo.Invoke(context, Nothing)
produces the same strange type.
I don't know if this is the problem, but this is the only strange thing I have noticed.
Can you help me to find out why this code doesn't work?
Thank you!
Entity framework, like many other ORMs will build a proxy type for your entities so that it can intercept calls to:
Lazy load the contents of any collection contained as properties within your entity, when you access those collection properties.
Detect that you have made changes to the properties of an instance as part of dirty checking, so that it will know which objects are dirty and need to be saved to the database when you invoke SaveChanges.
Refer for example to EF returning proxy class instead of actual entity or Working with Proxies.
If you want to find out the underlying type of your entity that is wrapped by the proxy, i.e. the one that would match with the type you are looking for (e.g. MyObject), you can do that using a method in the object context:
var underlyingType = ObjectContext.GetObjectType(entity.GetType());
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.
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
Rather than giving the very specific case (which I did earlier), let me give a general example. Let's say that I have a function, called callingFunction. It has one parameter, called parameter. Parameter is of an unknown type. Let us then say that I wish to copy this parameter, and return it as a new object. For example, in pseudo code, something along the lines of...
Function callingFunction(ByVal parameter As Object) As Object
Dim newObj As New Object
'newObj has the same value as parameter, but is a distinctly different object
'with a different reference
newObj = parameter
return newObj
End Function
EDIT: Additional Information
The first time I posted this question, I received only one response - I felt that perhaps I made the question too specific. I guess I will explain more, perhaps that will help. I have an ASP page with 10 tables on it. I am trying, using the VB code behind, to come up with a single solution to add new rows to any table. When the user clicks a button, a generic "add row" function should be called.
The difficulty lies in the fact that I have no guarantee of the contents of any table. A new row will have the same contents as the row above it, but given that there are 10 tables, 1 row could contain any number of objects - text boxes, check boxes, etc. So I want to create a generic object, make it of the same type as the row above it, then add it to a new cell, then to a new row, then to the table.
I've tested it thoroughly, and the only part my code is failing on lies in this dynamic generation of an object type. Hence why I asked about copying objects. Neither of the solutions posted so far work correctly, by the way. Thank you for your help so far, perhaps this additional information will make it easier to provide advice?
You can't do this in general. And it won't be a good idea, for example, if parameter is of a type which implements the singleton pattern. If parameter is of a type which supports copying, it should implement the ICloneable interface. So, your function could look like this:
Function MyFunc(ByVal parameter As Object) As Object
Dim cloneableObject As ICloneable = TryCast(parameter, ICloneable)
If Not cloneableObject Is Nothing Then
Return cloneableObject.Clone()
Else
Return Nothing
End If
End Function
You could implement something like this:
Dim p1 As Person = New Person("Tim")
Dim p2 As Object = CloneObject(p1)
Dim sameRef As Boolean = p2 Is p1 'false'
Private Function CloneObject(ByVal o As Object) As Object
Dim retObject As Object
Try
Dim objType As Type = o.GetType
Dim properties() As Reflection.PropertyInfo = objType.GetProperties
retObject = objType.InvokeMember("", System.Reflection.BindingFlags.CreateInstance, Nothing, o, Nothing)
For Each propertyInfo As PropertyInfo In properties
If (propertyInfo.CanWrite) Then
propertyInfo.SetValue(retObject, propertyInfo.GetValue(o, Nothing), Nothing)
End If
Next
Catch ex As Exception
retObject = o
End Try
Return retObject
End Function
Class Person
Private _name As String
Public Property Name() As String
Get
Return _name
End Get
Set(ByVal value As String)
_name = value
End Set
End Property
Public Sub New()
End Sub
Public Sub New(ByVal name As String)
Me.Name = name
End Sub
End Class
Here's a simple class that will work for most objects (assumes at least .Net 2.0):
Public Class ObjectCloner
Public Shared Function Clone(Of T)(ByVal obj As T) As T
Using buffer As MemoryStream = New MemoryStream
Dim formatter As New BinaryFormatter
formatter.Serialize(buffer, obj)
buffer.Position = 0
Return DirectCast(formatter.Deserialize(buffer), T)
End Using
End Function
End Class