I am converting DataTables to a generic list and need a quick and easy way to implement a Find function. It seems I am going to have to use a Predicate. Upon further investigation, I still can't seem to re-create the functionality. I have this predicate...
Private Function ByKey(ByVal Instance As MyClass) As Boolean
Return Instance.Key = "I NEED THIS COMPARISON TO BE DYNAMIC!"
End Function
And then calling it like this...
Dim Blah As MyClass = MyList.Find(AddressOf ByKey)
But I have no way to pass in a key variable to this predicate to do the comparison, as I used to do with DataTable...
Dim MyRow as DataRow = MyTable.Rows.Find(KeyVariable)
How can I setup a predicate delegate function in VB.NET to accomplish this?
Do not recommend LINQ or lambdas because this is question is regarding .NET version 2.0.
Just put your predicate in a class instance:
Public Class KeyMatcher
Public Sub New(ByVal KeyToMatch As String)
Me.KeyToMatch = KeyToMatch
End Sub
Private KeyToMatch As String
Public Function Predicate(ByVal Instance As MyClass) As Boolean
Return Instance.Key = KeyToMatch
End Function
End Class
and then:
Dim Blah As MyClass = MyList.Find(AddressOf New KeyMatcher("testKey").Predicate)
We can even get a little fancy and make this generic:
Public Interface IKeyed(Of KeyType)
Public Key As KeyType
End Interface
Public Class KeyMatcher(Of KeyType)
Public Sub New(ByVal KeyToMatch As KeyType)
Me.KeyToMatch = KeyToMatch
End Sub
Private KeyToMatch As KeyType
Public Function Predicate(ByVal Instance As IKeyed(Of KeyType)) As Boolean
Return Instance.Key = KeyToMatch
End Function
End Class
And then make your MyClass type implement the new IKeyed interface
Related
In VB, I have a class that does some standard validations. What I'd LIKE to do is to declare some variables, then create instances of a validator class that include pointers to the variables, and then at some later time execute the validators to test the values in the fields that are pointed to.
Something like this:
public class MyData
public property foo as string
public property bar as string
dim vfoo as validator
dim vbar as validator
public sub new()
vfoo=new validator(&foo) ' i.e. & operator like in C
vbar=new validator(&bar)
end sub
public sub validate()
vfoo.validate
vbar.validate
end sub
end class
public class validator
dim _field as string* ' i.e. * like in C
public sub new(field as string*)
_field=field
end sub
public sub validate
if string.isnullorempty(_field) then
throw SomeException
else if not SomeOtherTest(_field) then
throw SomeOtherException
end sub
The catch is that, to the best of my knowledge, there is nothing like C pointers in VB. Is there any reasonably easy way to do this?
At present I am passing in the field values at the time I call the validate() function, but this is not ideal because I would like to be able to create a List of validators specific to a given caller, and then loop through the List. But at the time I loop, how would I know which value from MyClass to pass in, unless I had a giant select statement keying off some "field code"? (And of course in real life, there are not just two fields like in this example, there are quite a few.)
Am I just having a brain freeze and there's an easy way to do this? Or can this not be done in VB because there are no such thing as pointers?
Like Java, VB doesn't make direct use of pointers (it compensates where it can with library/framework calls). In the context of a garbage-collected language, I can't imagine that this style of validation would work out well.
