Why Does C# Set Private Variables Before The Base Constructor While .NET Does The Opposite
Mar 3, 2011
comparing C# code and VB.NET and the results between the seemingly identical code were entirely different.(Why C# is always winning over VB.NET?)The explanation given is that C# will initialize the class fields, then call the base constructor, but VB.NET does the exact opposite. Is there a technical reason for the languages to be different? At first glance, it seems that either approach is equally valid, but I can't fathom why they wouldn't have selected the SAME approach.
EDIT: As 'Jeffrey L Whitledge' has pointed out, VB6 did not have inheritance, so I don't think we can say 'to keep VB.NET and VB6 more closely related'.
After debugging a particularly tricky issue in VB.NET involving the order in which instance variables are initialized, I discovered that there is a breaking discrepancy between the behavior that I expected from C# and the actual behavior in VB.NET.Nota bene: This question concerns a slight discrepancy in the behaviors of VB.NET and C#. If you're a language bigot that is unable to provide an answer other than "that's why you should use C#, noob", there is nothing for you to see here; kindly move along.Specifically, I expected the behavior outlined by the C# Language Specification (emphasis added):
When an instance constructor has no constructor initializer, or it has a constructor initializer of the form base(...), that constructor implicitly performs the initializations specified by the variable-initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor. The variable initializers are executed in the textual order in which they appear in the class declaration.
Contrast that with the portion of the VB.NET Language Specification concerning Instance Constructors, which says (emphasis added): When a constructor's first statement is of the form MyBase.New(...), the constructor implicitly performs the initializations specified by the variable initializers of the instance variables declared in the type. This corresponds to a sequence of assignments that are executed immediately after invoking the direct base type constructor. Such ordering ensures that all base instance variables are initialized by their variable initializers before any statements that have access to the instance are executed.
The discrepancy here is immediately obvious. C# initializes class-level variables before calling the base constructor. VB.NET does exactly the reverse, apparently preferring to call the base constructor before setting the values of instance fields.If you want to see some code, this related question provides a more concrete example of the divergent behavior. Unfortunately, it does not provide any hints as to how one might coerce VB.NET into following the model established by C#.
I'm less interested in why the designers of the two languages chose such divergent approaches than I am in possible workarounds for the problem. Ultimately, my question is as follows: Is there any way that I can write or structure my code in VB.NET to force instance variables to be initialized before the base type's constructor is called, as is the standard behavior in C#?
if for example i have two private sub I declared a local variable on private sub a... is there a way that private sub b could use the variable created on private sub a? Im asking because im in a problem in my app were using global variable is not an option to make the long story short
I am trying to write some unit tests on a class that looks like this, using Moq [code] the parameterized constructor is private or internal one of the two methods relies on the result of the other.I want to check that when GetThisOrThat is called with a value, it actually calls GetThis. But I also want to mock GetThis so that it returns a specific well-known value.To me this is an example of Partial Mocking, where we create a Mock based on a class, passing the parameters for the constructor. The problem here is that there is no public constructor, and therefore, Moq can not call it..I tried using the Accessors generated by Visual Studio for MSTest, and use those accessors for the mocking, and this is what I came up with [code]
I don't know if this has been asked before, but we're having a discussion about it today at my job. Should private variables (that are shared/static) be instantiated when they are dimensioned/defined, or is it a better practice to do this inside of a constructor?
Public Class IpCam Private Const HOST As String = "http://test.com/url/example" Private Shared _Example As New OurClass(HOST)which one to use?
I have a base class that contains a fairly large number of parameters in it's New constructor. I have 7 subclasses that inherit the Super base class. My question/issue is, all of the subclasses use the same values for most of the parameters in the New constructor of the base class and these subclasses can be called one after the other. I would like to populate the common values for the superclass and then use those for each of the subclasses, but I can't come up with a good way to do this. If I could figure out how to do this, I wouldn't have to pass all of those parameters in the MyBase.New call for each subclass. I'll try to illustrate with some code .
In the inherited class I use the base constructor, but I can't use the class's members calling this base constructor. In this example I have a PicturedLabel that knows its own color and has an image. A TypedLabel : PictureLabel knows its type but uses the base color.
The (base) image that uses TypedLabel should be colored with the (base)color, however, I can't obtain this color
Error: Keyword 'this' is not available in the current context`
Public Class Form1 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim example1 As New ExampleClass[code]......
Public Class Utilities Public Shared Function blah(userCode As String) As String 'doing some stuff End Function End Class
I'm running FxCop 10 on it and it says:"Because type 'Utilities' contains only 'static' Shared' in Visual Basic) members, add a default private constructor to prevent the compiler from adding a default public constructor."
