Why It Is Not Necessary To Initialize All Members In Parameterized Constructor Of A Structure
Jan 19, 2012
When it comes to constructors in a Structure type ,it is fact that we can not explicitly provide the default constructor .It is provided by the runtime. Ok, another point that is applicable to C# language is that if we are defining a parametrized constructor then it is compulsory to initialize all fields . We can not leave any field uninitialized , since the runtime will not call default constructor if we are calling parametrized constructor .Its logically right , but in VB.NET language , actually you are free to leave any of the fields uninitialized in your parametrized constructor.
I was looking on the interweb to see if there were any good examples on how to initialize shared members within a class while still initializing instance variables. I did find an expression that might fit to the answer:
[code]...
How do I initialize both instance and shared members without re-initializing the shared members every time an object is created from a class? Thanks!
I was looking on the interweb to see if there were any good examples on how to initialize shared members within a class while still initializing instance variables.[code]How do I initialize both instance and shared members without re-initializing the shared members every time an object is created from a class?
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#?
I'm having some headaches using Structures and functions that return Nothing in VB.NET. [Code] In the previous code, when I return Nothing as result of Foo function I'd expect that st is Nothing. But this is not what happens. Then I found in MSDN documentation: Assigning Nothing to a variable sets it to the default value for its declared type. If that type contains variable members, they are all set to their default values.
So I discovered that when I assign Nothing to a structure, all its members are set to their default values, instead of the structure itself.
Also, I tried to make st a Nullable type by declaring: Dim st As Nullable(Of Test) = Foo()
but, still I can't check if st is Nothing by using: If st Is Nothing Then
or If st.Equals(Nothing) Then
So, questions:
1 - Is it possible to assign Nothing to the structure and not to its members? 2 - How can I check if a return structure value is Nothing?
I am trying to figure out how to print specific members of a structure when I only have to pass the structure itself i.e.
Dim Point1 as PointF = New PointF(15,30) Debug.Print(Point1) ' This will output "{X = 15, Y = 30}" by default
I am trying to figure out how I can do the same with my own strucutres, for example:
Dim Point2 as PointF2D = New PointF2D(15,30) Debug.Print(Point2) ' This will output "ClassName.PointF2D.Point2"
How can I get my structures, when passed to a print statement, to print specific members of that structure by default? P.S. Please give another way other than overloading the .ToString function.
how you init a variable when it is created by assigning the value on the same line in which it is declared. Is there a way to do this with members of a structure?
I have a main array that is a structure with three fields which consists of id_code(int)members(int) and annual_income(dec). I've get everything entered and I want to go to a procedure that prints a report giving the total percentage of families below a particular income and the level is different depending on how many members there are in the family.
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)
Module Module1 Public Structure structure1 Public TRANS() As structure2 End Structure Public Structure structure2 Public X() As Integer End Structure End Module
I'm new to VB 2008 after having spent a long time with VB6, so I apologize if this is a stupid question. But I'd really like to have this straightened out.
Let's say I have a pretty large structure that has lots of properties.
Code:
Now say that I want an internal database with about 10 instances of this structure total, describing, say, 10 different products that a store sells. When these values are loaded from a database, they remain totally static. (However, they can be different each time a program loads)
Now say that I have a class. Each instance of this class is a type of that BaseProduct structure. Meaning, each instance of the class pertains to one of the 10 types of products that the store sells. However, this class has additional properties that pertain specifically to each instance, which are not static.
Code:
Now, the problem here is... If I have 200 different transactions, each one contains an instance of BaseProduct. BaseProduct is HUGE, and is largely redundant (only 10 types possible), so I think it's a little silly to include a whole copy of it with EVERY transaction. However, the Transaction class really needs information regarding the base product it pertains to. Is there a way to, instead of declaring a New BaseProduct in the Transaction class, to simply make one of the properties of the Transaction class a pointer to a BaseProduct variable?
In VB6, I would accomplish this by making a BaseProduct(10) array, and then giving each Transaction an ID number referring to an entry in that array. But in VB 2008, using class structure, this is impossible. I can't define the BaseProduct(10) array outside of a class in a namespace, and if I define it in the actual application's form, then the class loses modularity since it relies on the application that's using it.
