I have a list which is filled with a data fetched from DB.
My Code:
Dim lst As New List(Of MyClass)
lst = GetData()
MyClass looks like the following:
Public Class MyClass
Public Overridable Property Id as Integer
Public Overridable Property Questions as String
Public Overridable Property Comments as String
End Class
I am trying to loop the lst
For Each item As String In lst
'Some data manipulation
Next
But I am unable to loop it through using the above code.It throws following error:
Value of Type 'MyClass' cannot be converted to 'String'
Whats arong here? Any Help?
Thanks in advance.
Try this
For Each item As MyClass In lst
'Some data manipulation
Next
Related
I want to create a list of items in each room of a text adventure game, but I want each item to have both the name of the item, and the description of the item, so I can call either/both strings for the item depending on what I need the item for.
Dim StartRoom As RoomInfo
Public Class Items
Public ItemName As String
Public ItemDescription As String
End Class
Public Structure RoomInfo
Dim RoomName As String
Dim BasicDescription As String
Dim RoomDescription As String
Dim ValidExists As List(Of String)
Dim ObjectsInRoom As List(Of String)
Dim ItemsInRoom As List(Of Items)
End Structure
Sub InitializeRooms()
StartRoom.RoomName = "Start Room"
StartRoom.RoomDescription = "Test Description"
StartRoom.BasicDescription = "Test Basic Description"
Dim TestItem As New Items With {
.ItemName = "Test Item",
.ItemDescription = "This is a Test Item"
}
StartRoom.ItemsInRoom.Add(TestItem)
End Sub
I've created a list, ItemsInRoom, which is a list of Items, a Class. This way, each room can have a unique list of items that can be changed as the player picks up or interacts with them. That's what the Initialize Rooms sub is for- I can run it when the game loads, and set up all the information for each room in the game.
Anyway, when I try to add the TestItem to the list, I get an 'Object reference not set to an instance of an object.' error. I'm extremely dumb and this is probably the most obvious error in the book, but I can't find the solution to this particular case.
Thanks for the help.
This is what your type declarations ought to look like:
Public Class Item
Public Property Name As String
Public Property Description As String
End Class
Public Class Room
Public Property Name As String
Public Property BasicDescription As String
Public Property RoomDescription As String
Public ReadOnly Property ValidExists As New List(Of String)
Public ReadOnly Property Objects As New List(Of String)
Public ReadOnly Property Items As New List(Of Item)
End Class
Better names for the types and members, class rather than structure, properties rather than fields, collections created where they are declared, collection properties read-only.
I have the following class:
Public Class CodeList
Public Code As String
End Class
I am using Entity Framework.
I have the following table in a generated class:
Partial Public Class ContentTable
Public Property ID As Long
Public Property Type As Nullable(Of Integer)
Public Property Code As String
Public Property Active As Nullable(Of Boolean)
End Class
I am using this lambda expression to select distinct Code from ContentTable
Dim db As New ModelCodeEntities
Dim result = db.ContentTables.Select(Function(m As ContentTable) m.Code).Distinct().ToList
I want to convert the result of this lambda to a List(of CodeList)
Any help would be appreciated.
Dim result As List(Of CodeList)=db.ContentTables.Select(Function(item) item.Code).Distinct().Select(Function(code) new CodeList with{.Code=code}).ToList()
Given the scenario where I am building a wrapper for the data I pass to jquery datatables and I have for example, three Classes Orders, OrderDetails and Customers. In datatables, lists of objects are denoted by the variable name aoData, so to construct that I have to pass a List(Of someclass). Since I don't know which class until I pass it how do you do it? is There a GetType that takes the class name as a string? I couldn't find one.
Public Class DataTablesWrapper
Public Sub New(ByRef data As List(Of String))
Me.aaData = data
End Sub
Public Sub New(ByRef data As List(Of some class depending on what data I want wrapped))
Me.aoData = data
End Sub
Public Property aaData As List(Of String)
Public Property aoData As List(Of some class depending on what data I want wrapped)
End Class
I would recommend the use of generics.