Which always calls a method with no params or return. The idea is that the user of my object (transactional processing of business logic) inherits the base class adds in a load of private methods to fire. They then add these method names to a list in the order they would like them fired and the code above will take care of firing them.
It works fine with public methods but not with private or protected methods in the same class (Protected because I have some 'standard' pre built methods to add to the base class). Realistically I could make the methods public and be done with it but my inner nerd wont allow me to do so...
I am assuming this is a security feature. Is there a way to get around this or does anyone have any suggestions on how to proceed but keep my tasty, tasty visibility modifiers in tact?
(NOTE: ITS IN VB.NET but a C# Answer is fine if that is what you are comfortable with).
In the past when I have written classes and constructors, I named the variables in the constructor parameter something different than what would have been stored in the actual class itself.What I do now is name them the same, and reference the internal variables with Me.varname.Here is a class I just started building.Is my naming convention incorrect? [code]
I am a little new to .NET Developing, and have learned a great deal recently, but I'm stuck with something. I come from an ASP Background, I'm trying to subtract 2 variables, but they don't seem to 'see' one another.
I am wondering if you have a class and in the class you have a function which executes a sqlcommand. Which would u use ? the local variable declaration or the private member command variable.Assuming that the class has a private field called m_Cmd as OracleCommand..i am using oracle db so i use oraclecommand , if sql server then it should be sqlCommand.[code]
I'm trying to make an application that calls a function to create a timer, but I cant use WithEvents or Private inside of a function or sub, Is there any way to do this?
I thought I had this figured out because it presented no errors, but when i tried to run it, I got this exception error:
Object reference not set to an instance of an object.
I am trying to set two private variables:
Private mdatStart As DateTime Private mdatEnd As DateTime
I need to set these dates from a datetimepicker (dtpPickup, dtpReturn), but I don't now how to set it. I have tried using:
Private mdatStart As DateTime = New Date(dtpPickup.Value.Year, dtpPickup.Value.Month, dtpPickup.Value.Day) 'Private mdatEnd As DateTime = New DateTime(dtpReturn.Value.Year,
I'm trying to get a handle on creating my own Interfaces for a project I'm working on. I know the MSDN help shows that all member signatures will become Public when implmented, etc, etc. However, if I implement the IDisposable interface provided by the framework, comments and private members and fields show up! Not only that, but some of the methods have comments and code within!
How did they do that? And can comments, private fields and members, and predetermined code be declared as part of an interface by the average developer?
I am programmer from some time only, I have certain doubts in fundamentals, could you please clarify on the following:Case 1:
[Code]...
Does case 1 and case 2 yield same result, I mean is a private value necessarily in there?, can we use property itself to use its own value in its Set and get statements?
I have a bunch of private variables I've typed out and I want to put all of my corresponding Public ReadOnly Properties in a bunch below them.Is there some way of copying ten lines of
Private _myVar As String
and pasting in ten sets of
Public Readonly Property MyVar As String Get Return _myVar[code]....
I'm currently copying the whole bunch of variable declarations, Find+Replacing Private _ into Public ReadOnly Property then going line-by-line expanding the definitions and writing return statements.how to avoid all this nonsense in the future, as I'm developing on a virtual terminal server, and the input lag on my little copy/paste/type operations on the code is driving me up the wall.
I would like to create an interface that brings with it it's own private variables and regions.for example, if you create a class and implement IDisposable, you get all of the following:[code]
give me a good resource that explains the difference between a Private, Public, Shared Functions/Sub/Variables? I normally use Public for Subs/Functions inside of Modules I call from other parts of the program. But I'd like to get more of an understanding of how and when to use them. I want as little as impact to a system that is running my programs as possible, so i guess the key here is I'm trying to just get more proficient in my coding.
I came across a number of new Private Shared variables (of type Hashtables(Of String), initialized in the declaration) added to a partial class for a very large (DataContext-derived) class. This seems sensible to me in one sense because they never change, and making these shared variables ensures that they won't get re-initialized every time a function is called. However, these variables are only used within the scope of one function in the class, and I fear the private namespace of this DataContext-derived class is getting rather polluted, and having these sorts of things exposed at such a high level might be confusing to others reading the code in the future.
Would there be negative performance impact to making these local variables within the function where they are used, or is there some better way to handle this? Basically we are using these 3 hashtables to determine whether anything within particular subsets of properties changed (using GetModifiedMembers and then using the Overlaps function of the hashset to see if any of the modified members correspond to members we care about).
