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I have learned quite a bit browsing through Hidden Features
of C# and was surprised when I couldn't find something
similar for VB.NET.
So what are some of its hidden or lesser known features?
The Exception When clause is largely unknown.
Consider this:
Public Sub Login(host as string, user as String, password as string, _
Optional bRetry as Boolean = False)
Try
ssh.Connect(host, user, password)
Catch ex as TimeoutException When Not bRetry
''//Try again, but only once.
Login(host, user, password, True)
Catch ex as TimeoutException
''//Log exception
End Try
End Sub
Custom Enums
One of the real hidden features of VB is the completionlist XML documentation tag that can be used to create own Enum-like types with extended functionality. This feature doesn't work in C#, though.
One example from a recent code of mine:
'
''' <completionlist cref="RuleTemplates"/>
Public Class Rule
Private ReadOnly m_Expression As String
Private ReadOnly m_Options As RegexOptions
Public Sub New(ByVal expression As String)
Me.New(expression, RegexOptions.None)
End Sub
Public Sub New(ByVal expression As String, ByVal options As RegexOptions)
m_Expression = expression
m_options = options
End Sub
Public ReadOnly Property Expression() As String
Get
Return m_Expression
End Get
End Property
Public ReadOnly Property Options() As RegexOptions
Get
Return m_Options
End Get
End Property
End Class
Public NotInheritable Class RuleTemplates
Public Shared ReadOnly Whitespace As New Rule("\s+")
Public Shared ReadOnly Identifier As New Rule("\w+")
Public Shared ReadOnly [String] As New Rule("""([^""]|"""")*""")
End Class
Now, when assigning a value to a variable declared as Rule, the IDE offers an IntelliSense list of possible values from RuleTemplates.
/EDIT:
Since this is a feature that relies on the IDE, it's hard to show how this looks when you use it but I'll just use a screenshot:
Completion list in action http://page.mi.fu-berlin.de/krudolph/stuff/completionlist.png
In fact, the IntelliSense is 100% identical to what you get when using an Enum.
Have you noticed the Like comparison operator?
Dim b As Boolean = "file.txt" Like "*.txt"
More from MSDN
Dim testCheck As Boolean
' The following statement returns True (does "F" satisfy "F"?)'
testCheck = "F" Like "F"
' The following statement returns False for Option Compare Binary'
' and True for Option Compare Text (does "F" satisfy "f"?)'
testCheck = "F" Like "f"
' The following statement returns False (does "F" satisfy "FFF"?)'
testCheck = "F" Like "FFF"
' The following statement returns True (does "aBBBa" have an "a" at the'
' beginning, an "a" at the end, and any number of characters in '
' between?)'
testCheck = "aBBBa" Like "a*a"
' The following statement returns True (does "F" occur in the set of'
' characters from "A" through "Z"?)'
testCheck = "F" Like "[A-Z]"
' The following statement returns False (does "F" NOT occur in the '
' set of characters from "A" through "Z"?)'
testCheck = "F" Like "[!A-Z]"
' The following statement returns True (does "a2a" begin and end with'
' an "a" and have any single-digit number in between?)'
testCheck = "a2a" Like "a#a"
' The following statement returns True (does "aM5b" begin with an "a",'
' followed by any character from the set "L" through "P", followed'
' by any single-digit number, and end with any character NOT in'
' the character set "c" through "e"?)'
testCheck = "aM5b" Like "a[L-P]#[!c-e]"
' The following statement returns True (does "BAT123khg" begin with a'
' "B", followed by any single character, followed by a "T", and end'
' with zero or more characters of any type?)'
testCheck = "BAT123khg" Like "B?T*"
' The following statement returns False (does "CAT123khg" begin with'
' a "B", followed by any single character, followed by a "T", and'
' end with zero or more characters of any type?)'
testCheck = "CAT123khg" Like "B?T*"
Typedefs
VB knows a primitive kind of typedef via Import aliases:
Imports S = System.String
Dim x As S = "Hello"
This is more useful when used in conjunction with generic types:
Imports StringPair = System.Collections.Generic.KeyValuePair(Of String, String)
Oh! and don't forget XML Literals.
Dim contact2 = _
<contact>
<name>Patrick Hines</name>
<%= From p In phoneNumbers2 _
Select <phone type=<%= p.Type %>><%= p.Number %></phone> _
%>
</contact>
Object initialization is in there too!
Dim x as New MyClass With {.Prop1 = foo, .Prop2 = bar}
DirectCast
DirectCast is a marvel. On the surface, it works similar to the CType operator in that it converts an object from one type into another. However, it works by a much stricter set of rules. CType's actual behaviour is therefore often opaque and it's not at all evident which kind of conversion is executed.
