I know strings are immutable, so the minute you change a string reference's value .NET makes a brand new string on the heap.
But what if you don't change the value of a string reference; rather, you simply pass it into a function ByVal -- does this operation copy the string value on the heap as well? My inclination is "no," but I'd like to confirm.
For example:
Public Function IsStringHello(ByVal test As String) As Boolean
Return (String.Compare(test, "Hello") = 0)
End Function
Calling program:
Dim myWord as String = "Blah"
Dim matchesHello as Boolean = IsStringHello(myWord)
I know passing myWord by value makes a copy of the reference to "Blah", but since I have not tried to change the string itself, would it make another copy of the string on the heap?
By the way, string interning is completely unrelated to that. The rule for passing parameters to functions is the same for all reference types (and really, all types), no matter how they are managed internally.
The rule is simple and you have stated it correctly: pass by value copies the reference, not the target. No heap space is copied here.
No. it still uses the copy of the reference to the "Blah".
What makes you think, it will?
On a side note, string are interned.
string s = "hello";
string t = "hello";
s & t both refer to the same string (because it is interned). If you modify s or t, it will create a new string, then.
Passing objects ByVal creates a copy of the pointer, not the object itself. Here's a demonstration:
Module Module1
Dim original As String = "Hello world"
Sub PassByReferenceTest(ByVal other As String)
Console.WriteLine("object.ReferenceEquals(original, other): {0}", _
Object.ReferenceEquals(original, other))
End Sub
Sub Main()
PassByReferenceTest(original)
Console.ReadKey(True)
End Sub
End Module
This program outputs the following:
object.ReferenceEquals(original, other): True
So, the original string and the string we passed by value exist at the same address in memory address. You're not making a copy of the string itself.
Is short, no. It passes a ref to the string. Only one instance of the string itself.
string is a reference type. If you pass it by value, what you are passing is the value of the reference.
The only way you'd get another copy on the heap would be to change the variable's value.
A variable of type System.String effectively holds an "object-ID". Suppose that Object #1934 is a string with the characters "Blah", and you say Dim myWord As String = "Blah". The compiler will then store Object #1934 into myWord. Calling IsStringHello(myWord) would then cause that function to be called with its test parameter equal to Object #1934. In your example, there would be two variables of type System.String in memory--myWord and test, and both would hold the content Object #1934.
Related
I've heard advice to change from User Defined Type (UDT) to a regular Class in order to overcome the limitations of UDT, such as not being able to use For Each with a UDT.
I've also heard advice to change from a regular Class to UDT to overcome the Class limitation where you can't pass things BYREF, like...
'Function:
Public Function RemoveArticle (ByRef strMovieTitle As String)
'Expected input is like "Terminator, The"
strMovieTitle = Left(... 'removes the article.
End Function
That works fine for this call:
Dim strMovieTitle As String
strMovieTitle = "Terminator, The"
RemoveArticle strMovieTitle
But not this call:
Dim objMovie As MovieClass
objMovie.strMovieTitle = "Terminator, The"
objMovie.strMovieGenre = "Sci-Fi"
InvertArticle objMovie.strMovieTitle
Even though MovieClass defines
strMovieTitle As String
I can't go changing RemoveArticle (and every simple little function like it) to take a MovieClass parameter instead of a String parameter because there are other UDTs or Classes and String Variables that also need to use RemoveArticle.
What do I do if I need to use For Each and I also need to pass ByRef?
Is there a way a Class can work around the parameter problem?
(Using Excel 2010.)
Now I have understood your concern.
You simply can't take that approach to meet your goal. As Tim Williams has commented in your question, your best bet would be something like this:
Dim objMovie As MovieClass
Dim strMovieTitle As String
strMovieTitle = "Terminator, The"
objMovie.strMovieTitle = InvertArticle(strMovieTitle)
However, I see that this still does not satisfy your need.
My suggestion is as follows:
make your object internal, target properties Private and expose them with Property Let and Property Get. This way you can do the modifications you want to the properties either on set or on get (from within the class... rather than fixing things from outside the class).
Aside note, in regards to create a helper class (as someone has recommended to you): you could join into one class all those functions you use widely, such as RemoveArticle or InvertArticle. However, it requires to create an instance object every time you want to use them and, therefore, does not combine well with the recommendation I am giving to you (if you want just to simplify code). So having them in a Module as you do now is fine. Just to clarify: those recommendations they gave to you are unrelated to your question here.
