I have defined a structure in my code and have a list of this structures"
Structure Parcel
Public name As String
Public type As String
End Structure
Dim ParcelList As New List(Of Parcel)
Then I'm trying to set some values to an element of the list which name is known to me
For Each myParcel As Parcel In ParcelList
If (myParcel.name = "Parcel1") Then
myParcel.type="Type1"
End If
Next
Unfortunately values in my list don't change at all. what am I doing wrong?
As Parcel is a Structure, it is passed by value so when iterating through collection, you are modifying a copy of your structure.
To better understand this case, you should understand what For Each really is. Your code can be translated into:
Dim enumerator As List(Of Parcel).Enumerator = ParcelList.GetEnumerator()
While enumerator.MoveNext()
' Here you have a local copy of your Structure
Dim myParcel As Parcel = enumerator.Current
Dim flag As Boolean = Operators.CompareString(myParcel.name, "Parcel1", False) = 0
If flag Then
' Here you modify your local copy
myParcel.type = "Type1"
End If
End While
If Parcel was a Class, it would be passed by reference so no local copy would be created and line myParcel.type = "Type1" would change proper object existing in your collection.
As already Stated this is because you are modifying a local copy of a value type. One way round this is to access the items in the list by ordinal and replace the ordinal value type with a new type:
For i As Integer = 0 To ParcelList.Count - 1
If ParcelList(i).name = "Parcel1" Then
ParcelList(i) = New Parcel With {.name = ParcelList(i).name, .type = "Type1"}
End If
Next
But really you should change the Sturcture to a Class
When checking for strings use Equals instead of '='.
If (myParcel.name.equals("Parcel1")) Then
myParcel.type="Type1"
End If
Strings are in fact 'Objects'. When you compare Strings (example StringA = StringB), you check the allocation of the String in Memory instead of the contents of the string.
Even better would be:
If (myParcel.name.ToUpper().equals(Cstr("Parcel1").toUpper())) Then
myParcel.type="Type1"
End If
That way you ignore any difference case-wise.
example:
myParcel.name = "teST"
myParcel.name.equals("test")
is False
myParcel.name.ToUpper().equals(Cstr("test").toUpper())
is true
Related
I'm trying to pass a dynamic amount of variables to a Sub by using ByRef;
Essentially I'm trying to create a module that I can easily import into my projects and make handling the file saving/loading process automated.
The Sub/Function would take a number of variables as references and then loop through them changing each one's value.
I realize I'm missing a crucial point in how visual basic's syntax works but I haven't been able to figure out what I need to do.
The code I've written for this is:
Public Sub LoadSaveToVars(ByRef KeyNamesAndVars() As Object, ByVal FileLoc As String = "")
If isEven(KeyNamesAndVars.Length) Then
Dim Contents As String = My.Computer.FileSystem.ReadAllText(FileLoc)
Dim isOnName As Boolean = True
Dim CurrentVal As String = ""
For i = 0 To KeyNamesAndVars.Length - 1
If isOnName Then
CurrentVal = GetStringValue(KeyNamesAndVars(i), Contents) 'Get the value of the key with the key name in the array
isOnName = False
Else
KeyNamesAndVars(i) = CurrentVal 'Set the variable referenced in the array to the value
isOnName = True
End If
Next
Else
Throw New ArgumentOutOfRangeException("The key names and variables supplied are not even.", "Error loading to variables!")
End If
End Sub
And here's how I try to use this function:
Dim TestVar1 As String = ""
Dim TestVar2 As String = ""
LoadSaveToVars({"key1", TestVar1, "key2", TestVar2})
To keep this question clean I did not include the other functions, but I did make a poor attempt at drawing what I want to happen: https://gyazo.com/eee34b8dff766401f73772bb0fef981a
In the end, I want TestVar1 to be equal to "val1" and TestVar2 to be equal to "val2" and to be able to extend this to a dynamic number of variables. Is this possible?
I have some code that populates a SOAP request in VB.NET. I get the data from a SQL query and run through the objects members to populate each one. I'm trying to find a quick(ish) way to not provide the member when it is empty rather than checking each value before applying it. As the SQL data comes in from a pipe delimited file in the first place we never get NULL just empty cells which get sent in the SOAP request as "".
Is there a way to define an objects member using a variable rather than its literal name?
UpLB_request_Items.Spare8 = Convert.ToString(SourceDataSet.tables(0).Rows(i)(colSPARE8))
UpLB_request_Items.Spare9 = Convert.ToString(SourceDataSet.tables(0).Rows(i)(colSPARE9))
UpLB_request_Items.Spare10 = Convert.ToString(SourceDataSet.tables(0).Rows(i)(colSPARE10))
Call IterateObject(UpLB_request_Items)
So this populates each member with the value from the SQL data (SourceDataSet), then I could send the completed object down to a Sub to check each value before I actually send it to the webservice
Sub IterateObject(objName)
Dim CollName = ""
For Each m As System.Reflection.PropertyInfo In objName.GetType().GetProperties()
If m.CanRead Then
If m.PropertyType.Name = "String" Then
CollName = m.Name
Dim CollVal = CallByName(objName, CollName, CallType.Get)
If CollVal = "" Then
objName.CollName = Nothing 'this is the tricky bit
End If
End If
End If
Next
End Sub
The bit where it obviously breaks is objName.CollName = Nothing as it will then say that the object doesn't have a member called CollName and I'm not sure if there is a way to force the code to evaluate CollName to its value (e.g. SPARE8 rather than the string "CollName")
I went with this in the end
UpLB_request_Items.Spare4 = Strings.Replace(Convert.ToString(SourceDataSet.Tables(0).Rows(i)(colSPARE4)), "", Nothing)
The code I have is:
Dim Dbase() As String = Nothing
Dbase(0) = Db_ComboBox.Text
I have declared Dbase as array and assigned Nothing, Db_ComboBox is a combobox.