DirectCast only supports two distinct operations:
Unboxing of a value type, and
upcasting in the class hierarchy.
Any other cast will not work (e.g. trying to unbox an Integer to a Double) and will result in a compile time/runtime error (depending on the situation and what can be detected by static type checking). I therefore use DirectCast whenever possible, as this captures my intent best: depending on the situation, I either want to unbox a value of known type or perform an upcast. End of story.
Using CType, on the other hand, leaves the reader of the code wondering what the programmer really intended because it resolves to all kinds of different operations, including calling user-defined code.
Why is this a hidden feature? The VB team has published a guideline1 that discourages the use of DirectCast (even though it's actually faster!) in order to make the code more uniform. I argue that this is a bad guideline that should be reversed: Whenever possible, favour DirectCast over the more general CType operator. It makes the code much clearer. CType, on the other hand, should only be called if this is indeed intended, i.e. when a narrowing CType operator (cf. operator overloading) should be called.
1) I'm unable to come up with a link to the guideline but I've found Paul Vick's take on it (chief developer of the VB team):
In the real world, you're hardly ever going to notice the difference, so you might as well go with the more flexible conversion operators like CType, CInt, etc.
(EDIT by Zack: Learn more here: How should I cast in VB.NET?)
If conditional and coalesce operator
I don't know how hidden you'd call it, but the Iif([expression],[value if true],[value if false]) As Object function could count.
It's not so much hidden as deprecated! VB 9 has the If operator which is much better and works exactly as C#'s conditional and coalesce operator (depending on what you want):
Dim x = If(a = b, c, d)
Dim hello As String = Nothing
Dim y = If(hello, "World")
Edited to show another example:
This will work with If(), but cause an exception with IIf()
Dim x = If(b<>0,a/b,0)
This is a nice one. The Select Case statement within VB.Net is very powerful.
Sure there is the standard
Select Case Role
Case "Admin"
''//Do X
Case "Tester"
''//Do Y
Case "Developer"
''//Do Z
Case Else
''//Exception case
End Select
But there is more...
You can do ranges:
Select Case Amount
Case Is < 0
''//What!!
Case 0 To 15
Shipping = 2.0
Case 16 To 59
Shipping = 5.87
Case Is > 59
Shipping = 12.50
Case Else
Shipping = 9.99
End Select
And even more...
You can (although may not be a good idea) do boolean checks on multiple variables:
Select Case True
Case a = b
''//Do X
Case a = c
''//Do Y
Case b = c
''//Do Z
Case Else
''//Exception case
End Select
One major time saver I use all the time is the With keyword:
With ReallyLongClassName
.Property1 = Value1
.Property2 = Value2
...
End With
I just don't like typing more than I have to!
The best and easy CSV parser:
Microsoft.VisualBasic.FileIO.TextFieldParser
By adding a reference to Microsoft.VisualBasic, this can be used in any other .Net language, e.g. C#
AndAlso/OrElse logical operators
(EDIT: Learn more here: Should I always use the AndAlso and OrElse operators?)
Static members in methods.
For example:
Function CleanString(byval input As String) As String
Static pattern As New RegEx("...")
return pattern.Replace(input, "")
End Function
In the above function, the pattern regular expression will only ever be created once no matter how many times the function is called.
Another use is to keep an instance of "random" around:
Function GetNextRandom() As Integer
Static r As New Random(getSeed())
Return r.Next()
End Function
Also, this isn't the same as simply declaring it as a Shared member of the class; items declared this way are guaranteed to be thread-safe as well. It doesn't matter in this scenario since the expression will never change, but there are others where it might.
In vb there is a different between these operators:
/ is Double
\ is Integer ignoring the remainder
Sub Main()
Dim x = 9 / 5
Dim y = 9 \ 5
Console.WriteLine("item x of '{0}' equals to {1}", x.GetType.FullName, x)
Console.WriteLine("item y of '{0}' equals to {1}", y.GetType.FullName, y)
'Results:
'item x of 'System.Double' equals to 1.8
'item y of 'System.Int32' equals to 1
End Sub
I really like the "My" Namespace which was introduced in Visual Basic 2005. My is a shortcut to several groups of information and functionality. It provides quick and intuitive access to the following types of information:
My.Computer: Access to information related to the computer such as file system, network, devices, system information, etc. It provides access to a number of very important resources including My.Computer.Network, My.Computer.FileSystem, and My.Computer.Printers.
