Background Info For Lambda Expressions?
Mar 6, 2009
I'm trying to expand my programming horizons, and LINQ and Lambda Expressions have popped up as some apparently very important topics. As I began reading articles on Lambda Expressions, it appears that Lambda Expressions are built heavily on the concept of Delegates. As I began reading articles on Delegates, it appears that Delegates are built heavily on the concept of callback functions. As I began reading articles on callback functions, .[code]Can anyone out there give me a simple, clear, concise definition of callback functions? Do I need to learn C or C++ to fully understand these concepts? Maybe a better question is: What do I need to know to be able to understand Delegates (they've always somewhat mystified me) and LINQ/Lambda Expressions (I haven't looked into these too much, but they're mystifying, too)?I don't mind doing research, but google searches are giving me way too much information.
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Nov 11, 2009
I see lambda expressions have become a very useful tool at some points in the language. I've been using them a lot and most of the time they fit really nice and make the code shorter and perhaps clearer.
Now.. I've seen some , I would say excessive use of them. Some people like them so much that try to use them everywhere they can.. Some times the C# code looks like a functional language. Other factors against are the cost using reflection by lambda and that not friendly to debugging.I would like to hear opinions about how good and how code clear it is to use more or less the lambda expressions. (this is not the better example, but let's say it was the trigger)I was writing the following code. The use of the delegate { return null; } helps me avoid having to ask if the event is null or not every time I have to use it.
public delegate ContactCellInfo.Guest AddGuest();
public event AddGuest GuestRequest = delegate { return null;}
Im using resharper and the wise resharper( even it some times literaly eats the memory) made me the following suggestion
public delegate ContactCellInfo.Guest AddGuest();
public event AddGuest GuestRequest = () => null;
At my point of view the code using the delegate looks clearer. I am not against the Lamdba expression just would like to hear some advices on how and when to use them.
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Jan 20, 2010
Functionally, is there any difference (apart from syntax onbviously) between lambda expressions in C# and VB.Net?EDIT: following up on CraigTP's answer: any references to the situation in .Net 4?EDIT: I'm asking because I'm used to C#, but for a next project the customer asks VB.Net. We're not a priori against that. We realize that most language constructs are supported in both languages. However, we're particularly fond of the way C# implements lambda expressions. We would like to have an overview of the differences with VB.Net
EDIT: accepted CraigTP's answer for pointing out what I currently consider the most important difference.
So to summarize: VB.Net 9 does not support multiline statements in a lambda expression, and a lambda must always return a value. Both of these issues are addressed in VB.Net 10
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Apr 20, 2012
Async Sub like this: Dim f As Func(Of Task) = Async Sub() End Sub Produces compiler error: error BC36670: Nested sub does not have a signature that is compatible with delegate 'System.Func(Of System.Threading.Tasks.Task)'.Equivalent C# code compiles fine:
Func<Task> f = async () => { };Rewriting Async Sub into Async Function make code works.
Why does Async Sub() is not convertible to delegate types with return value of type Task?
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Jul 4, 2010
how to convert this code to VB.net, can anybody help?
public static readonly DependencyProperty CommandProperty =
DependencyProperty.RegisterAttached(
"Command",
[code]...
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Feb 3, 2012
I have a List(of T) where T has a property that is a list of checkboxes, what I need is a Lambda expression that will count all the checked checkboxes in the list. I tried with:
[Code]...
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Jul 19, 2011
I just came across a bug in NHibernate which happens to already be raised: [url] I'm not sure if this applies to anything else other than enums but when using a Lambda from VB, it looks different to the same Lambda from C#.
C#:
Where(x => x.Status == EmployeeStatus.Active)
VB
Where(Function(x) x.Status = EmployeeStatus.Active)
They are the same as far as I'm aware? (My VB isn't great) If I put a break point on the same line of code, where the above code is passed into. In C# I get: On the same line when VB version is passed in, I get: Is this something I'm doing wrong? Is the result's the same, just displayed different between C#/VB?