But for fun, maybe a lambda-based solution could suit?:
Public Class MyData
Public Property foo As String
Public Property bar As String
Dim vfoo As validator
Dim vbar As validator
Public Sub New()
vfoo = New validator(Function() foo)
vbar = New validator(Function() bar)
End Sub
Public Sub validate()
vfoo.validate()
vbar.validate()
End Sub
End Class
Public Class validator
ReadOnly _fieldFunc As Func(Of String)
Public Sub New(fieldFunc As Func(Of String))
_fieldFunc = fieldFunc
End Sub
Public Sub validate()
Dim _field = _fieldFunc()
If String.IsNullOrEmpty(_field) Then
Throw New Exception("NullOrEmpty")
ElseIf Not SomeOtherTest(_field) Then
Throw New Exception("SomeOtherTest")
End If
End Sub
Public Function SomeOtherTest(f As String) As Boolean
Return True
End Function
End Class
So this is how I would design my Lazy class (From this SO):
Public NotInheritable Class MySingleton
Private Shared ReadOnly _instance As New Lazy(Of MySingleton)(Function() New _
MySingleton(), System.Threading.LazyThreadSafetyMode.ExecutionAndPublication)
Private Sub New()
End Sub
Public Shared ReadOnly Property Instance() As MySingleton
Get
Return _instance.Value
End Get
End Property
Private _MyString As String
Public Property MyString As String
Get
Return _MyString
End Get
Set(value As String)
_MyString = value
End Set
End Property
End Class
To access the _MyString value, I do the following:
Dim MyString = MySingleton.Instance.MyString
In fact, I always have to type the ".Instance."
Does it have any disadvantage if I design the Property the following way:
Public Property MyString As String
Get
Return instance._MyString
End Get
Set(value As String)
instance._MyString = value
End Set
End Property
So I can access it without always writing the ".Instance."
Dim MyString = MySingleton.MyString
Yes you can (of course with Public Shared Property), but you are losing some of the benefits from singleton over static classes.
Lets say you have another class MyWorker
Public Class MyWorker
Public Sub Work(instance as MySingleton)
Dim value as String = instance.MyString
' Do something ...
End Sub
End Class
I would not do this. This may not look like a big issue, but on the long run you have a tight coupling in your code base and a hard time mocking your class for unit testing, one of the reasons for using singeltons over static classes in the first place.
I often use this approach, when accessing Singelton values mutiple times:
Dim instance as MySingelton = MySingelton.Value
If instance.MyString = "something" Then
instance.MyString = "something else"
End If
much cleaner approach.
Is there any way to make a class can be only initialized at declaration.
Public Class AnyValue
Private value As Int32
Public Sub New(ByVal aValue As Int32)
value = aValue
End Sub
End Class
'I want to be able to do this:
Dim val As New AnyValue(8)
'But not this.
val = New AnyValue(9)
Or it is possible to stop the assignment or detect when the operator = is used.
Lets just say this - No, you can't do what you want. The closest thing to it that I can think of, is to hide the constructor and give static access to the consumer as follows:
Public Class AnyValue
Private value As Int32
Private Sub New(ByVal aValue As Int32) ' Note - private constructor
value = aValue
End Sub
Public Shared Function Create(ByVal aValue As Int32) As AnyValue
Return New AnyValue(aValue)
End Function
End Class
'This will not work
Dim val As New AnyValue(8)
'This will not work
val = New AnyValue(9)
' This will work
Dim val As AnyValue = AnyValue.Create(8)
Now, if you look at this method of object creation, you can see that you can set all sort of rules for object construction. So, the client has very little input on the construction itself because how you construct the object is totally controlled by the object itself.
I've created a request class. Here is an abbreviated version of it:
Public Class Request(Of T)
Private _Account As String
Public Property Account() As String
Get
Return _Account
End Get
Set(ByVal value As String)
_Account = value
End Set
End Property
Private _InnerRequest As T
Public Property InnerRequest() As T
Get
Return Me._InnerRequest
End Get
Set(ByVal value As T)
Me._InnerRequest = value
End Set
End Property
End Class
And then I have two other classes that I intend to use with this one - again, abbreviated
Public Class Individual
Public FirstName As String
Public LastName As String
Friend Sub New()
End Sub
End Class
And
Public Class Commercial
Public EntityName As String
Friend Sub New()
End Sub
End Class
Again, both of these are pretty abbreviated. The issue comes in when I attempt to use the properties of individual or commercial:
Dim Req As New Request(Of Individual)()
Req.InnerRequest.FirstName = "Herman" <-- Null Ref Exception
So... how do I get my inner request null ref exception kicked? I tried simply using Me._InnerRequest = New T in the New sub of Request, but no dice. Is there a way to handle this?
Req.InnerRequest must be set to an object instance of Individual first.