In your case it would look like this:
Public Class DataTablesWrapper(Of T)
Public Sub New(ByRef data As List(Of T))
Me.aaData = data
End Sub
Public Property aaData As List(Of T)
End Class
Creating your instance would then look like this:
Sub Main()
Dim stringList As New List(Of String)({"foo", "bar"})
Dim tablesWrapper As New DataTablesWrapper(Of String)(stringList)
End Sub
You don't want to use List(Of Object) due to this reasons.
For the problem I have these classes:
Public MustInherit Class BaseLeaf(Of T)
Implements IBaseLeaf
// etc
Public Class WebsiteLeaf
Inherits BaseLeaf(Of Headline)
// etc
Public Class WebsiteCollection
Inherits BaseCollection(Of WebsiteLeaf)
// etc
Public Class SubscriptionList
Private mCollection As BaseCollection(Of IBaseLeaf)
Public Sub LoadSubscriptions(ByVal collection As BaseCollection(Of IBaseLeaf))
mCollection = collection
End Sub
In the Main class I am trying to call the following function:
Private Sub FetchSubscriptions(ByVal websites As WebsiteCollection)
gUser.SubscriptionList.LoadSubscriptions(websites)
// code
End Sub
This will access LoadSubscriptions with passing the "websites" variable.
As you can see LoadSubscriptions expects a BaseCollection(Of IBaseLeaf).
The "websites" variable is a WebsiteCollection, which is a BaseCollection(Of WebsiteLeaf(Of Headline)).
Now I am getting the error: Value of type 'WebsiteCollection' cannot be converted to 'BaseCollection(Of IBaseLeaf)'.
What I am doing wrong here?
If class B is a descendent of class A this does not mean that Collection(Of B) is a descendant of Collection(Of A)!
If you have these definitions
Dim stringList As List(Of String)
Public Sub DoSomthingWithList(list As List(Of object))
list.Add(Date.Now)
End Sub
and you could call the method like this
DoSomthingWithList(stringList)
then the method would try to add a Date to the list which is actually a list of strings! Since the parameter is typed as object the date would be boxed (i.e. converted to an object) but not converted to string.
Therefore the collections List(Of X) and List(Of Y) are never compatible, even if Y inherits X.
Let's look at another example
Public Class TestClass
Implements IBaseLeaf
// etc
What happens if you call LoadSubscriptions with a WebsiteCollection?
Public Sub LoadSubscriptions(ByVal collection As BaseCollection(Of IBaseLeaf))
collection.Add(New TestClass())
End Sub
The code below works for the class that I hard coded "XCCustomers" in my RetrieveIDandName method where I use CType. However, I would like to be able to pass in various classes and property names to get the integer and string LIST returned. For example, in my code below, I would like to also pass in "XCEmployees" to my RetrieveIDandName method. I feel so close... I was hoping someone knew how to use CType where I can pass in the class name as a string variable.
Note, all the other examples I have seen and tried fail because we are using Option Strict On which disallows late binding. That is why I need to use CType.
I also studied the "Activator.CreateInstance" code examples to try to get the class reference instance by string name but I was unable to get CType to work with that.
When I use obj.GetType.Name or obj.GetType.FullName in place of the "XCCustomers" in CType(obj, XCCustomers)(i)
I get the error "Type 'obj.GetType.Name' is not defined" or "Type 'obj.GetType.FullName' is not defined"
Thanks for your help.
Rick
'+++++++++++++++++++++++++++++++
Imports DataLaasXC.Business
Imports DataLaasXC.Utilities
Public Class ucCustomerList
'Here is the calling method:
Public Sub CallingSub()
Dim customerList As New XCCustomers()
Dim customerIdAndName As New List(Of XCCustomer) = RetrieveIDandName(customerList, "CustomerId", " CustomerName")
'This code below fails because I had to hard code “XCCustomer” in the “Dim item...” section of my RetrieveEmployeesIDandName method.