Edit: Ok so they are displayed different, but they can't be the same because NHibernate cannot handle it. The C# version is handled perfectly fine by NHibernate, the VB version resolves in the following exception being thrown: [Code]
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Jul 25, 2011
I have an issue with building a project using the MSBuild (ver 4) from the command line when declaring lambda expression like this:
Private Sub Foo(ByVal s As String)
Dim WL = Sub(str As String)
If Not String.IsNullOrEmpty(str) Then[code]......
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Jul 7, 2010
what are delegates?How are the lambda expressions related to the delegates?
reading the msdn about the delegates is really confusing me ;so i need a start off from here before moving on to the msdn.
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Jul 1, 2011
To streamline code, I increasingly rely on lambda expressions. But I may have hit against the wall in the attempt of removing an event handler set by a lambda expression. This really eludes me. The code below shows two samples: Alt1 uses a plain lambda expression as delegate and works fine: the object size increases as the numeric control varies. However, the drawback is that the handler cannot be removed:
RemoveHandler Me.upDnScale.ValueChanged, Sub(sender As Object, e As Object) ScaleObject1(sender, e, objTgt)
According to this MSDN article, a variable should be used to set/remove lambda based handlers. Alt2 shows a couple of attempts, but none works.
[Code]...
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Feb 22, 2010
I have a type lets call it "MyType". I have a List(Of MyType). Here is what i'm doing: MyList.Sum(Function(x) x.MyFieldToTotal) "MyFieldToTotal" is a decimal. For the life of me i can't figure out why x above is an object rather than a type of "MyType". Shouldn't Type Inferencing be working in this case? Even in intellisense i get "selector as System.Func(Of MyType) as Decimal"
[Code]...
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May 21, 2012
Currently I am searching through my list to find Customers that match on Address.
I need to match on both address and city. How do I rewrite my lambda Expression to match both criteria?
CustomerList.FindAll(Function(c) c.Address = addressToMatch)
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Jul 2, 2011
How to remove a handler set up by a lambda expression? According to this MSDN article, lambda expression should be assigned to variables which should be used to define handler operators. So far, I have used lambda expessions w/o variables.
For instance, the following works:
Private Sub ScaleObjectInit1(ByVal objSrc As NumericUpDown, ByVal objTgt As FrameworkElement) ...
AddHandler Me.upDnScale.ValueChanged, Sub (sender As Object, e As Object) ScaleObject1 (sender, e, objTgt)
End Sub
Private Sub ScaleObject1(ByVal sender As Object, ByVal e As Object, ByVal objTgt As FrameworkElement)
[Code] .....
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Jun 30, 2011
Can someone who is using a 64 bit machine help me pintpoint this crash to either the VS2010 IDE itself or one of the plugins I have installed?
It's the code fragment below. When I copy & paste it into the Main() function of a new VB.Net console app on my Win7 64 bit machine, the VS IDE crashes and dies onthe spot, every time I try it.
Doing exactly the same on a 32 bit XP machine, nothing abnormal happens.
The 64 bit machine does have some IDE plug-ins installed, the biggest of which is DevExpress (the free version), so i think either one of those or the fact that the IDE is running on 64 bit must be the culprit.
This is the code. The static variable can also be made a module-level variable, with the same result.
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Nov 16, 2009
I'm still trying to get my head around the whole "Lambda Expressions" thing.
Can anyone here give me an example ordering by multiple columns using VB.Net and Linq-to-SQL using a lambda expression?
Here is my existing code, which returns an ordered list using a single-column to order the results:
Return _dbContext.WebCategories.OrderBy(Function(c As WebCategory) c.DisplayOrder).ToList
Note: The WebCategory object has a child WebPage object (based on a foreign key). I'd like to order by WebPage.DisplayOrder first, then by WebCategory.DisplayOrder.
I tried chaining the order bys, like below, and though it compiled and ran, it didn't seem to return the data in the order I wanted.
Return _dbContext.WebCategories.OrderBy(Function(c As WebCategory) c.DisplayOrder).OrderBy(Function(c As WebCategory) c.WebPage.DisplayOrder).ToList
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Mar 16, 2009
I have an untyped dataset returned from my WebService. The user wants to dynamically construct a query referencing tables and columns and specifying values to test for.I have looked at Lambda Expressions and the Expressions.Expression Namespace. I think these provide the answer I'm looking for, but I'm not certain how to go about contructing the linq expressions to extract the datarows I'm looking for.