My.Application: Access to information related to the particular application such as name, version, current directory, etc.
My.User: Access to information related to the current authenticated user.
My.Resources: Access to resources used by the application residing in resource files in a strongly typed manner.
My.Settings: Access to configuration settings of the application in a strongly typed manner.
Custom Events
Though seldom useful, event handling can be heavily customized:
Public Class ApplePie
Private ReadOnly m_BakedEvent As New List(Of EventHandler)()
Custom Event Baked As EventHandler
AddHandler(ByVal value As EventHandler)
Console.WriteLine("Adding a new subscriber: {0}", value.Method)
m_BakedEvent.Add(value)
End AddHandler
RemoveHandler(ByVal value As EventHandler)
Console.WriteLine("Removing subscriber: {0}", value.Method)
m_BakedEvent.Remove(value)
End RemoveHandler
RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
Console.WriteLine("{0} is raising an event.", sender)
For Each ev In m_BakedEvent
ev.Invoke(sender, e)
Next
End RaiseEvent
End Event
Public Sub Bake()
''// 1. Add ingredients
''// 2. Stir
''// 3. Put into oven (heated, not pre-heated!)
''// 4. Bake
RaiseEvent Baked(Me, EventArgs.Empty)
''// 5. Digest
End Sub
End Class
This can then be tested in the following fashion:
Module Module1
Public Sub Foo(ByVal sender As Object, ByVal e As EventArgs)
Console.WriteLine("Hmm, freshly baked apple pie.")
End Sub
Sub Main()
Dim pie As New ApplePie()
AddHandler pie.Baked, AddressOf Foo
pie.Bake()
RemoveHandler pie.Baked, AddressOf Foo
End Sub
End Module
I just found an article talking about the "!" operator, also know as the "dictionary lookup operator". Here's an excerpt from the article at: http://panopticoncentral.net/articles/902.aspx
The technical name for the ! operator
is the "dictionary lookup operator." A
dictionary is any collection type that
is indexed by a key rather than a
number, just like the way that the
entries in an English dictionary are
indexed by the word you want the
definition of. The most common example
of a dictionary type is the
System.Collections.Hashtable, which
allows you to add (key, value) pairs
into the hashtable and then retrieve
values using the keys. For example,
the following code adds three entries
to a hashtable, and looks one of them
up using the key "Pork".
Dim Table As Hashtable = New Hashtable
Table("Orange") = "A fruit"
Table("Broccoli") = "A vegetable"
Table("Pork") = "A meat"
Console.WriteLine(Table("Pork"))
The ! operator can be used to look up
values from any dictionary type that
indexes its values using strings. The
identifier after the ! is used as the
key in the lookup operation. So the
above code could instead have been
written:
Dim Table As Hashtable = New Hashtable
Table!Orange = "A fruit"
Table!Broccoli = "A vegetable"
Table!Pork = "A meat"
Console.WriteLine(Table!Pork)
The second example is completely
equivalent to the first, but just
looks a lot nicer, at least to my
eyes. I find that there are a lot of
places where ! can be used, especially
when it comes to XML and the web,
where there are just tons of
collections that are indexed by
string. One unfortunate limitation is
that the thing following the ! still
has to be a valid identifier, so if
the string you want to use as a key
has some invalid identifier character
in it, you can't use the ! operator.
(You can't, for example, say
"Table!AB$CD = 5" because $ isn't
legal in identifiers.) In VB6 and
before, you could use brackets to
escape invalid identifiers (i.e.
"Table![AB$CD]"), but when we started
using brackets to escape keywords, we
lost the ability to do that. In most
cases, however, this isn't too much of
a limitation.
To get really technical, x!y works if
x has a default property that takes a
String or Object as a parameter. In
that case, x!y is changed into
x.DefaultProperty("y"). An interesting
side note is that there is a special
rule in the lexical grammar of the
language to make this all work. The !
character is also used as a type
character in the language, and type
characters are eaten before operators.
So without a special rule, x!y would
be scanned as "x! y" instead of "x !
y". Fortunately, since there is no
place in the language where two
identifiers in a row are valid, we
just introduced the rule that if the
next character after the ! is the
start of an identifier, we consider
the ! to be an operator and not a type
character.
This is built-in, and a definite advantage over C#. The ability to implement an interface Method without having to use the same name.