For that assignment statement, I'm getting the following error: "Reference 'Dbase' has a value of 'Nothing'"
What is the reason for this error, and how can I take the value from the combobox and save it in the array?
You need to change this:
Dim Dbase() As String = Nothing
to this (declare an array of 1 element):
Dim Dbase(0) As String
And then this line will work:
Dbase(0) = Db_ComboBox.Text
If you need to change your array size you can use Redim or Redim preserve, as required.
If you anticipate contents of Dbase to change often, I am all with #Joel's suggestion about switching to List(Of String) instead of handling array sizes manually.
Let's look at your code:
Dim Dbase() As String = Nothing
Dbase(0) = Db_ComboBox.Text
Especially the first line. That first line creates a variable that can refer to an array, but the = Nothing portion explicitly tells it, "Do not create a real array here yet". You have, effectively, a pointer that doesn't point to anything.
I get here that what you really need is a List collection that you can append to over time:
Dim Dbase As New List(Of String)()
Dbase.Add(Db_ComboBox.Text)
Dbase() IS NOTHING. Look at this example:
cargoWeights = New Double(10) {}
atmospherePressures = New Short(2, 2, 4, 10) {}
inquiriesByYearMonthDay = New Byte(20)()() {}
That's how you declare arrays.
More examples: http://msdn.microsoft.com/en-us/library/vstudio/wak0wfyt.aspx
Public Structure testStruct
Dim blah as integer
Dim foo as string
Dim bar as double
End Structure
'in another file ....
Public Function blahFooBar() as Boolean
Dim tStrList as List (Of testStruct) = new List (Of testStruct)
For i as integer = 0 To 10
tStrList.Add(new testStruct)
tStrList.Item(i).blah = 1
tStrList.Item(i).foo = "Why won't I work?"
tStrList.Item(i).bar = 100.100
'last 3 lines give me error below
Next
return True
End Function
The error I get is: Expression is a value and therefore cannot be the target of an assignment.
Why?
I second the opinion to use a class rather than a struct.
The reason you are having difficulty is that your struct is a value type. When you access the instance of the value type in the list, you get a copy of the value. You are then attempting to change the value of the copy, which results in the error. If you had used a class, then your code would have worked as written.
try the following in your For loop:
Dim tmp As New testStruct()
tmp.blah = 1
tmp.foo = "Why won't I work?"
tmp.bar = 100.100
tStrList.Add(tmp)
Looking into this I think it has something to do with the way .NET copies the struct when you access it via the List(of t).
More information is available here.
Try creating the object first as
Dim X = New testStruct
and setting the properties on THAT as in
testStruct.blah = "fiddlesticks"
BEFORE adding it to the list.
I have a user-defined structure in a list that I am trying to change the value for in an individual element within the list of structures. Accessing the element is not a problem. However, when I try to update the value, the compiler complains:
"Expression is a value and therefore cannot be the target of the
assignment"
For example:
Public Structure Person
Dim first as String
Dim last as String
Dim age as Integer
End Structure
_
Public Sub ListTest()
Dim newPerson as Person
Dim records as List (Of Person)
records = new List (Of Person)
person.first = "Yogi"
person.last = "bear"
person.age = 35
records.Add(person)
records(0).first = "Papa" ' <<== Causes the error
End Sub
As the other comments said, when you refer to records(0), you get a copy of the struct since it is a value type. What you can do (if you can't change it to a Class) is something like this:
Dim p As Person = records(0)
p.first = "Papa"
records(0) = p
Although, I think it's just easier to use a Class.
There are actually two important concepts to remember here.
One is that, as Hans and Chris have pointed out, Structure Person declares a value type of which copies are passed between method calls.
You can still access (i.e., get and set) the members of a value type, though. After all, this works:
Dim people(0) As Person
people(0).first = "Yogi"
people(0).last = "Bear"
people(0).age = 35
So the other important point to realize is that records(0) accesses the List(Of Person) class's special Item property, which is a sugary wrapper around two method calls (a getter and setter). It is not a direct array access; if it were (i.e., if records were an array), your original code would actually have worked.
I had the same problem, and I fixed it by adding a simple Sub to the structure that changes the value of the property.
Public Structure Person
Dim first as String
Dim last as String
Dim age as Integer
Public Sub ChangeFirst(value as String)
me.first = value
End Sub
End Structure