VB2010. I am trying to populate a ComboBox with the contents of an Enumeration of units. I have managed to do this with a Dictionary. Something like
Dim dUnits As New Dictionary(Of String, Integer)
Dim da As String
For Each enumValue As eUnits In System.Enum.GetValues(GetType(eUnits))
da = ConvertEnumToCommonName 'gets unique name for an enumeration
dUnits.Add(da, enumValue)
Next
cbo.DisplayMember = "Key" 'display the the common name
cbo.ValueMember = "Value" 'use the enumeration as the value
cbo.DataSource = New BindingSource(dUnits, Nothing)
When I load up my form that works well. Now the user can choose to select a default unit to display. So then I try
Dim defUnits As eUnits = eUnits.Feet
Dim idx As Integer = cbo.Items.IndexOf(defUnits) 'doesnt work, returns a -1
cbo.SelectedIndex = idx
I have been doing some research for some time and am fairly sure that this has to do with the ComboBox storing Values as a string and in reality I'm searching for an enumeration which is an integer. Don't know if I have that right or not. Anyway, I just cant seem to get the default item selected. Is there another approach I can try?
First of all you have a collection of integers and you're searching for the enum value. For that, try one of the following:
Store the enum value in the dictionary instead of a string:
Dim dUnits As New Dictionary(Of String, eUnits)
Keep the integers in the Dictionary but use the integer value of the enum when searching the ComboBox:
Dim idx As Integer = cbo.Items.IndexOf(CInt(defUnits))
But this is not going to work yet. You are data-bound to a Dictionary, which means the items in cbo.Items are not of the enum type, but of the type of the elements in the Dictionary (KeyValuePair(Of String, eUnits) assuming #1 above).
The easiest solution is just to set the SelectedValue property of the combo box instead of the SelectedIndex. Assuming you used option #1 above, this would be:
cbo.SelectedValue = defUnits
If you used option #2 instead, you'll have to convert this to an integer first:
cbo.SelectedValue = CInt(defUnits)
Related
Needed code is something like this:
Dim myArray(0) As String
Dim ay As String = "ay"
myArr & ay(0) = "asd"
I've tried but did not worked
Dim classlist1(0) As String
Dim classlist2(0) As String
Dim classlist3(0) As String
Dim classlist4(0) As String
Dim count As Integer = 0
For _year As Integer = 1 To 4
("classlist" & _year)(count) = "hi"
count += 1
Next
Any time you see something like this:
Dim classlist1(0) As String
Dim classlist2(0) As String
Dim classlist3(0) As String
' etc.
It's an indication that you're using the wrong data structure. Instead of trying to dynamically build variable names (which isn't really possible in a static language, at least not without some really ugly reflection code with a high potential for runtime errors), just use a collection.
For example, if you want a collection of strings:
Dim classList As New List(Of String)()
And if you want a collection of collections of strings:
Dim classLists As New List(Of List(Of String))()
Then you can reference the nested lists within the parent list. So to add your first "year" of classes:
classLists.Add(new List(Of String))
And add a class to that year:
classLists(0).Add("some value")
As you can see, it starts to get a little difficult to keep track of the data structures. This is where creating custom types and structures becomes very useful. For example, rather than representing a "year" as a list of strings, create an actual Year class. That class can internally hold a list of strings, and other logic/data.
Try Dictionary<TKey, TValue> Class From MSDN.
Dim classLists As New Dictionary(Of String, String)()
'Add items with keys
For _year As Integer = 1 To 4
classLists.Add(String.Format("classlist{0}",_year), "hi")
Next
And you can get value by key later
Dim key As String = "classlist2"
Dim value As String = classLists(key)
I have need in a module for a dictionary of dictionaries but for some reason it is not producing the results That I was expecting. I have made up an example that illustrates the problem , shown below.
Dim mydict As New Dictionary(Of Int32, Dictionary(Of Int32, Decimal))
Dim outerkey As Integer = 2
Dim innerkey As Integer = 7
Dim innerDictionary As Dictionary(Of Int32, Decimal) = Nothing
Dim innerkeyvalue As Decimal = CDec(12.3)
If Not mydict.TryGetValue(outerkey, innerDictionary) Then
' ' So we need to create one
innerDictionary = New Dictionary(Of Int32, Decimal)
mydict.Add(outerkey, innerDictionary)
End If
If Not innerDictionary.TryGetValue(innerkey, innerkeyvalue) Then
' So we need to create it
innerDictionary.Add(innerkey, innerkeyvalue)
Else
Dim retrievedvalue As Decimal = innerDictionary.Item(innerkey)
innerkeyvalue += retrievedvalue
innerDictionary.Add(innerkey, innerkeyvalue)
End If
If I place a breakpoint on the 'If Not innerDictionary.TryGetValue(innerkey, innerkeyvalue) Then
' line the value of innerkey is 7 and the value of innerkeyvalue is 12.3, which is exactly as I would expect. Those values aren't yet in the inner dictionary so I would then expect the code to go to the next line (which it does) to add those values to the inner dictionary.