Such as:
Public Sub GetISCSIAdmInfo(ByRef xDoc As System.Xml.XmlDocument) Implements IUnix.GetISCSIInfo
End Sub
Forcing ByVal
In VB, if you wrap your arguments in an extra set of parentheses you can override the ByRef declaration of the method and turn it into a ByVal. For instance, the following code produces 4, 5, 5 instead of 4,5,6
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Dim R = 4
Trace.WriteLine(R)
Test(R)
Trace.WriteLine(R)
Test((R))
Trace.WriteLine(R)
End Sub
Private Sub Test(ByRef i As Integer)
i += 1
End Sub
See Argument Not Being Modified by Procedure Call - Underlying Variable
Passing parameters by name and, so reordering them
Sub MyFunc(Optional msg as String= "", Optional displayOrder As integer = 0)
'Do stuff
End function
Usage:
Module Module1
Sub Main()
MyFunc() 'No params specified
End Sub
End Module
Can also be called using the ":=" parameter specification in any order:
MyFunc(displayOrder:=10, msg:="mystring")
The Using statement is new as of VB 8, C# had it from the start. It calls dispose automagically for you.
E.g.
Using lockThis as New MyLocker(objToLock)
End Using
Import aliases are also largely unknown:
Import winf = System.Windows.Forms
''Later
Dim x as winf.Form
Consider the following event declaration
Public Event SomethingHappened As EventHandler
In C#, you can check for event subscribers by using the following syntax:
if(SomethingHappened != null)
{
...
}
However, the VB.NET compiler does not support this. It actually creates a hidden private member field which is not visible in IntelliSense:
If Not SomethingHappenedEvent Is Nothing OrElse SomethingHappenedEvent.GetInvocationList.Length = 0 Then
...
End If
More Information:
http://jelle.druyts.net/2003/05/09/BehindTheScenesOfEventsInVBNET.aspx
http://blogs.msdn.com/vbteam/archive/2009/09/25/testing-events-for-nothing-null-doug-rothaus.aspx
If you need a variable name to match that of a keyword, enclose it with brackets. Not nec. the best practice though - but it can be used wisely.
e.g.
Class CodeException
Public [Error] as String
''...
End Class
''later
Dim e as new CodeException
e.Error = "Invalid Syntax"
e.g. Example from comments(#Pondidum):
Class Timer
Public Sub Start()
''...
End Sub
Public Sub [Stop]()
''...
End Sub
There are a couple of answers about XML Literals, but not about this specific case:
You can use XML Literals to enclose string literals that would otherwise need to be escaped. String literals that contain double-quotes, for instance.
Instead of this:
Dim myString = _
"This string contains ""quotes"" and they're ugly."
You can do this:
Dim myString = _
<string>This string contains "quotes" and they're nice.</string>.Value
This is especially useful if you're testing a literal for CSV parsing:
Dim csvTestYuck = _
"""Smith"", ""Bob"", ""123 Anywhere St"", ""Los Angeles"", ""CA"""
Dim csvTestMuchBetter = _
<string>"Smith", "Bob", "123 Anywhere St", "Los Angeles", "CA"</string>.Value
(You don't have to use the <string> tag, of course; you can use any tag you like.)
DateTime can be initialized by surrounding your date with #
Dim independanceDay As DateTime = #7/4/1776#
You can also use type inference along with this syntax
Dim independanceDay = #7/4/1776#
That's a lot nicer than using the constructor
Dim independanceDay as DateTime = New DateTime(1776, 7, 4)
You can have 2 lines of code in just one line. hence:
Dim x As New Something : x.CallAMethod
Optional Parameters
Optionals are so much easier than creating a new overloads, such as :
Function CloseTheSystem(Optional ByVal msg AS String = "Shutting down the system...")
Console.Writeline(msg)
''//do stuff
End Function
Title Case in VB.Net can be achieved by an old VB6 fxn:
StrConv(stringToTitleCase, VbStrConv.ProperCase,0) ''0 is localeID
Properties with parameters
I have been doing some C# programming, and discovered a feature that was missing that VB.Net had, but was not mentioned here.
An example of how to do this (as well as the c# limitation) can be seen at: Using the typical get set properties in C#... with parameters
I have excerpted the code from that answer:
Private Shared m_Dictionary As IDictionary(Of String, Object) = _
New Dictionary(Of String, Object)
Public Shared Property DictionaryElement(ByVal Key As String) As Object
Get
If m_Dictionary.ContainsKey(Key) Then
Return m_Dictionary(Key)
Else
Return [String].Empty
End If
End Get
Set(ByVal value As Object)
If m_Dictionary.ContainsKey(Key) Then
m_Dictionary(Key) = value
Else
m_Dictionary.Add(Key, value)
End If
End Set
End Property