|
It's just like the compiler told you... you can only "point" to a lambda via a delegate type. Dems da rules. Sorry.
BillWoodruff wrote: private void MethodTakesAFunc(Func<dynamic, dynamic, string> theFuncParam)
{
string evalFunc = theFuncParam.DynamicInvoke();
}
I don't get it... this works just fine:
private void MethodTakesAFunc(Func<dynamic, dynamic, string> theFuncParam)
{
string s = theFuncParam(someRandomVar1, someRandomVar2);
}
As for the uses of a lambda... well, some people "var them all up". What I mean by that is, I've seen a lot of C# code where people go hog wild with var...
var x = 0;
var y = new List<int>();
etc. Not what a var is intended for at ALL. I've seen people similarly write several page long functions in an inline lamda just cuz they are too lazy too create a function. Style wise, I think if your lambda code is < 5 lines or so, its fine to put it inline, but anything more then that it should be a separate function.
One trick you can do with lambdas is to change a methods signature. Typical UI event handlers are object sender, EventArgs e. Lets say you don't find any of those params useful, and you instead want to pass in a structure. That would require making that structure a class variable so the event handler can access it, but instead you can do:
button.Clicked += (x, y) => { MyEventHandler(someLocalObject); }
|
|
|
|
|
Thanks for your response, SledgeHammer01,
I've revised the section about which you commented, "I don't get it... this works just fine," in a way where I hope my sense of the weirdness of getting a passed in Func to evaluate is clearer.
I have no problem at all with the use of 'var as a way to save repetitious entry of complex object Types on "both sides" of an assignment statement: MS's documentation clearly tells you that is one of two primary uses 'var is for. The other use, of course, is to conveniently hold complex "unknown" Types returned by, for example, a Linq operation.
What I think is "bad practice," with using 'var, is to create a variable name that's not mnemonic, that doesn't "shout" at you, wherever you are in the code, what kind of object it contains, and/or how it's used, but you could say that about any variable name, imho.
Your example of "changing a method's signature" in a lambda used as an EventHandler is interesting, but I don't really see how you are in any way changing the signature of any method. In my view you are just inserting a call to another Method in the executable code body of the EventHandler. Perhaps if I saw your code that creates 'MyEventHandler, I'd "get" what you're demonstrating.
yours, Bill
Humans are amphibians: half spirit, half animal; as spirits they belong to the eternal world; as animals they inhabit time. While their spirit can be directed to an eternal object, their bodies, passions, and imagination are in continual change, for to be in time, means to change. Their nearest approach to constancy is undulation: repeated return to a level from which they repeatedly fall back, a series of troughs and peaks.” C.S. Lewis
|
|
|
|
|
I'm not sure what you are getting at with passing in nulls to execute the lamdba. If you have two dynamics as parameters to the function, that means you can pass anything into them, but you should pass something.
What I was demonstrating with changing the signature of a function is:
private void SomeMethod()
{
// random code here
// some more random code here
SomeComplexObject o = ...; // SomeComplexObject is built from the above random code
theButton.Click += MyEventHandler;
}
Ok, so without lamdas, how do you get "o" to MyEventHandler? You'd have to make it a class member, right? Seems lame to store a local object as a class member just for the sake of passing it to an event handler.
private void SomeMethod()
{
// random code here
// some more random code here
SomeComplexObject o = ...; // SomeComplexObject is built from the above random code
theButton.Click += (x, y) => { MyCoolerEventHandler(o); }
}
No, I didn't really "change the sig of the method" per say if you want to nitpick, but I passed o to my event handler in what I think is a much cleaner way then making it a class member.
One use of that is lets say you have a static object for whatever reason that has an event that takes an x & y. Without lamdas, you have no way to pass additional stuff into that function.
class MyClass
{
private static SomeClassWithEvents;
MyClass()
{
SomeClassWithEvents.theEvent += theHandler;
}
static void theHandler(x, y)
{
}
}
what if I want the handler to have access to SomeClassWithEvents... I'd either have to add a static "this" pointer:
static MyClass classPtr;
and init that.. or I could just do:
SomeClassWithEvents.theEvent += (x, y) => { theHandler(x, y, this); }
modified 28-Apr-13 2:13am.
|
|
|
|
|
You can't assign a lambda expression to a variable declared as var or dynamic because the compiler doesn't know whether you want an Action<> or an Expression<Action<>>.
