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I just received a mail from Adam Machanic that pointed me to this bug (I would call it a performance issue) about the construction of SQL statements generated by the LINQ to SQL engine. The issue: every string passed as a constant in the query will be auto-parameterized using the length of the passed string, even when you used a string variable into the LINQ query. If you write something like string s = "Wine";
var query =
from x in db.Products
where x.ProductName == s
select x; you will see that a parameter of type NVARCHAR(4) will be passed to the generated SQL query. The next execution of query might contain a different value in the s parameter, and for this reason a different parameter type might be used: if the length of the string in the s variable changes, then the same query will be sent to SQL Server, but using a different type in the sp_executesql parameters .For example, a NVARCHAR(5) would be used whether s contains"Bread". The consequence of this behavior is that you could have a non-optimal performance from SQL Server and, more important, the procedure cache could be filled up with several copies of the same query, differing each other only in the length of the parameter type. I agree with Adam: this is something to be fixed. But my suspect is that we will get a "by design" answer another time...
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In the Programming Microsoft LINQ book we dedicated two whole chapters (76 pages) about the writing of a IQueryable LINQ provider: one is about expression trees and the other covers the several ways to extend LINQ, including the writing of an IQueryable provider. I know that the subject is complex and probably is not necessary to every programmer. However, a good understanding of what happens under the cover of an IQueryable provider is good for everyone using any flavor of LINQ: when you debug your code, it might help you in finding issues faster. I wrote this introduction just to explain why you should read this post of Bart De Smet, which is undoubtedly shorter than the corresponding chapter of our book and gives you a very good step-by-step introduction of the inner workings of an IQueryable LINQ provider. Then, if you really like this kind of things, you have another good reason to read the book :-)
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Dinesh Kulkarni wrote an important post about changes in LINQ introduced by .NET 3.5 SP1 that has been released yesterday. One of the interesting changes is in the Cast<T> operator and its behavior is better described in this post by Ed Maurer. I think that the side effects of this change should be limited, because the use of explicit type for the range variable in a query expression (i.e. from int n in numbers select... instead of from n in numbers select...) is not very common. In fact, I don't remember examples of its usage in our Programming LINQ book. Take care of this change if you used (or will use) this syntax.
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I have just validated this bug posted on Connect. It seems a compiler issue, I'd like to read a Microsoft answer about this. However, the general issue is that using ArrayList in a lambda expression with a collection initializer could be dangerous. There are not so many reasons to use an ArrayList in a lambda expression, unless you are refactoring or working with legacy code that cannot be modified upgrading ArrayList to generic collections.
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Eric White has written an interesting post titled "Are developers using LINQ?" - there are interesting considerations about the adoption of functional programming too, but the most interesting part for me is the list of comment of the post. A lot of people described the adoption of LINQ into their team or company, and there is a spread variety of comments (good and bad). An interesting comment is about the future adoption of F# when it will be shipped, because of the complete adoption of functional programming (C# 3 is not a complete functional programming like F# is). I suggest you to take a look at this post and its comments, because it gives you an idea of what is going on out there.
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I just read a post about getting multiple results with LINQ to SQL without using stored procedures. This technique is interesting when you have multiple queries returning a few rows each one and you want to save time by skipping some roundtrip between your program and SQL Server. Looking at the post, I immediately thought that it would be interesting comparing this solution with an asynchronous one, executing each query in a different thread. I don't have time to make some benchmark, but it would be interesting to make a comparison between these two techniques.
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A few days ago I found a bug in a program written using LINQ to SQL, which was caused by years of use of SQL. The requirement was something like: get the distinct values of (bla bla bla) sorted alphabetically. An example of the required query with Northwind would be the following one: SELECT DISTINCT
e.LastName
FROM Orders o
LEFT JOIN [Employees] e
ON e.[EmployeeID] = o.[EmployeeID]
ORDER BY e.LastName
Fundamentally, we are using both a DISTINCT and an ORDER BY statement in SQL.
Now, if you create a NorthwindDataContext importing the Order and Employee tables, you can try to write a similar statement in LINQ to SQL. Unfortunately, the Distinct clause is not part of the query syntax and the most intuitive path could be the one of calling Distinct at the end of your statement, like in the following query:
var queryA =
(from o in db.Orders
orderby o.Employee.LastName
select o.Employee.LastName)
.Distinct();
However, the Distinct clause is removing the sort condition defined by the orderby keyword. In fact, the SQL statement sent to the database is the following one:
SELECT DISTINCT
[t1].[LastName]
FROM [dbo].[Orders] AS [t0]
LEFT OUTER JOIN [dbo].[Employees] AS [t1]
ON [t1].[EmployeeID] = [t0].[EmployeeID]
This behavior might appear strange. The problem is that the Distinct operator does not grant that it will maintain the original order of values. Applied to LINQ to SQL, this mean that a sort constraint can be ignored in the case of a query like queryA.