What happens though is that the value of the innerkey remains as it should (7) but the value of innervalue changes from 12.3 to 0.
I'm obviously doing something stupid, or I have failed to understandstand a fundamental principle of dictionaries within dictionaries. Either way I'd be grateful if someone could explain why I'm seeing what I'm seeing.
if you read the MSDN doc
if the key is not found, then the value parameter gets the appropriate default value for the type TValue; for example, 0 (zero) for integer types, false for Boolean types, and null for reference types.
default value for decimal is 0
you would need another variable for the second parameter of the trygetvalue
I know that it is not possible to get keys (How to get the Key and Value from a Collection VB.Net) and it is better to use other classes. However I am debugging 10 years old code and I cannot change it.
In the Watch Window I see:
So it seems to be possible to deal the collection as "List(of KeyValuePair)". Can I do it in code or is it only an internal translation.
I basically need to list all KEYs in a Collection.
VisualBasic.Collection has an undocumented private method InternalItemsList which allows reading keys besides values. The type of InternalItemsList is Microsoft.VisualBasic.Collection.FastList which does not seem to be Enumerable but has a method Item which returns items with a zero based index. The type of those items is Microsoft.VisualBasic.Collection.Node. An item has two private properties m_Value and m_Key. And here we are.
Following code illustrates how to convert VisualBasic.Collection to List(Of KeyValuePair.
Dim flg As BindingFlags = BindingFlags.Instance Or BindingFlags.NonPublic
Dim InternalList As Object = col.GetType.GetMethod("InternalItemsList", flg).Invoke(col, Nothing)
Dim res As New List(Of KeyValuePair(Of String, Object))
For qq As Integer = 0 To col.Count - 1
Dim Item As Object = InternalList.GetType.GetProperty("Item", flg).GetValue(InternalList, {qq})
Dim Key As String = Item.GetType.GetField("m_Key", flg).GetValue(Item)
Dim Value As Object = Item.GetType.GetField("m_Value", flg).GetValue(Item)
res.Add(New KeyValuePair(Of String, Object)(Key, Value))
Next
I know that using undocumented functions is not safe but it is for internal use only and only solution I have managed to find.
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
How do you get the key value from a vb.net collection when iterating through it?
Dim sta As New Collection
sta.Add("New York", "NY")
sta.Add("Michigan", "MI")
sta.Add("New Jersey", "NJ")
sta.Add("Massachusetts", "MA")
For i As Integer = 1 To sta.Count
Debug.Print(sta(i)) 'Get value
Debug.Print(sta(i).key) 'Get key ?
Next
Pretty sure you can't from a straight Microsoft.VisualBasic.Collection.
For your example code above, consider using a System.Collections.Specialized.StringDictionary. If you do, be aware that the Add method has the parameters reversed from the VB collection - key first, then value.
Dim sta As New System.Collections.Specialized.StringDictionary
sta.Add("NY", "New York")
'...
For Each itemKey in sta.Keys
Debug.Print(sta.Item(itemKey)) 'value
Debug.Print(itemKey) 'key
Next
I don't recommend using the Collection class, as that is in the VB compatibility library to make migrating VB6 programs easier. Replace it with one of the many classes in the System.Collections or System.Collections.Generic namespace.
It is possible to get a key with using Reflection.
Private Function GetKey(Col As Collection, Index As Integer)
Dim flg As BindingFlags = BindingFlags.Instance Or BindingFlags.NonPublic
Dim InternalList As Object = Col.GetType.GetMethod("InternalItemsList", flg).Invoke(Col, Nothing)
Dim Item As Object = InternalList.GetType.GetProperty("Item", flg).GetValue(InternalList, {Index - 1})
Dim Key As String = Item.GetType.GetField("m_Key", flg).GetValue(Item)
Return Key
End Function
Not using VB.Collection is recommended but sometimes we are dealing with code when it was used in past. Be aware that using undocumented private methods is not safe but where is no other solution it is justifiable.
More deailed information can be found in SO: How to use reflection to get keys from Microsoft.VisualBasic.Collection
Yes, it may well, but I want recomend that you use another Collection.
How to do you do with Reflection, the type Microsoft.VisualBasic.Collection contains some private fields, the field one should use in this case is "m_KeyedNodesHash" the field, and the field type is System.Collections.Generic.Dictionary(Of String, Microsoft.VisualBasic.Collection.Node), and it contains a property called "Keys", where the return type is System.Collections.Generic.Dictionary(Of String, Microsoft.VisualBasic.Collection.Node).KeyCollection, and the only way to get a certain key is to convert it to type IEnumerable(Of String), and the call ElementAt the function.
Private Function GetKey(ByVal col As Collection, ByVal index As Integer)
Dim listfield As FieldInfo = GetType(Collection).GetField("m_KeyedNodesHash", BindingFlags.NonPublic Or BindingFlags.Instance)
Dim list As Object = listfield.GetValue(col)
Dim keylist As IEnumerable(Of String) = list.Keys
Dim key As String = keylist.ElementAt(index)
Return key
End Function