Req.InnerRequest = new Individual()
Req.InnerRequest.FirstName = "Herman"
Or create an instance for InnerRequest with the following modifications
Public Class Request(Of T As {New}) 'Classes of type T must have a public new constructor defined
::
Private _InnerRequest As New T() 'Creates a new class of type T when an instance is created of Request
And make the constructors of the other classes Public instead of Friend.
Than you can directly do
Dim Req As New Request(Of Individual)()
Req.InnerRequest.FirstName = "Herman"
#Barry already answered what the main problem is, but here's an alternate syntax if you prefer object initializers:
Req.InnerRequest = new Individual() With { FirstName = "Herman" }
Or, if you prefer, you could overload the constructor for your Individual class:
Dim individual As New Individual("Herman")
Req.InnerRequest = individual
With the Individual class looking like:
Public Class Individual
Public FirstName As String
Public LastName As String
Friend Sub New()
End Sub
Friend Sub New(firstName As String)
Me.FirstName = firstName
End Sub
End Class
You probably should consider restricting the T to some Entity class:
Public Class Request(Of T As Entity)
From which both Individual and Commercial will inherit:
Public Class Individual : Inherits Entity
Then maybe declare an overridable property Name of type String on this Entity class (which can be abstract/MustInherit), this should provide some flexibility. Otherwise you'd be having a hard time consuming your design pattern.
I'm completely stuck in a situation and I have no idea on where to go from here. I'm creating a very large project, so my goal is to keep the code itself as clean as possible and keeping as many hacks as possible out of the mix.
Here is the situation.
I have a class called Woo_Type, it is the parent of my many derived classes.
Public MustInherit Class Woo_Type
Private Shared TypeList As New Dictionary(Of String, Woo_Type)
Public MustOverride Sub SetValue(ByVal val As Object)
Public MustOverride Function GetValue() As Object
Public Shared Function GetTypeFromName(ByVal name As String) As Woo_Type
Return TypeList(name)
End Function
Public Shared Sub AddType(ByVal name As String, ByVal def As Woo_Type)
TypeList.Add(name, def)
End Sub
End Class
I have many classes that Inherit from Woo_Type that have similar structures to this:
Public Class Woo_tpInt
Inherits Woo_Type
Private value As Integer = 0
Public Overrides Function GetValue() As Object
Return value
End Function
Public Overrides Sub SetValue(val As Object)
value = val
End Sub
End Class
I want to be able to do things like:
Woo_Type.GetTypeFromName("int")
And have it return something like the class or something...
At this point I'm really confused as to what I want and I didn't know if anybody had any suggestions. To make sure that GetTypeFromName worked correctly, I had in an Initializer sub the following:
Public Sub InitializeTypes()
Woo_Type.AddType("int", Woo_tpInt)
Woo_Type.AddType("String", Woo_tpInt)
End Sub
But I quickly realized that-that obviously doesn't work either.
So this may seem confusing but I'm basically wondering how to better structure this so that everything works...
What do you want to do with the result? Are you sure you don't simply need generics?
Public Class WooType(Of T)
Public Property Value As T
End Class
Public Class Test
Public Sub Foo()
Dim int As New WooType(Of Integer)
int.Value = 42
Dim str As New WooType(Of String)
str.Value = "Forty-Two"
End Sub
End Class
If what you want to do is get the type itself (as opposed to an object), I would recommend using reflection rather than trying to reinvent the wheel. For instance, to get the Woo_tpInt type, you could do this:
Dim a As Assembly = Assembly.GetExecutingAssembly()
Dim t As Type = a.GetType("WindowsApplication1.Woo_tpInt") ' Change WindowsApplication1 to whatever your namespace is
If you want to use a shorter name like "int" to mean "WindowsApplication1.Woo_tpInt", you could create a dictionary to store the translation table, for instance:
Dim typeNames As New Dictionary(Of String, String)
typeNames.Add("int", GetType(Woo_tpInt).FullName)
Dim a As Assembly = Assembly.GetExecutingAssembly()
Dim t As Type = a.GetType(typeNames("int"))