Action<object> action = x => MessageBox.Show(x.ToString());
Expression<Action<object>> expression = x => MessageBox.Show(x.ToString());
var whatAmI = (object x) => MessageBox.Show(x.ToString());
You can help the compiler out with a few simple helper methods:
public static class Lambda
{
public static Action<T> Action<T>(Action<T> action)
{
return action;
}
}
var action = Lambda.Action((object x) => MessageBox.Show(x.ToString()));
dynamic myAction = Lambda.Action((dynamic x) => x.DoSomethingCool());
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
|
|
|
|
|
+5
Hi Richard,
Thanks very much for this tightly focused, very useful, response to my question ! I will be studying your code with great interest.
It's interesting to me I've never seen this information (that I can remember) in Jon Skeet's "C# in Depth," second edition; perhaps his upcoming 3rd. edition (due out sometime this summer) will cover this. I'm going to go take another look at the 2nd. edition today, to see if it has content on "Expression" that might, if I had understood it, in contrast with "Action," have cleared things up for me.
yours, Bill
Humans are amphibians: half spirit, half animal; as spirits they belong to the eternal world; as animals they inhabit time. While their spirit can be directed to an eternal object, their bodies, passions, and imagination are in continual change, for to be in time, means to change. Their nearest approach to constancy is undulation: repeated return to a level from which they repeatedly fall back, a series of troughs and peaks.” C.S. Lewis
|
|
|
|
|
I am running into a peculiar problem. I want to extract only date part from DateTime and pass the Date as DateTime datatype to stored procedure. I tried with the following code:
DateTime datetime = DateTime.Now.Date;
Console.WriteLine("Date is : " + datetime);
This is always returns with date and time as
4/27/2013 12:00:00 AM . I want only date, no time and should pass this as Date Datatype.
Even though I tried with lots of examples, I am able to do it if I convert to string which is not possible.
Is there any way which I can achieve this? Am I doing something wrong here?
|
|
|
|
|
Look at the name of the structure you're using: DateTime
The time component is not optional there. It will always have a time because the value for 12:00 AM is 0. There is no Date structure or class in .NET.
If you want to pass the date to a stored procedure, pass in a DateTime to a parameter of a parameterized query. When SQL evaluates a date, the time is still there even though it's not specified. Guess what time it is if it's not specified? Yeah, 12:00 AM.
You're basically trying to do something that you don't really need to do at all.
If you pass a DateTime into a, for example, SqlParameter object and the parameter type is defined as an SqlDbType.Date, the conversion is done automatically.
|
|
|
|
|
Besides the prior comment you should also be aware that MS SQL Server only has one data type 'Date' which only holds a date component.
If the database has a different data type then it will ALWAYS have a time component regardless of what you want. You might treat it like it doesn't, with care, but you better understand exactly how values go into and what impact timezones might have.
|
|
|
|
|
Olá pessoal, boa noite! estou com um probleminha simples, mais não estou conseguindo resolver,
preciso da ajuda de vocês!! Olha só no evento clique do meu datagridview eu tenho o código para jogar os dados nos textboxs apartir de um clique em uma linha...consigo fazer para todos os campos menos para o campo imagem, que no caso é um picturibox. agradeço qualquer ajuda...
segue o ex:
//exemplo que está correto
txtEstoqueAtual.Text = Convert.ToString(dataGridView1.CurrentRow.Cells[20].Value);
//exemplo que dá erro
|
|
|
|
|
The translation into English does not make any sense.
Post messages in English, use a good translation engine, check that reverse translation makes sense.
A tradução para o inglês não faz qualquer sentido.
Postar mensagens em Português, use um motor de boa tradução, verificar que a tradução inversa faz sentido.
Recomendar Bing
"It's true that hard work never killed anyone. But I figure, why take the chance." - Ronald Reagan
That's what machines are for.
Got a problem?
Sleep on it.
modified 26-Apr-13 21:00pm.
|
|
|
|
|
Hello guys, good night! I have a little problem Simple, More'm not getting resolved,
I need help from you! just look at the click event of my datagridview I have a code to play data in textbox starting from a click on a line ... can do for all fields, less than for the Image field, which in the case is a picturibox. I appreciate any help ...