Example A: on set
In you class MovieClass, rename first all the instances of strMovieTitle to pStrMovieTitle and add this to your code:
Private pStrMovieTitle As String
Public Property Let strMovieTitle (strIn As String)
pStrMovieTitle = InvertArticle(strIn)
End Property
Public Property Get strMovieTitle As String
strMovieTitle = pStrMovieTitle
End Property
The usage would be something like this:
Dim objMovie As MovieClass
objMovie.strMovieTitle = "Terminator, The" ' the article position gets rectified on assignation
objMovie.strMovieGenre = "Sci-Fi"
'InvertArticle objMovie.strMovieTitle ' => you don't need to do this call
Example B: on get
To keep your original string as it comes, and do apply your helpers when you get the property value. That way you always preserve the original string. However, this approach will need more rework and it's only worthy in cases where you have lots of ways to use that String in different parts of your code.
Private pStrMovieTitleSource As String
Public Property Let strMovieTitle (strIn As String)
pStrMovieTitleSource = Trim(strIn)
End Property
Public Property Get strMovieTitleSource () As String
strMovieTitleSource = pStrMovieTitleSource
End Property
Public Property Get strMovieTitleRoot () As String
strMovieTitleRoot = RemoveArticle(pStrMovieTitleSource)
End Property
Public Property Get strMovieTitle () As String
strMovieTitle = InvertArticle(pStrMovieTitleSource)
End Property
Hope it helps
I have read that String was a "reference type", unlike integers. MS website
I tried to test its behavior.
Sub Main()
Dim s As New TheTest
s.TheString = "42"
Dim z As String = s.GimmeTheString
z = z & "000"
Dim E As String = s.TheString
s.GimmeByref(z)
end sub
Class TheTest
Public TheString As String
Public Function GimmeTheString() As String
Return TheString
End Function
Public Sub GimmeByref(s As String)
s = TheString
End Sub
End Class
So I expected :
z is same reference as TheString, thus TheString would be set to "42000"
Then Z is modified by reference by GimmeByref thus Z is set to whatever TheString is
Actual result:
Z="42000"
E="42"
TheString="42"
What point am I missing?
I also tried adding "ByRef" in GimmeByRef : yes obviously the GimmeByRef does work as expected, but it also does if I put everything as Integer, which are said to be "Value type".
Is there any actual difference between those types?
The confusion comes about because regardless of type, argument passing in VB is pass by value by default.
If you want to pass an argument by reference, you need to specify the argument type as ByRef:
Public Sub GimmeByref(ByRef s As String)
You also need to understand the difference between mutating a value and re-assigning a variable. Doing s = TheString inside the method doesn’t mutate the value of the string, it reassigns s. This can obviously be done regardless of whether a type is a value or reference type.
The difference between value and reference types comes to bear when modifying the value itself, not a variable:
obj.ModifyMe()
Strings in .NET are immutable and thus don’t possess any such methods (same as integers). However, List(Of String), for instance, is a mutable reference type. So if you modify an argument of type List(Of String), even if it is passed by value, then the object itself is modified beyond the scope of the method.
Strings are immutable, every time you do a change it creates a new "reference" like if New was called.
A String object is called immutable (read-only), because its value
cannot be modified after it has been created. Methods that appear to
modify a String object actually return a new String object that
contains the modification. Ref
Your code basically does something like this:
Sub Main()
Dim a, b As String
a = "12"
b = a
a = a & "13"
Console.WriteLine(a) ' 1213
Console.WriteLine(b) ' 12
Console.ReadLine()
End Sub
I would like to to copy that values of an array into a Structure.
Example:
' The Array
Dim Columns(2) As String
' The Structure
Private Structure Fields
Public FName As String
Public LName As String
Public Email As String
End Structure
' I would like to map it like so:
Fields.FName = Columns(0)
Fields.LName = Columns(1)
Fields.Email = Columns(2)
Obviously I could write a function if it was so simple, but really there are over 25 columns and it's a pain to write a function that would map it.
Is there some way to do this?
There really is no simple way that will work in all cases. What you are complaining is too much effort is the only way to guarantee that it will work in all cases.
That said, if you can guarantee that the number of elements in the array matches the number of properties/fields in the structure/class and that they are in the same order and of the same types then you could use Reflection in a loop, e.g.
