I'm looking for good tutorials on how to create LINQ accessors/APIs to my business classes. So that someone could eventually enter something like this in a program--
var potentialCustomers = from people in county
where people.NumberOfCats > 2
select people
I've used LINQ often enough with the .Net collections, but have never done it before on my own classes. Is it just a matter of implementing IEnumerable, or are there additional steps needed?
LINQ is an interesting beast.
Immediately IEnumerable<T> comes to mind when discussing LINQ. It seems that IEnumerable<T> is LINQ, but it is not. IEnumerable<T> is one implementation of the LINQ methods that allow LINQ queries to be written against objects that implement IEnumerable<T>.
Another implementation is IObservable<T> which powers the Microsoft's Reactive Extensions. This is a set of extensions that allow LINQ queries to be written against events (or streams of data). Nothing to do with IEnumerable<T>.
LINQ also can be written directly in your objects - it doesn't have to be extension methods at all.
For example, define classes A and B like so:
public class A
{
public B Select(Func<A, B> selector)
{
return selector(this);
}
}
public class B
{
public B(A a) { }
}
Now I can write this code:
B query =
from x in a
select new B(x);
It's LINQ, Jim, but not as we know it.
All of the LINQ operators can be defined this way. So long as the compiler gets to see methods with the right signature you're golden.
Having said this LINQ queries feel natural when working with a series of values - and hence this is why IEnumerable<T> and IObservable<T> are good examples of LINQ in action. But it certainly is possible to define LINQ against any type you like just by implementing the right methods.
You just need to implement IEnumerable interface in your class and then you can use LINQ.
Because LINQ is a set of extensions for IEnumerable objects
Imagine i have an extension method that operates of an interface of abstract objects:
public static void BlowTheirWhoHoovers(this ICollection<Image> source)
{
...
}
This extension knows how to blow the who hoovers of a list of images.
Note:
it's a list of Images, which is abstract. As long as the lists contain anything that descends from Image (e.g. Bitmap, Metafile, PngImage) the extension can handle it
source list can by any kind of list as long as it exposes ICollection
since source list is going to potentially have items added or removed, it is ICollection rather than IEnumeration (IEnumeration doesn't support modifying the list)
That's all well and good, except it doesn't work, the extension method is not found:
Collection<Bitmap> whoHonkers = new Collection<Bitmap>();
whoHonkers.BlowTheirWhoHoovers();
i assume this fails because:
Visual Studio doesn't think Collection implements ICollection<T>
Visual Studio doesn't think Bitmap descends from Image
Now i can change my extension:
public static void BlowTheirWhoHoovers(this Collection<Bitmap> source)
{
...
}
which does compile:
except it's no longer useful (it requires Collection, and it requires Bitmap), e.g. it fails to work on anything except Collection<T>:
List<Bitmap> whoHonkers = new List<Bitmap>();
whoHonkers.BlowTheirWhoHoovers();
It also fails on anything that it's Bitmap:
List<Metafile> whoHonkers = new List<Metafile>();
whoHonkers.BlowTheirWhoHoovers();
or
List<PngImage> whoHonkers = new List<PngImage>();
whoHonkers.BlowTheirWhoHoovers();
How can i apply an extension to anything that is:
ICollection<Image>
?
i can't force anyone to change their declaration, e.g.:
List<Bitmap>
to
Collection<Image>
Update:
Possible avenues of solution exploration:
finding the syntax that allows extensions on things that the compiler should be able to do
manually casting the list; doing the compiler's job for it
Update#2:
i could have simplified some of the distractions around ICollection, and made it IEnumerable (since it's a hypothetical example i can make up whatever i want). But the screenshots already show Collection, so i'm going to leave it as it is.
Update#3:
As foson pointed out, changing it to IEnumerable allows a way to cheat the answer. So it definitely should stay as ICollection.
Sounds like a covarient problem.
If you change the signature to
public static void BlowTheirWhoHoovers(this IEnumerable<Image> source)
you should be able to do it in .NET 4 (not SL).
The problem with varience is that if you passed into the method an ICollection<JpegImage>, the method itself wouldn't know that it could only add JpegImages and not Bitmaps, defeating type saftey. Therefore the compiler does not let you do it. IEnumerable's type parameter is covarient, as seen by the out keyword, while ICollection's type parameter is not (since it is both in return positions and method parameter positions in the ICollection interface)
public interface IEnumerable<out T>
http://msdn.microsoft.com/en-us/library/dd799517.aspx
I am developing a Windows Phone 7 silverlight application but, i can't use the session values to "navigate" between different pages on windows phone 7.
I also used "Isolated Storage" but i couldn't get the values.
This sample shows some persistence mechanisms:
http://www.scottlogic.co.uk/blog/colin/2011/05/a-simple-windows-phone-7-mvvm-tombstoning-example/
You can also use Query Strings to pass information between two pages. The values that make up a query string are appended to the URI.
Personally, I have a centralised controller class that gets instantiated with the main App class. Any values that need passing are placed in here, in one way or another.
