Assume we have a class called "MyClass"
Public Class MyClass
End Class
this class has a function called "My function"
Public Class MyClass
Public Function MyFunction()
End Function
End Class
This class has been implemented for some time and its been working fine. Now we need to change the implementation of the function "MyFunction". One option would be to open the source code and change it there. But I'm guessing there has to be a better approach.
Inheritance comes to mind but I don't want to change the derived classes name. I want the name of the class to still remain "MyClass", But I'm guessing the code below will cause an error:
Public Class MyClass
Inherits MyClass
Public Function MyFunction()
End Function
End Class
In other words I'm trying to create a new version of the old class by keeping most of the members the same but just changing a few functions.
To explain the project as a whole, The program is meant for structural design. What it does it designs structural components (i.e columns, beams, slabs, ...). The design procedures are specified by 3rd parties (government regulations). For example:
In the year 2007 government regulations specified that column dimensions are to satisfy the equation F:
H*B < Fy^2/L
In the year 2008 they introduced a new function G and they say column dimensions must satisfy this new function:
H*B^2 < Fy^0.5/E+Alpha^2/L
Where H and B are column dimensions.
What I don't want to do is to open the source code every year and make these changes. I want to somehow override the functions that need to be changed without opening the source.
Any Ideas?
The code is generally not supposed to be changed over time. That is - if you wrote code that is guaranteed to break after 2 weeks by itself, you probably should reconsider your design.
As you rules/regulations come out, you usually update your input data (in a form of XML, or a relational database for large amounts of data), and your program would automatically pick those up.
The only case you would update your program under this scenario is when new type of regulations come out. But even in this case the changes are usually minimal.
A good anti-pattern example for this - you have 500 forms, each of them has 500 lines of code, so that's 250000 lines of code in your UI layer. New regulations come out that requires changing 50% of the code in each form. Your impact is 125000, which at 40 lines of code per day would take 8.5 developer-years.
A solution to this would be having a change of 100 lines spread across all forms, adding 1 line in each, or leaving everything as is. Also there will be a data load/conversion procedure from a government/other file, which populates your database in the proper format, updating the values or adding new ones. There may be 10 lines of change in that program, but that's about it, 3 days worth of work, if you believe in 40 LoC per day. Otherwise it still falls under 2 weeks of developer's time.
Depending on how you implement it, the benefit of this approach could be that you support old standards as well, so older input can be matched and production reports can be generated. It is a good practice to be able to back-date your reports, cause sometimes there are issues in report code left unnoticed for months before being discovered.
EDIT: A more structured approach to what I suggested in the comments would be storing expression trees in the DB. Most simple form of it is just a linear workflow, using postfix notation (single table). For example A, B, + C - is equivalent to A + B - C. You can then have a user interface for some configuration tool, which only allows user to input values and functions that are applicable. This is assuming applicable values are also stored in DB as parameters (one structural component can have 0...N of them).
Inheritance can do what you want but you need to create a new ancestor, not descendant.
Change the name of the original class to something that denotes that it is a base class. Also, add the MustInherit modifier to the class and Overridable to any of the methods or properties that you may need to override.
One thing to watch for is Private members in this base class. Any members that need to be accessible from the descendant class cannot be Private and must be changed to Protected.
The original class looks like this.
Public MustInherit Class MyBaseClass
Public Overridable Function MyFunction() As String
' code...
End Function
Public Overridable Function AnotherFunction() As String
' code...
End Function
End Class
Now create a new class with the original class's name which inherits from the base class. Override just the members that need to be different.
Public Class MyClass
Inherits MyBaseClass
Public Overrides Function MyFunction() As String
' new code...
End Function
End Class
That will get you started. The Template Pattern will allow you to do more fine grained code changes where only parts of a method need to be changed.
The formula is a bit complicated and you'll still need to change some code unless you store these in a database somehow.
An option would be to use inheritance with a factory method.
Public Class BaseClass
Public MustOverride Function MyFunction()
Public Function GetInstance(ByVal year As Integer) As BaseClass
If year = 2007 Then Return New Class2007()
If Year = 2008 Then Return New Class2008()
End Function
End Class
Public Class Class2007
Inherits BaseClass
Public Overrides Function MyFunction()
' H*B < Fy^2/L
End Function
End Class
Public Class Class2008
Inherits BaseClass
Public Overrides Function MyFunction()
' H*B^2 < Fy^0.5/E+Alpha^2/L
End Function
End Class
then, everywhere in your code you use BaseClass never knowing that Class2007 and Class2008 exists
Dim o As BaseClass
o = BaseClass.GetInstance(2007)
o.MyFunction()
Depending on the need, this can also be done with interface.