The solution is pretty sample: put the OrderBy operator after the Distinct one, like in the following queryB definition:
var queryB =
(from o in db.Orders
select o.Employee.LastName)
.Distinct().OrderBy( n => n );
This will result in the following SQL statement sent to Northwind:
SELECT [t2].[LastName]
FROM ( SELECT DISTINCT
[t1].[LastName]
FROM [dbo].[Orders] AS [t0]
LEFT OUTER JOIN [dbo].[Employees] AS [t1]
ON [t1].[EmployeeID] = [t0].[EmployeeID]
) AS [t2]
ORDER BY [t2].[LastName]
If you remove some syntax redundancy, this is exactly the same query I wrote at the beginning of my post.
The lesson is: in a SQL query, the position of an operator is not relevant until operators belong to the same SELECT/FROM statement. In LINQ, this is not true and the conversion to SQL could remove LINQ operators when their operation might be ignored by other operators in the same LINQ query.
Final consideration: initially I considered that the compiler could emit some warning in case a query reduction is done like in the queryA case. Unfortunately, the query reduction operation is done by the LINQ to SQL provider at execution time and not during compilation. A warning could still be possible, but it's something that I would move to tools like FxCop.
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Look at this excellent blog post written by K. Scott Allen. I completely agree with him: don't try to optimize a LINQ query until you measure its performance and understand it is really a bottleneck that needs to be improved.
An interesting consideration I never made before is that you can call the OrderBy extension method after the Select and not before. Yes, using the query syntax of C# you are used to put the Select after the OrderBy, but sometime it could be better to invert this order (the reasons are already well explained in the Scott Allen's post).
And, of course, consider that performance have to be evaluated in two dimension: time and space. And, sooner than later, a third dimension (parallelism) will gain the same importance.
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Samuel Jack wrote two interesting posts discussing possible extension methods for LINQ. One is to implement progress reporting of a LINQ query. The other is to implement a way to cancel a running LINQ query. Both implementations are very simple and they are very good to illustrate how LINQ can be extended and manipulated in a simple way by using extension methods.
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If you are using the Object Relational Designer of LINQ to SQL creating an entity of an existing table that has some VARCHAR(1) fields, you are going into this issue. The data member created in C# is char instead of string. If the field is always filled with one char, this works. But if you try to read a row from the table containing an empty string (not a NULL field, but a field of zero characters) you will get this exception: String must be exactly one character long. This behavior has been already described in this post and in the LINQ forum. But one more warning could be important: this is a latent error that will express yourself only at runtime if you don't fix. Thus, be careful whenever you have some VARCHAR(1) fields in your tables.
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While you have few reviews, a single bad review lower the overall rate. Today we got a bad review, probably because there was a misunderstanding about the scope of the book. I feel the need to give some information to help other possible readers to make a good choice. First of all, I suggest everyone to take a look at the book contents. It already describes pretty well what you should expect to find in this large book. In the same place, you will find the links to download two sample chapters of the book. They are not the toughest ones, but they show you the general approach that is the one of explaining LINQ, addressing its use with other libraries (like ASP.NET, WCF, WPF, WCF and so on). This does not mean that we cover how to make data binding works of how to write an application in WPF. We assume that a particular library for communication or presentation is already in your skills. We only concentrate our attention on data query and manipulation. Another point is the language. We used C# as a language of choice, and used VB.NET only in chapters where the features and/or syntaxes are significantly different (XML integration is one of the most important area for this). Converting existing C# samples in VB is very simple, and we always highlighted when major differences are expected. There are parts where only the VB syntax is available (see XML) and other parts where C# doesn't have correspondent VB syntax. All these differences are well explained in two appendixes, one for C# and the other for VB. We had to make this decision because space was limited and we had a lot of content to put into the book. I hope this will help you. Please contact me if you have any doubt and/or would like to give other feedback.
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Eric White shows some interesting use of LINQ to XML to query an Open XML document. Something that is not immediate to learn when you use LINQ is that you can define your own extension methods to make your queries smarter and more readable. This post is a good exercise to think in a more flexible way: even for me, it's the first time I see an example of "extension" applied to LINQ to XML.