Private Function Map(source As Object()) As SomeType
Dim result As New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return result
End Function
EDIT:
The code I have provided works as is if SomeType is a class but, as I missed the first time around, not for a structure. The reason is that structures are value types and therefore a copy of the original object is being sent to SetValue, so the field value never gets set on that original object. In theory, to prevent a copy being created, you should be able to simply box the value, i.e. wrap it in an Object reference:
Private Function Map(source As Object()) As SomeType
Dim result As Object = New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return DirectCast(result, SomeType)
End Function
As it turns out though, the VB compiler treats that a little differently than the C# compiler treats the equivalent C# code and it still doesn't work. That's because, in VB, the boxed value gets unboxed before being passed to the method, so a copy is still created. In order to make it work in VB, you need to use a ValueType reference instead of Object:
Private Function Map(source As Object()) As SomeType
Dim result As ValueType = New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return DirectCast(result, SomeType)
End Function
I have written the following code in VB.NET:
Dim obj As Object
obj = "00"
Test(obj)
MsgBox(obj)
Private Sub Test(ByRef num As Integer)
End Sub
Private Sub Test(ByVal num As Integer)
End Sub
When the value "00" is passed "ByRef" in the method "Test" it converts to 0. But if the value "00" is passed "ByVal" it keeps the same value as "00". How the passed value is being converted only depending of the signature?
In VB6 although the default passing type is "ByRef", still the same code keeps the same value("00")
Could anybody explain the reason behind this contradictory behaviour in VB6 and VB.NET?
The way you are doing it, the ByRef changes the type of the object from string to integer. By default, integer do not have trailling "0" when covnerted to strings.
This example below might help you understand what is hapenning.
Sub Main()
Dim o1 As Object = "00"
Dim o2 As Object = "00"
Console.WriteLine(o1.GetType().ToString())
Test1(o1)
Console.WriteLine(o1.GetType().ToString())
Console.WriteLine(o2.GetType().ToString())
Test2(o2)
Console.WriteLine(o2.GetType().ToString())
Console.ReadLine()
End Sub
Sub Test1(ByVal num As Integer)
End Sub
Sub Test2(ByRef num As Integer)
End Sub
Output
System.String
System.String
System.String
System.Int32
I suggest you always turn Option Strict On, this will remove a lot of confusion.
The object is of type System.String. It cannot be passed ByRef to a method, it is of the wrong type. So the compiler has to work around it and rewrites the code:
Dim obj As Object
obj = "00"
Dim $temp As Integer
$temp = CInt(obj)
Test($temp)
obj = $temp '' <=== Here
MsgBox(obj)
The indicated statement is the one that changes the object from a string to an integer. Which, converted again to a string by the MsgBox() call, produces "0" instead of "00".
Notable is that C# does not permit this and generate a compile error. This rewriting trick is rather nasty, if the method itself changes the original object then you'll have a very hard time guessing what is going on since that doesn't change the passed argument value.
ByRef means that value passes by reference and in function will be used the same value what has been sent.
ByVal means that value passes by value (function creates a copy of passed value) and you use only copy of value.
I have to create a class in Visual Basic called StringWork.
Public Class StringWork
Now i wrote a shared function in the class called Working that can take a string or a string and Boolean.
Public Shared Function Working(ByVal SingleString as string, optional BValu as Boolean = true)as string
if(working(SingleString))then
'The handling of the string
else if (working(SingleValue, BValue) then
'do something else with string
end if
end function
The function I wrote is returning a string.
Can I access the string passed and edit characters in the string or change the position of characters?
You use that optional parameter to determine how you handle that string:
Public Shared Function Working(ByVal singleString as string, _
Optional bValue as Boolean = True) As String
If bValue Then
'Handle the true part manipulating the result string
Else
'Handle the false part manipulating the result string
End If
End Function
If you call this function like this:
Dim test As String = StringWork.Working("I am Spartacus")
it will call that Working function with bValue = true.
What bValue is suppose to represent isn't very clear from the code nor the post.
In VB.NET, and other .NET languages, strings are immutable. Typically, if you need to modify a string that is passed to your method, you would return the modified string. If however, you need it to modify the parameter, you can specify that it is a "ByRef" argument, in which case you will be able to set it to point to a new string object which will affect the variable that was passed into the method as a parameter. If you need a truly mutable string, you will need a character array or a StringBuilder object.