Edit: I caved and took the time to write my own test program, which confirmed that there is a cost to using local variables (which I assume applies generally to all cases -- I doubt there's any case where a shared variable would be slower unless using the shared variable requires some additional logic to do so properly): [Code]
So in this particular case, using the local variable costs about 200%. But in most cases (including my own), the time is probably negligible compared to the overall task. So I guess the question now becomes, how do people generally feel about improving code maintainability at the cost of negligible but known performance impacts?
I have these list of RS-232 strings declared in my public class Public Class TouchInterface Dim WatchTVbtnCmd As String = "rs232command1" Dim VolUpbtnCmd As String = "rs232command2" Dim VolDownbtnCmd As String = "rs232command3" Dim SystemOffbtnCmd As String = "rs232command4" Dim RadiobtnCmd As String = "rs232command5" Dim MusicbtnCmd As String = "rs232command6"
I then have this private sub. For the sake of this thread, what I would like to do is get the name of the label that was clicked which in the code below is already done, convert it to string (not sure if this needs to be done), this is also done in the code below. Next I want to compare the labelnamestring to all the variables in the public class to find out which one it is equal to and then disply that variable which should be my rs232 string in the message box. I have also added "Cmd" to the variable names above so I also need to work out how to add the text "Cmd" to the string to properly compare.
Private Sub Musicbtn_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles WatchTvbtn.Click, VolUpbtn.Click, VolDownbtn.Click, SystemOffbtn.Click, Radiobtn.Click, Musicbtn.Click, GardenVolUpbtn.Click, GardenVolDownbtn.Click, GardenRadiobtn.Click, GardenOffbtn.Click, GardenMusicbtn.Click, GardenMediaCenterbtn.Click Dim labelname As Label = DirectCast(sender, Label) [Code] .....
In re-engineering a windows forms application, I find that a lot of code-behind in the various form classes is duplicative and I'm trying to centralize as many procedures as possible into a base class which can be inherited and used by the subclassed forms.
This process seems to be going well and is making the code in my subclasses much simpler and hopefully easier to maintain, but I'm not sure where to draw the line between leaving code in the subclasses and engineering for generic resusability and moving it to the base class.
Specifically, in some subclasses I have code which manipulates variables and objects specific to the subclass, and although I could move the code-behind into the base class, the base class code references specific objects which are needed to compile. For example, each subclass manipulates a databound datagridview and form detail controls which allows the user to select between multi-record and detailed single-record views of a datatable.
In Visual Basic 2008 do I need to declare dummy data objects in the base class so that the base class will compile? Or is there a way to indicate that the data objects will be provided by the subclass?
I have an abstract class which requires a delegate to function. I pass the delegate into the constructor. Now that I have a non default constructor I need to call the abstract class's constructors from the concrete class which means that I need to use MyBase.New(...). I have included a quick example below.
Public MustInherit Class BaseClass Public Delegate Sub WorkMethod() Private _Work As WorkMethod
[code]....
I have tried to do this but I keep getting the following error: "Implicit reference to object under construction is not valid when calling another constructor".Can I not do what I am trying to do above? I initially had the delegate setup in its own setter method. But then I am creating a deceptive API because it does require a point to a method to work properly.
what's the rationale behind this limitation: Constructor call is valid only as the first statement in an instance constructor i want to pass an argument to my constructor which validates this argument and calls mybase.new according to this argument but it doesn't let me
example:
Public Class prob Inherits System.ApplicationException Public Sub New(ByVal problem As String, ByRef inner_exception As Exception)
I'm sure there is an answer to this somewhere but I'm clearly using the wrong terminology in my searches, so I apologise in advance for this inevitably being a duplicate.Take the function CType. Clearly I can cast (or at least try) a given object to a given reference type. The function will not work if trying to cast to a structure, i.e.CType(myObject, Integer)
will generate a compiler error. This I'm sure most often crops up when working with generics:
[Code]...
What is the opposite? I want only reference types so that CType doesn't fail. I can't overload the T As Structure with a plain T because it considers them identical signatures, so surely there's a keyword I'm missing somewhere?
I can remember reading some time ago that there is some way to set the opposite value of a variable depending on the value that it is set to but, I can't remember the syntax of it and I'm not sure what the correct terminology is to describe what I'm looking for. Suppose that I have a boolean variable that can be either true or fule. I need to some way set the variable to the opposite value that is set.
Off of the top of my head it was written something like this (excuse the wrong syntax below, it's what I'm trying to figure out :-) ). Dim myBool as Boolean = False myBool = True : False I know this can be done with if else statements but I'm sure I've seen a more compact way to write it.