/ / correct code
txtEstoqueAtual.Text = Convert.ToString(dataGridView1.CurrentRow.Cells[20].Value);
/ / This does not work for the image picturibox
//pctubImagemProd.Image = Convert.ToString(dataGridView1.CurrentRow.Cells[21].Value);
|
|
|
|
|
That still doesn't make any sense.
The code makes even less sense. You're converting the content of a DGV cell to a string and you expect that string to work in place of an Image object? Never going to happen.
I have no idea what that string represents. If it's a filepath to an image file, you have to load the image in order for it to be displayed in a control. It's not going to happen automatically.
|
|
|
|
|
Actually I do not avia realized it, the more I've sure could do with that format! was a mistake to send the code so ...
though of course unknown, but I figured averia somehow playing well in the picture from a different picturibox click the image in datagridview cell.
if someone has an idea I'm waiting ...
Thank you.
|
|
|
|
|
I have no idea what you're saying. Your English makes no sense at all.
|
|
|
|
|
You cannot use Convert.ToString to get an image; and why would you? The Value method returns an object, so you just need to cast it to the correct type.
Use the best guess
|
|
|
|
|
Actually I do not avia realized it, the more I've sure could do with that format! was a mistake to send the code so ...
|
|
|
|
|
I store almost all of my data for my applications in xml-files. Works fine, especially with Linq-Support
But I continuously ask myself, if it is not better to store data in a database (SqlExpress or MySql) with much advanced data-handling-capabilities
One advantage of xml is, that files can be easily moved around via simple copy/paste. Do I loose this feature with a database?
modified 26-Apr-13 14:05pm.
|
|
|
|
|
It's more to do with what you're doing with the data that should determine this. Also, XML is not a database, though too many noobs treat it like it is one.
If you're not doing any reporting or anything that requires shared access to the data, you're probably fine with XML.
You can move it to a database, but you make it harder to move the files around. You can attach and detach database files, but it may not be as easily as just a straight XML file depending on the database engine you chose.
|
|
|
|
|
Thank you for your answer
I continuously see some code-examples, where a database-table is bound to some control and data are displayed in any control almost "magically". Is this also possible with xml?
|
|
|
|
|
You can't bind a database table to a control.
You bind the control to any object that implements (If I remember correctly) IList, IListSource, IBindingList, or IBindingListView. This includes, but is not limited to, any one dimensional array, any collection (i.e.: List, List<t>), DataSet, DataTable, DataView, BindingList<t>, BindingSource, ...
You can put your database data into any suitable object and bind the grid to it.
|
|
|
|
|
Features are never loose with a database, they are always tight.
Your "advantage" that you say with XML is actually a problem if you are talking about shared data. The problem is User 1 edits some data and sends it to User 2 - then User 2 edits some data and sends it to User 3 - at this point, User 1 has WRONG DATA, because they are still holding an old copy of the file and they have no way of knowing that User2 made changes. So, as a general rule, any data which will be used by multiple users needs to be in a database. Well, it needs to be in a central location that everyone is using - so it could be an XML file on the network, right?
Your problem here isn't about the technology being used to store the data, the issues you need to solve are about how the data is used. Files can cause major problems if the data is used by multiple users, while databases are specifically designed for that situation. Many companies have an internal problem knowing what the truth is about the company, because they don't all pull their data from a central source. Don't be like that.
On the other hand, users love files! They like to have control over 'their' data and when it's in a file on their machine, they feel comfortable with that. This is good practice for data that is work product, such as images produced by graphic designers, or engineering drawings from AutoCad, things like that. They control it while they want to, and they control how that information is shared. If you need user-level control like that, staying with files might be more appropriate.
|
|
|
|
|
Jasmine2501 wrote: Features are never loose with a database, they are always tight.
That isn't true - using your definition.
It is a common problem that an update to a database does not instantly appear in all clients. So one must account for that. Usually by just ignoring it because actual business cases (not hypothetical development cases) do not allow for it.
Jasmine2501 wrote: Many companies have an internal problem knowing what the truth is about the company, because they don't all pull their data from a central source
Which of course isn't solved by technology but rather architecture (people).
|
|
|
|
|
You missed the joke, and the heinous spelling error.
Your pants can be loose. Your database can't.
|
|
|
|
|
Jasmine2501 wrote: You missed the joke
So you are stating that your entire posting was a joke?
|
|
|
|
|
No just the part you didn't get.
|
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