If you need to store the formulas in the database as string, you'll need to get a parser and this can also be found using 3rd party library. https://stackoverflow.com/questions/1387430/recommended-math-library-for-c-net
Related
I have a class that I would like to extend by defining a new class that contains the first class as a public property, as well as additional added properties. However, the class that I'm extending has multiple derived types, which should be treated the same in the extension class.
Below is an example of what I am trying to do:
Public Class ClassA
End Class
Public Class ClassB
Inherits ClassA
End Class
Public Class ClassC
Inherits ClassA
End Class
Public Class BaseExtended
Public Property Foo As ClassA
Public Property ExtendedMetaData1 As Double
Public Property ExtendedMetaData12 As Integer
End Class
Public Class DerivedExtendedB
Inherits BaseExtended
Public Property Foo As ClassB
End Class
Public Class DerivedExtendedC
Inherits BaseExtended
Public Property Foo As ClassC
End Class
The code that uses an instance of any of the 'extended' classes would then need use that instance appropriately depending on it's type. There would be many cases where the property 'Foo' needs to be accessed and modified outside of the class that it belongs to.
If I were to implement something like what I have shown above, that would require that I first cast it to the required type before accessing or modifying it. Ideally I would like to do that inside the 'DerivedExtended' class; The alternative, I think, would be to duplicate code to cast that property would [hundreds of times] in the client code.
Private Sub ClientUsesObject(bar As BaseExtended)
' Perform a task that is agnostic Foo type
' Would not require that Foo be cast to any specific type
If bar.GetType() Is GetType(DerivedExtendedB) Then
Dim barCast As DerivedExtendedB = DirectCast(bar, DerivedExtendedB)
' Perform task that requires Foo to be of type ClassB
ElseIf bar.GetType() Is GetType(DerivedExtendedC) Then
Dim barCast As DerivedExtendedC = DirectCast(bar, DerivedExtendedC)
' Perform task that requires Foo to be of type ClassC
End If
End Sub
What I'm looking for is advice outlining or describing a design pattern that can handle this situation. I've searched for quite a while, and have not been able to find any examples that solve this problem.
I realize that this may be somewhat of an "XY" problem. I'm working with existing code that simply assumes all instances are of the same derived type (when in fact some instances are of the other derived type). As such, the existing code does not work. To me what I've tried to outline above seems like the most straightforward path, but I'm open to alternative if this is just the wrong approach.
This pattern of type covariance in derived classes is the canonical reason for what is called in C++ the "Curiously Recurring Template Pattern" and has been called in .NET the "Curiously Recurring Generic Pattern." I believe it's also sometimes referred to as "F-Bounded Polymorphism" (not a computer scientist, so I might have the reference wrong).
You can write a base class like this:
Public Class Base(Of TDerived As Base)
Public Overridable Property foo As TDerived
End Class
And then use it like this:
Public Class MyDerived
Inherits Base(Of MyDerived)
End Class
Then, the derived class has a property foo whose type is MyDerived. No casting required by clients.
However, this has some limitations. It works best when you don't need to switch back and forth between derived and base. There is no one Base, so you can't declare instances of it. If you want to be able to declare something as Base, then you end up needing to fall back on a non-generic base class. This will still work well for certain usage patterns where you don't need to convert from base to derived, but otherwise you run right back into the casting problems you are trying to avoid.
Eric Lippert has written a bit about this pattern. He's always interesting to read, so I'd recommend looking up his commentary.
Another alternative to consider, if the generic approach doesn't work for you, is code generation. You can use T4 templates to process a compact description of what your code should be, and generate the code files from them. A long list of casts is less tedious if you only write the machinery to generate it, you don't write them all out explicitly.
Using asp.net/vb.net. Created a solution with 2 projects, "MainProject" and "MyCommonStuff". The 2nd project ("MyCommonStuff") is really a common utilities class, the resulting dll I hope to use for other projects as well.
MyCommonStuff is defined very simply....
Public Class MyCommonStuff Stuff
Public Shared Function GetInfo() as string
:
:
End Function
Public Shared Sub Test
:
:
End Sub
:
End Class
In MainProject I set a reference to this MyCommonStuff project.
I want to access some of the MyCommonStuff methods in my code. But for some reason the methods are not being recognized.
For example, in a button in the MainProject I tried this....
dim m as new MyCommonStuff
x = m.GetInfo()
Intellisense doesn't pick up any of the subs/functions for m. What am I doing wrong? Thanks!
The thing is that you've made your methods static ("Shared" in VB). You need to either remove the Shared keywords:
Public Function GetInfo() As String
':
':
End Function
Public Sub Test()
':
':
End Sub
or keep the Shared keywords and use it like this:
x = MyCommonStuff.GetInfo()
Here is some information about Shared members. Most notably:
Specifies that one or more declared programming elements are
associated with a class or structure at large, and not with a specific
instance of the class or structure.