Thanks Adam Houldsworth for your response, it really helped me. However i found a simpler solution.
We can create a Global Variables Class in "App.xaml.cs" file and put the variables in it. The class is accessible from everywhere.
Example:
public static class GlobalVariables
{
public static string my_string = "";
public static int my_int = "";
}
Then we access the Global Variables class like this:
project_Name.GlobalVariables.variable_name;
I have defined a GenericRepository class which does the db interaction.
protected GenericRepository rep = new GenericRepository();
And in my BLL classes, I can query the db like:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => a.AccessLevel.BinaryAnd(accessLevel)).ToList();
}
BinaryAnd is an extension method which checks two int values bit by bit. e.g. AccessLevel=5 => AccessLevel.BinaryAnd(5) and AccessLevel.binaryAnd(1) both return true.
However I cannot use this extension method in my LINQ queries. I get a runtime error as follows:
LINQ to Entities does not recognize the method 'Boolean BinaryAnd(System.Object, System.Object)' method, and this method cannot be translated into a store expression.
Also tried changing it to a custom method but no luck. What are the workarounds?
Should I get all the albums and then iterate them through a foreach loop and pick those which match the AccessLevels?
I realize this already has an accepted answer, I just thought I'd post this in case someone wanted to try writing a LINQ expression interceptor.
So... here is what I did to make translatable custom extension methods: Code Sample
I don't believe this to be a finished solution, but it should hopefully provide a good starting point for anyone brave enough to see it through to completion.
You can only use the core extension methods and CLR methods defined for your EF provider when using Entity Framework and queries on IQueryable<T>. This is because the query is translated directly to SQL code and run on the server.
You can stream the entire collection (using .ToEnumerable()) then query this locally, or convert this to a method that is translatable directly to SQL by your provider.
That being said, basic bitwise operations are supported:
The bitwise AND, OR, NOT, and XOR operators are also mapped to canonical functions when the operand is a numeric type.
So, if you rewrite this to not use a method, and just do the bitwise operation on the value directly, it should work as needed. Try something like the following:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => (a.AccessLevel & accessLevel > 0)).ToList();
}
(I'm not sure exactly how your current extension method works - the above would check to see if any of the flags come back true, which seems to match your statement...)
There are ways to change the linq query just before EF translates it to SQL, at that moment you'd have to translate your ''foreign'' method into a construct translatable by EF.
See an previous question of mine How to wrap Entity Framework to intercept the LINQ expression just before execution? and mine EFWrappableFields extension which does just this for wrapped fields.
I have an Enumerable<T> and am looking for a method that allows me to execute an action for each element, kind of like Select but then for side-effects. Something like:
string[] Names = ...;
Names.each(s => Console.Writeline(s));
or
Names.each(s => GenHTMLOutput(s));
// (where GenHTMLOutput cannot for some reason receive the enumerable itself as a parameter)
I did try Select(s=> { Console.WriteLine(s); return s; }), but it wasn't printing anything.
A quick-and-easy way to get this is:
Names.ToList().ForEach(e => ...);
You are looking for the ever-elusive ForEach that currently only exists on the List generic collection. There are many discussions online about whether Microsoft should or should not add this as a LINQ method. Currently, you have to roll your own:
public static void ForEach<T>(this IEnumerable<T> value, Action<T> action)
{
foreach (T item in value)
{
action(item);
}
}
While the All() method provides similar abilities, it's use-case is for performing a predicate test on every item rather than an action. Of course, it can be persuaded to perform other tasks but this somewhat changes the semantics and would make it harder for others to interpret your code (i.e. is this use of All() for a predicate test or an action?).
Disclaimer: This post no longer resembles my original answer, but rather incorporates the some seven years experience I've gained since. I made the edit because this is a highly-viewed question and none of the existing answers really covered all the angles. If you want to see my original answer, it's available in the revision history for this post.
The first thing to understand here is C# linq operations like Select(), All(), Where(), etc, have their roots in functional programming. The idea was to bring some of the more useful and approachable parts of functional programming to the .Net world. This is important, because a key tenet of functional programming is for operations to be free of side effects. It's hard to understate this. However, in the case of ForEach()/each(), side effects are the entire purpose of the operation. Adding each() or ForEach() is not just outside the functional programming scope of the other linq operators, but in direct opposition to them.
But I understand this feels unsatisfying. It may help explain why ForEach() was omitted from the framework, but fails to address the real issue at hand. You have a real problem you need to solve. Why should all this ivory tower philosophy get in the way of something that might be genuinely useful?