Dim employeeList As New XCEmployees()
Dim employeeIdAndName As New List(Of XCEmployee) = RetrieveIDandName(employeeList, "EmployeeId", " EmployeeName")
'doing stuff here...
End Sub
'Here is the method where I would like to use the class name string when I use CType:
Private Function RetrieveIDandName(ByVal obj As Object, ByVal idPropName As String, ByVal namePropName As String) As List(Of IntStringPair)
Dim selectedItems As List(Of IntStringPair) = New List(Of IntStringPair)
Dim fullyQualifiedClassName As String = obj.GetType.FullName
Dim count As Integer = CInt(obj.GetType().GetProperty("Count").GetValue(obj, Nothing))
If (count > 0) Then
For i As Integer = 0 To count - 1
'Rather than hard coding “XCCustomer” below, I want to use something like “obj.GetType.Name”???
Dim Item As IntStringPair = New IntStringPair(CInt(CType(obj, XCCustomers)(i).GetType().GetProperty("CustomerId").GetValue(CType(obj, XCCustomers)(i), Nothing)), _
CStr(CType(obj, XCCustomers)(i).GetType().GetProperty("CustomerName").GetValue(CType(obj, XCCustomers)(i), Nothing)))
selectedItems.Add(Item)
Next
End If
Return selectedItems
End Function
End Class
'+++++++++++++++++++++++++++++++
' Below are the supporting classes if you need to see what else is happening:
Namespace DataLaasXC.Utilities
Public Class IntStringPair
Public Sub New(ByVal _Key As Integer, ByVal _Value As String)
Value = _Value
Key = _Key
End Sub
Public Property Value As String
Public Property Key As Integer
End Class
End Namespace
'+++++++++++++++++++++++++++++++
Namespace DataLaasXC.Business
Public Class XCCustomer
Public Property CustomerId As Integer
Public Property CustomerName As String
End Class
End Namespace
'+++++++++++++++++++++++++++++++
Namespace DataLaasXC.Business
Public Class XCCustomers
Inherits List(Of XCCustomer)
Public Sub New()
PopulateCustomersFromDatabase()
End Sub
Public Sub New(ByVal GetEmpty As Boolean)
End Sub
End Class
End Namespace
'+++++++++++++++++++++++++++++++
Namespace DataLaasXC.Business
Public Class XCEmployee
Public Property EmployeeId As Integer
Public Property EmployeeName As String
End Class
End Namespace
'+++++++++++++++++++++++++++++++
Namespace DataLaasXC.Business
Public Class XCEmployees
Inherits List(Of XCEmployee)
Public Sub New()
PopulateEmployeesFromDatabase()
End Sub
Public Sub New(ByVal GetEmpty As Boolean)
End Sub
End Class
End Namespace
From MSDN
CType(expression, typename)
. . .
typename : Any expression that is legal
within an As clause in a Dim
statement, that is, the name of any
data type, object, structure, class,
or interface.
This is basically saying you can't use CType dynamically, just statically. i.e. At the point where the code is compiled the compiler needs to know what typename is going to be.
You can't change this at runtime.
Hope this helps.
Since List(Of T) implements the non-generic IList interface, you could change your function declaration to:
Private Function RetrieveIDandName(ByVal obj As System.Collections.IList, ByVal idPropName As String, ByVal namePropName As String) As List(Of IntStringPair)
And then your troublesome line would become (with also using the property name parameters):
Dim Item As IntStringPair = New IntStringPair(CInt(obj(i).GetType().GetProperty(idPropName).GetValue(obj(i), Nothing)), _
CStr(obj(i).GetType().GetProperty(namePropName).GetValue(obj(i), Nothing)))
Of course, you could still have the first parameter by Object, and then attempt to cast to IList, but that's up to you.
ctype is used to convert in object type.