I am trying to communicate with an external device and i am trying to send a byte array to the external device via sockets but i am always getting a response the message size is too small so i am not sure what i have done wrong. Between the data type there should be no alignment present and all numbers are represented in little endian format. The char array is not null terminated as mentioned in the protocol specifications.
I have to send data based on a struct that embeds 2 other struct. So here's my vb.net code for the struct used to convert to byte array and the sending part.
Public Structure MESSAGETYPE_OIP_Login Dim Header() As COMMANDHEADER Dim UserName() As PSTRING
In my project I need to log all queries executed against my database. As an example we can use the staff user data here. In that class I have a function generating the command with the parameters as follows:
How to use parameterized queries in vb.net? Because I always wanted to make a program that could read an ms access database and display results based on your query.
statement = "SELECT OrderID, (SELECT VendName FROM Vendors WHERE Vendors.VendorID = Orders.VendorID) " & ",OrderDt, RcvdDt, OrderTotal " & "FROM Orders " & "WHERE VendName=? " & "ORDER BY OrderDt DESC" Dim cmd As New OleDbCommand(statement, connection) cmd.Parameters.AddWithValue("VendName", txtVendorFilter.Text) Dim reader As OleDbDataReader = cmd.ExecuteReader(CommandBehavior.Default)
I was trying to do this before by simply concatenating the textbox value right into the SQL and I was getting a "No values given for required parameters", and read that that I should use parameterized queries instead. So I tried this and it doesn't give me any errors, but the reader never has anything in it. I've never used parameterized queries before, so I'm a bit lost as to why this isn't working. I've changed the above code to account for OLEDB from what I briefly read on how it should work, and it's giving me the "no values given for required parameters" again.
The parameterized query '(@AdminEmail nvarchar(4000),@AdminPassword nvarchar(4000))SELECT' expects the parameter '@AdminEmail', which was not supplied.
I am coming from the C# world to VB.NET and this puzzles me. Why are there 2 ways of doing the same thing? or is there some difference I am not aware of? What is the difference between the following: Public ReadOnly Property Test(ByVal v as String) As Integer Get Return SomeOperationOn(v) End Get End Property
And Public Function Test(ByVal v as String) As Integer Return SomeOperationOn(v) End Function When do you use one as opposed to the other?
How do I make a parameterized query in the database with VB.NET? remember to mark the replies Welcome to the All-In-One Code Framework! If you have any feedback,
I'm trying to LINQ two tables based on a dynamic key. User can change key via a combo box. Key may be money, string, double, int, etc. Currently I'm getting the data just fine, but without filtering out the doubles. I can filter the double in VB, but it's slooooow. I'd like to do it in the LINQ query right out of the gate.
[Code]...
"Range variable name can be inferred only from a simple or qualified name with no arguments."
I solved this issue in this thread but there still are issues with special chars. ie 'I thought by doing parameters that it would take care of any special chars for you. Well i am still getting mysql exceptions about syntax. I look at the string it is working with and it has a "'" in it.Why are they not working as I expect them to?
As you may have seen in my other post, I have to switch over an all text SQL statement based database interaction program to using parameterized queries instead.I remember doing it this way in college but I don't seem to be able to recall one part of it. The program I wrote back then and some code samples here both point to doing the code as I did so far:
m_strSQL = "UPDATE [OWNER DATA] SET [FIRST NAME] = @strFName" m_strSQL &= ",[LAST NAME] = @strLName" Dim upCMD As New OleDb.OleDbCommand(m_strSQL, m_Connection) upCMD.Parameters.Add("@strFName", OleDb.OleDbType.Char, 30, "[First Name]")
[Code]...
it ran the SQL statement and that's that. Can I just do that with a parameterized query too without using the adapter.update()? And for bonus points, why the heck did I use that command in my old college program cuz I sure don't know?
I'm having a problem that's driving me crazy; I can't understand how to convert the XML structure into a class structure (that I want to use to hydrate a XML document).
The XML document looks like this:
xml <?xml version="1.0" encoding="utf-8"?> <artists xmlns:opensearch="http://a9.com/-/spec/opensearch/1.1/" xmlns="http://www.spotify.com/ns/music/1">