I plan on limiting the result to one datatable that the query will result in and i"ll provide the joins from the user's constructs.For a simple example I have a DataTable with (n) rows called "Table", and in it there is a computed column called "b_IsActive" that has the expression "Convert(IsActive, 'System.Boolean')". The user wants to retrieve all rows in "Table" where field "b_IsActive" is true.
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May 20, 2009
I'm looking for an advanced level VB.NET book which covers LINQ and Lambda Expressions.Generally I read C# .NET books due to lack of good VB.NET books when it comes to generic .NET Framework related subjects. However Lambda and LINQ is quite different in C# and VB.NET I'm looking for an advanced level VB.NET book on this subjec
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Jun 11, 2011
I've noticed running in a PE environment and seeing the "Nerd Herd" MRI software, I was wondering if anyone can tell me now to recreate this using VB.NET. I want to show the system info over the background if possible.
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Jun 21, 2011
Im hearing about this, can someone explain it to me or please show some basic code of Lambda with explanation.
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Dec 14, 2009
Is it possible to shorten the following function to a lambda expression?Or (to do the trick by myself) what is the best and most understandable for beginners tutorial for lambda in vb.net?[code]
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Jul 31, 2010
I am working on a media player, and I would like to setup a local playlist feature. I am using a single ListBox and its "Display-" and "Value-Member" properties. The display is the song's Artist and Name, and the value is the song's file path (i.e 'C:UsersUSERMusicetc.').To write the file path to the .txt file, I believe I can use the following code:[code]
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Jul 23, 2009
how to create Quick info and/or parameter info for own code?
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Jun 22, 2011
I am making a program, obviously, and I require some assistance. I found an explanation on how to read information from a Text file and have modified it for my needs. Here is my (modded) version:
Public Function GetInfo(ByVal playlistname As String) As String
On Error Resume Next
Dim PlaylistInfo As String
[code].....
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Jan 7, 2010
I'm making a database app. I've finish the search, add new, and add function. What I'm trying to do is to expand the functionality of the add new item function.
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Nov 17, 2011
I am trying to do is fill out a form with certain info, put the info into an array, and write it to a listbox using loops, arrays, and maybe a function if I need one. This is for VB2010 and here is what I have so far... I think there are ways to make it shorter but I cannot figure it out.... maybe just brain dead from all the coffee I have been drinking!
[code]...
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Sep 6, 2011
under my project I have the following
However when I use this code.
lblABOUT3.Text = "Version : "
lblABOUT3.Text += My.Application.Info.Version.Major.ToString()
lblABOUT3.Text += "." & My.Application.Info.Version.Minor.ToString
[Code]....
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Mar 17, 2009
how to correlate info from a textbox based on a listbox item selection. For example the user will select an item from the listbox, enter say the length of a song in the textbox say 2:00 for song #1, select say song #2, enter the song time for that song say 3:00, click on song #1 again and have the same textbox display the length of that song and be able to do that until there are no more songs on the list.
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Feb 18, 2011
Background of form: black
background of char, bombs and shrooms: transparent
picbox behind char, bombs and shrooms is regular (marioland)
How can i fix it so the transparent parts dont see form as background but marioland as background?
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May 3, 2012
I am using query from a SQLite database as the AutocompleteCustomSource for a ComboBox. Also, I want to load data in separate thread. My LoadData method works fine when called directly, but it fails when it is called from the BackgroundWorker thread. When it is called from the background thread, it throws a Specified cast is not valid exception on the csearch.AutoCompleteCustomSource.Add(hh("Taj")) line. Below is my code:
Sub LoadData()
Dim connetionString As String
Dim cnn As SQLiteConnection
[Code].....
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Sep 17, 2011
Can I get info from stored cookie on the local machine. I have a Webbrowser control in a winform, and I would like to output data from a cookie to a label or so.
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