问题
I\'m trying to get a handle on if there\'s a good time to use standard linq keywords or linq extension methods with lambda expressions. They seems to do the same thing, just are written differently. Is it purely a matter of style?
var query = from p in Products
where p.Name.Contains(\"foo\")
orderby c.Name
select p;
// or with extension methods:
var query = Products
.Where(p => p.Name.Contains(\"foo\"))
.OrderBy(p => p.Name);
They\'re very similar with the second example being a bit more terse, but perhaps less expressive if you don\'t know what the => is doing.
Other than writing terse code, are there other advantages to using the extension methods as opposed to the LINQ syntax?
回答1:
Honestly, sometimes it can be situational once you start using Funcs and Actions. Say you are using these three funcs:
Func<DataClasses.User, String> userName = user => user.UserName;
Func<DataClasses.User, Boolean> userIDOverTen = user => user.UserID < 10;
Func<DataClasses.User, Boolean> userIDUnderTen = user => user.UserID > 10;
As you can see the first one replaces the lamdba expression to get the user name, the second replaces a lamdba expression used to check if the ID is lower than 10, and let's face it, the third should be pretty easy to understand now.
NOTE: This is a silly example but it works.
var userList =
from user in userList
where userIDOverTen(user)
select userName;
Versus
var otherList =
userList
.Where(IDIsBelowNumber)
.Select(userName)
In this example, the second is a little less verbose since the extension method can make full use of the Func, but he Linq expression can't since it is look just for a Boolean rather than a Func that returns boolean. However, this is where it might be better to use the expression language. Say you already had a method that takes in more than just a user:
private Boolean IDIsBelowNumber(DataClasses.User user,
Int32 someNumber, Boolean doSomething)
{
return user.UserID < someNumber;
}
Note: doSomething is just there because of the where extension method being ok with a method that takes in a user and integer and returns boolean. Kind of annoying for this example.
Now if you look at the Linq query:
var completeList =
from user in userList
where IDIsBelowNumber(user, 10, true)
select userName;
You're good for it. Now the Extension Method:
var otherList =
userList
.Where(IDIsBelowNumber????)
.Select(userName)
Without a lambda expression, I really can't call that method. So now what I have to do is create a method that creates a Func based off the original method call.
private Func<DataClasses.User, Boolean> IDIsBelowNumberFunc(Int32 number)
{
return user => IDIsBelowNumber(user, number, true);
}
And then plug it in:
var otherList =
userList
.Where(IDIsBelowNumberFunc(10))
.Select(userName)
So you can see, sometimes it may just be easier to use the query approach at times.
回答2:
One advantage to using LINQ extension methods (method-based queries) is that you can define custom extension methods and it will still read fine.
On the other hand, when using a LINQ query expression, the custom extension method is not in the keywords list. It will look a bit strange mixed with the other keywords.
Example
I am using a custom extension method called Into which just takes a string:
Example with query
var query = (from p in Products
where p.Name.Contains("foo")
orderby c.Name
select p).Into("MyTable");
Example with extension methods
var query = Products
.Where(p => p.Name.Contains("foo"))
.OrderBy(p => p.Name)
.Into("MyTable");
In my opinion the latter, using a method-based query, reads better when you have custom extension methods.
回答3:
I think it's a good idea not to use them together and choose one and stick with it.
Mostly it's personal taste, but in the query syntax (Comprehension method) not all operators are available as was said before.
I find the Extension Methods syntax more in line with the rest of my code. I do my SQL in SQL. It's also very easy to build your expression just by adding everything on top of eachother with the extension methods.
Just my two cents.
As I cannot make comments yet I want to make one here to the answer of Programming Tool: Why make a whole new method for the last example?? Can't you just use:
.Where(user => IDIsBelowNumber(user, 10, true))
回答4:
They compile the same, and are equivalent. Personally, I prefer the lambda (extension) methods for most things, only using the statements (standard) if I'm doing LINQ to SQL or otherwise trying to emulate SQL. I find that the lambda methods flow better with code, whereas the statements are visually distracting.
回答5:
I prefer the extension method syntax when I use Linq methods that have no query syntax equivalent, such as FirstOrDefault() or others like that.
回答6:
I like to use the query syntax when its really a query, ie a lazy expression which evaluates on demand.
A method that looks like regular method calls (method syntax or the lambda syntax) doesn't look lazy enough, so I use that as a convention. For eg,
var query = from p in Products
where p.Name.Contains("foo")
orderby p.Name
select p;
var result = query.ToList(); //extension method syntax
If it's a not a query, I like the fluent style which looks to me consistent with other eagerly executing calls.
var nonQuery = Products.Where(p => p.Name.Contains("foo"))
.OrderBy(p => p.Name)
.ToList();
It helps me to differentiate the two styles of calls better. Of course there are situations where you will be forced to use method syntax anyway, so my convention is not very compelling enough.
回答7:
One advantage of extension methods/lynda expressions is the additional operators that is offered like Skip and Take. For example, if you are creating a pagination method, being able to skip the first 10 records and take the next 10 is easy to implement.
来源：https://stackoverflow.com/questions/279701/extension-methods-syntax-vs-query-syntax