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Two sample chapters of my Programming Microsoft LINQ book are finally available. Links to download pages in the chapters title. Chapter 6 - Tools for LINQ to SQL In this chapter, we took a look at the tools that are available to generate LINQ to SQL entities and DataContext classes. The .NET Framework SDK includes the command-line tool named SQLMetal. Visual Studio 2008 has a graphical editor known as the Object Relational Designer. Both allow the creation of a DBML file, the generation of source code in C# and Visual Basic, and the creation of an external XML mapping file. The Object Relational Designer also allows you to edit an existing DBML file, dynamically importing existing tables, views, stored procedures, and user-defined functions from an existing SQL Server database. Chapter 16 - LINQ and ASP.NET This chapter showed you how to leverage the new features and controls available in ASP.NET 3.5 to develop data-enabled Web applications, using LINQ to SQL and LINQ in general. Consider that what you have seen is really useful for rapidly defining Web site prototypes and simple Web solutions. On the other hand, in enterprise-level solutions you will probably need at least one intermediate layer between the ASP.NET presentation layer and the data persistence one, represented by LINQ to SQL. In real enterprise solutions, you usually also need a business layer that abstracts all business logic, security policies, and validation rules from any kind of specific persistence layer. And you will probably have a Model-View-Controller or Model-View-Presenter pattern governing the UI. In this more complex scenario, chances are that the LinqDataSource control will be tied to entities collections more often than to LINQ to SQL results. The following is the complete list of the chapters included in the book. Programming Microsoft LINQ - Part I LINQ FOUNDATIONS
- 1 LINQ Introduction
- 2 LINQ Syntax Fundamentals
- 3 LINQ to Objects
- Part II LINQ to Relational Data
- 4 LINQ to SQL: Querying Data
- 5 LINQ to SQL: Managing Data
- 6 Tools for LINQ to SQL
- 7 LINQ to DataSet
- 8 LINQ to Entities
- Part III LINQ and XML
- 9 LINQ to XML: Managing the XML Infoset
- 10 LINQ to XML: Querying Nodes
- Part IV Advanced LINQ
- 11 Inside Expression Trees
- 12 Extending LINQ
- 13 Parallel LINQ
- 14 Other LINQ Implementations
- Part V Applied LINQ
- 15 LINQ in a Multitier Solution
- 16 LINQ and ASP.NET
- 17 LINQ and WPF/Silverlight
- 18 LINQ and the Windows Communication Foundation
- Appendixes
- A ADO.NET Entity Framework
- B C# 3.0: New Language Features
- C Visual Basic 2008: New Language Features
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A comment received by one reader of Programming LINQ suggested me to underline a concept that is not so intuitive using LINQ, especially if you come from years of SQL coding.
The idea is very simple. Two entities in LINQ might be related in the model. Whenever this happen, usually it is better to leverage on this existing relationship and not to write the join syntax in an explicit way. If you are using LINQ to SQL, the generated SQL code might be more performant or at least correspondant to the one generated by writing an explicit join in your LINQ query. The less constraints in your query, the better.
Let's look at an example on the Northwind database. Imagine you want to see a list of all categories with a flag set for the one which a particular product belongs to. This is a SQL query we could write:
SELECT
c.CategoryID,
c.CategoryName,
CASE WHEN p.ProductID IS NULL
THEN 0
ELSE 1
END AS Selected
FROM Categories c
LEFT JOIN Products p
ON p.CategoryID = c.CategoryID
AND p.ProductID = 10
ORDER BY CategoryName
Ok, we can write the same query in many other ways, but there are several more complex situations where a LEFT JOIN is used to test the presence of an element in a related table. A correspondant LINQ query might be the following one:
from c in dc.Categories orderby c.CategoryName join p in dc.Products.Where(p => p.ProductID == 10) on c.CategoryID equals p.CategoryID into pj from x in pj.DefaultIfEmpty() select new { c.CategoryID, c.CategoryName, Selected = x != null
};
The LINQ query above will generate a SQL query containing a LEFT JOIN statement. However, a relationship exists between Categories and Customer, and you can leverage on this relationship in the point where you really need to traverse the relationship (in the projection statement). The following one is a better way to get the same result:
from c in dc.Categories orderby c.CategoryName select new { c.CategoryID, c.CategoryName, Selected = c.Products.Any( p => p.ProductID == 10 ) ? true : false };
This new version has two advantages. First, it is shorter and express its intent more explicitly. Second, it generates a SQL query with an EXISTS statement, similar to the following one.
SELECT CategoryID, CategoryName,
(CASE
WHEN EXISTS(
SELECT NULL AS [EMPTY]
FROM Products AS p
WHERE (p.ProductID = 10) AND (p.CategoryID = c.CategoryID)
) THEN 1
ELSE 0
END) AS Selected
FROM Categories AS c
ORDER BY CategoryName
The execution plan used by SQL Server might be similar if not equal. However, using the implicit relationship between Categories and Products in the LINQ query is usually better, because it gives more freedom to the LINQ provider to generate a more efficient SQL code.
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I and Paolo have been interviewed at TechEd by Ken Rosen. We talk about our experience as book authors.
If you are interested in writing a book, or if you simply want to see our faces and hear our italian accent, you can watch the video available in both low resolution and high resolution. Enjoy!
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