In other words, if you want to use your methods from an instance of your MyCommonStuff class, e.g. m in m.GetInfo(), you need to leave the Shared keyword off. If, on the other hand, you have a method that is common across all instances of your class or for which you don't even need an instance, you would use the Shared keyword and access the method like I said above, e.g. MyCommonStuff.GetInfo().
Make sure you're importing you common stuff's namespace.
I have read several questions on here and it appears that the general consensus is that an interface is not required for every class in a project. I have read posts like this: Is it the best practice to extract an interface for every class?.
I want to know how this applies to the .NET framework classes. I believe that all of the classes I have looked at either inherit from an abstract class e.g. SQLConnection inherits from dbConnection or implement and interface e.g. the Component class implements the IComponent interface.
I have a copy of Reflector, which I downloaded two months ago and I am awaiting the license (paid the fee recently). When I start to step through the code (using Reflector); am I going to see code like this:
Public Class Foo
Public Name As String
Public Property NameProperty()
Get
Return Name
End Get
Set(value)
Name = value
End Set
End Property
Public Shared Sub Main()
Dim f As Foo = New Foo
f.NameProperty = "Ian"
End Sub
End Class
rather than code like this:
Public Class Foo
Implements IFoo
Public Name As String
Public Property NameProperty() Implements IFoo.NameProperty
Get
Return Name
End Get
Set(value)
Name = value
End Set
End Property
Public Shared Sub Main()
Dim f As IFoo = New Foo
f.NameProperty = "Ian"
End Sub
End Class
Public Interface IFoo
Property NameProperty()
End Interface
Notice that there is an Interface used in the second code fragment. I am still struggling to understand when it is suitable not to use interfaces. Some developers say never. I suppose some of it is subjective.
As someone who strives to do things the right way, I struggle with this every time I start a new project. I've come to realize that it's mostly subjective; like saying "on which days is OK to not take a shower".
Of course it provides abstraction, improves testability, etc., but it can lead to unnecessary "class explosion". Adding interfaces to ancillary internal classes does more harm than good in my experience. I occasionally open a small project I did years ago and am shocked my the endless list of classes ... over-engineering!
As a rule, I use interfaces for APIs in class libraries. Other than that, I add them if time allows, or I have a special need to clarify how a section of code is supposed to be called by client code.
I have a little Problem:
I have a method that parses an incoming string for certain values, if a value is found, a new class is instantiated. The class name is identical to the parsed string. At the moment, my code looks like this:
Public Class Test1
End Class
Public Class Important
End Class
Public Class DoWork
Public Sub DoWork(incoming as String)
Select case incoming
case "Test1"
dim myobj as new Test1
Case "Important"
dim myobj as new Important
End Select
End Sub
End Class
I do not like the string literals like "Test1" - i could store them in a constant, but if the class names change, they have to be changed too. Is there a way to replace the literals with the Name of class?
I know that me.gettype produces the result for instantiated objects, but what about the simple name for a class, which is no object at this moment?
If your string is in correct format you can use Type.GetType(string) method to retrieve type. Then you can use Activator class to create instance if you have default constructor on that type.
Rafal's answer is good if you're stuck with the current situation, with the incoming string parameter. But it's still a bit fragile. What if the incoming parameter changes? What if you want to restructure your code, moving some classes to different namespaces or assemblies? What if those strings change - do you now have to rename your classes and recompile? You don't see the magic strings explicitly now, but they're still there.
So ask yourself - where are those strings coming from? Are they generated internally by your code? If so, you might want to generate, instead of strings, an Enum value that corresponds to the class to be instantiated. If they're external strings that you map to your classes, consider having explicit mapping (in a configuration file, for instance) that map String->Type. It's a bit more cumbersome, but a lot more flexible.
I have been charged with porting a VB6 project into VB.NET. In vb6, if you were in a class separate to a particular variable, you could access that variable easily:
Public Class Foo
Public k As Integer
End Class
Public Class Bar
k = 12
End Class
In VB.NET, my understanding is that before you can use a variable in another class, you must declare a new instance of it:
Dim foobar As New Foo
This would be fine, but I have to access these variables from different classes and every time I declare a new instance, it wipes all old values from the variables, which I need. Can anybody help? I tried using Inherits statements but they presented many problems.
Thanks.
Nick
Your're looking for the shared keyword. This makes the member available to other classes without having to have an instance of your class. See MSDN for more info
For the port just use Public module like you would in vb6
Public Module Foo
Public k As Integer
End Module
Public Module Bar
Foo.k = 12
End Module
Its not good practice but it will help you do your first pass at the port. Ideally you would refactor out modules/shared functions as being able to access variable from any part in the system will produce code that is harder to maintain
Dim YourobjName As YourClassName = Me.DataContext
Now you can use public methods and functions with YourobjName. Here YourClassName will be the class you want to access the public objects.