Eric Lippert, who was on the C# design team at the time, can help us out here. He recommends using a traditional foreach loop:
[ForEach()] adds zero new representational power to the language. Doing this lets you rewrite this perfectly clear code:
foreach(Foo foo in foos){ statement involving foo; }
into this code:
foos.ForEach(foo=>{ statement involving foo; });
His point is, when you look closely at your syntax options, you don't gain anything new from a ForEach() extension versus a traditional foreach loop. I partially disagree. Imagine you have this:
foreach(var item in Some.Long(and => possibly)
.Complicated(set => ofLINQ)
.Expression(to => evaluate))
{
// now do something
}
This code obfuscates meaning, because it separates the foreach keyword from the operations in the loop. It also lists the loop command prior to the operations that define the sequence on which the loop operates. It feels much more natural to want to have those operations come first, and then have the the loop command at the end of the query definition. Also, the code is just ugly. It seems like it would be much nicer to be able to write this:
Some.Long(and => possibly)
.Complicated(set => ofLINQ)
.Expression(to => evaluate)
.ForEach(item =>
{
// now do something
});
However, even here, I eventually came around to Eric's point of view. I realized code like you see above is calling out for an additional variable. If you have a complicated set of LINQ expressions like that, you can add valuable information to your code by first assigning the result of the LINQ expression to a new variable:
var queryForSomeThing = Some.Long(and => possibly)
.Complicated(set => ofLINQ)
.Expressions(to => evaluate);
foreach(var item in queryForSomeThing)
{
// now do something
}
This code feels more natural. It puts the foreach keyword back next to the rest of the loop, and after the query definition. Most of all, the variable name can add new information that will be helpful to future programmers trying to understand the purpose of the LINQ query. Again, we see the desired ForEach() operator really added no new expressive power to the language.
However, we are still missing two features of a hypothetical ForEach() extension method:
It's not composable. I can't add a further .Where() or GroupBy() or OrderBy() after a foreach loop inline with the rest of the code, without creating a new statement.
It's not lazy. These operations happen immediately. It doesn't allow me to, say, have a form where a user chooses an operation as one field in a larger screen that is not acted on until the user presses a command button. This form might allow the user to change their mind before executing the command. This is perfectly normal (easy even) with a LINQ query, but not as simple with a foreach.
(FWIW, most naive .ForEach() implementations also have these issues. But it's possible to craft one without them.)
You could, of course, make your own ForEach() extension method. Several other answers have implementations of this method already; it's not all that complicated. However, I feel like it's unnecessary. There's already an existing method that fits what we want to do from both semantic and operational standpoints. Both of the missing features above can be addressed by use of the existing Select() operation.
Select() fits the kind of transformation or projection described by both of the examples above. Keep in mind, though, that I would still avoid creating side effects. The call to Select() should return either new objects or projections from the originals. This can sometimes be aided through the use of an anonymous type or dynamic object (if and only if necessary). If you need the results to persist in, say, an original list variable, you can always call .ToList() and assign it back to your original variable. I'll add here that I prefer working with IEnumerable<T> variables as much as possible over more concrete types.
myList = myList.Select(item => new SomeType(item.value1, item.value2 *4)).ToList();
In summary:
Just stick with foreach most of the time.
When foreach really won't do (which probably isn't as often as you think), use Select()
When you need to use Select(), you can still generally avoid (program-visible) side effects, possibly by projecting to an anonymous type.
Avoid the crutch of calling ToList(). You don't need it as much as you might think, and it can have significant negative consequence for performance and memory use.
Unfortunately there is no built-in way to do this in the current version of LINQ. The framework team neglected to add a .ForEach extension method. There's a good discussion about this going on right now on the following blog.
http://blogs.msdn.com/kirillosenkov/archive/2009/01/31/foreach.aspx
It's rather easy to add one though.
public static void ForEach<T>(this IEnumerable<T> enumerable, Action<T> action) {
foreach ( var cur in enumerable ) {
action(cur);
}
}
You cannot do this right away with LINQ and IEnumerable - you need to either implement your own extension method, or cast your enumeration to an array with LINQ and then call Array.ForEach():
Array.ForEach(MyCollection.ToArray(), x => x.YourMethod());
Please note that because of the way value types and structs work, if the collection is of a value type and you modify the elements of the collection this way, it will have no effect on the elements of the original collection.
Because LINQ is designed to be a query feature and not an update feature you will not find an extension which executes methods on IEnumerable<T> because that would allow you to execute a method (potentially with side effects). In this case you may as well just stick with
foreach(string name in Names)
Console.WriteLine(name);
Using Parallel Linq:
Names.AsParallel().ForAll(name => ...)
Well, you can also use the standard foreach keyword, just format it into a oneliner:
foreach(var n in Names.Where(blahblah)) DoStuff(n);
Sorry, thought this option deserves to be here :)
There is a ForEach method off of List. You could convert the Enumerable to List by calling the .ToList() method, and then call the ForEach method off of that.
Alternatively, I've heard of people defining their own ForEach method off of IEnumerable. This can be accomplished by essentially calling the ForEach method, but instead wrapping it in an extension method:
public static class IEnumerableExtensions
{
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> _this, Action<T> del)
{
List<T> list = _this.ToList();
list.ForEach(del);
return list;
}
}
As mentioned before ForEach extension will do the fix.
My tip for the current question is how to execute the iterator
[I did try Select(s=> { Console.WriteLine(s); return s; }), but it wasn't printing anything.]
Check this
_= Names.Select(s=> { Console.WriteLine(s); return 0; }).Count();
Try it!