Archive for the ‘VB.NET’ Category.
May 19, 2010, 7:15 pm
The title of this article is a pretty obvious statement, but it’s actually pretty easy to forget and it can lead to some painful bugs. Here’s a code snippet whose output may seem surprising:
Module Module1
Sub Main()
Dim query =
From name In {"one", "two", "three"}
Select New User(name)
Dim x = query.First
Dim y = query.First
Console.WriteLine(x Is y)
Console.ReadKey()
End Sub
End Module
Public Class User
Public Sub New(ByVal username As String)
Console.WriteLine("Creating user: " & username)
End Sub
End Class
At first glance it looks like x and y are the same object, but since query is just a query, not an actual collection, the result will be fetched independently each time you call First. When the code is run, the console output looks like this:
Creating user: one
Creating user: one
False
This shows that the constructor was called twice for the same string, which explains why x and y are actually different objects. In a case like this it is better to put ToList at the end of the query:
Dim users =
(From name In {"one", "two", "three"}
Select New User(name)).ToList()
Bugs like this can be particularly problematic when you pass around an object of type IEnumerable and the programmer assumes that they are dealing with a collection, when they are really dealing with a query. So… watch out!
April 2, 2010, 11:14 am
Up until Visual Studio 2010, simple property definitions were always ridiculously verbose. For example:
Private _title As String
Public Property Title() As String
Get
Return _title
End Get
Set(ByVal value As String)
_title = value
End Set
End Property
That’s 9 lines of code just to make a simple string property. Luckily, in VB 10 we will be able to write these simple properties in one line:
Public Property Title As String
This is great, but what if things get a little more complicated? Obviously this syntax just uses the default getter and setter, both of which have the same scope. When I write my properties I like avoid using ReadOnly and instead make the setter private so that I can still encapsulate the setting code within the class. With VB 10 you still have to define the property the old way in order to do this:
Private _title As String
Public Property Title() As String
Get
Return _title
End Get
Private Set(ByVal value As String)
_title = value
End Set
End Property
But now it’s back to 9 lines just because I wanted the setter to be private. In C#, programmers have the luxury of writing properties like this:
public string Title { get; private set; }
Furthermore, neither language allows you to use a default getter and custom setter (or vice versa) probably because it would make it difficult for them both to use the same backing store, but that’s a whole other tangent. Overall, the new abbreviated syntax for properties in VB 10 is great for the simple cases, but you have to revert to the old, verbose method if you want to deviate from the default in even the slightest way.
March 6, 2010, 9:17 am
A common pattern in database design is to use make a column required, give it a default value and then never think about it when doing INSERTs. A perfect example would be a createdDate column on the Users with a default value of GetDate(). Here’s the full table definition:
- userID (identity key)
- userName
- password
- ts (timestamp)
- createdDate (default value = GetDate())
In this case we can easily insert into the table without worrying about the createdDate, userID, or ts columns:
INSERT INTO Users (userName, password) VALUES ('asdf', 'qwer')
However, since this is the 21st century, we don’t want to do this in SQL, we want to do it with an ORM. Unfortunately, LINQ to SQL doesn’t do a very good job with this.
Using context = New TestDataContext
' Output SQL to the console for debugging
context.Log = Console.Out
' Attach a new user and submit the changes
Dim newUser As New User With {.userName = "NewUser", .password = "password"}
context.Users.InsertOnSubmit(newUser)
context.SubmitChanges()
End Using
The above code generates the following INSERT statement when SubmitChanges() is called (note: I replaced @p0, @p1, etc with their actual values to make the query more readable):
INSERT INTO [dbo].[Users]([userName], [password], [createdDate]) VALUES ('NewUser', 'password', NULL)
This query fails and we get a SqlTypeException because createdDate is NOT NULL and NULL cannot be converted to a valid date. Notice that the generated SQL does not attempt to explicitly set a value for userID or ts. It appears that LINQ to SQL knows how to deal with IDENTITY fields and TIMESTAMPs, but not how to deal with other required columns that happen to have a default value.
I would have expected LINQ to SQL to generate a query that does not explicitly set createdDate so that SQL Server could handle it, but no such luck. You can easily set the createdDate manually like this:
Dim newUser As New User With {.userName = "NewUser", .password = "password", .createdDate = Date.Now}
It really sucks to have to do this every time though, especially if you have many fields to fill in. A possible alternative is to put a partial class on either your DataContext or just on the User class and write some code that will automatically initialize fields like createdDate. If you want to make generic behaviour for this (eg: automatically set columns named “createdDate” to Date.Now when SubmitChanges is called) you can do something like this in the DataContext partial class:
Public Overrides Sub SubmitChanges(ByVal failureMode As ConflictMode)
' NOTE: this is just a sample to get you started
For Each insert In GetChangeSet().Inserts
Dim createdDateProp = insert.GetType.GetProperty("createdDate")
If createdDateProp IsNot Nothing Then
createdDateProp.SetValue(insert, Date.Now, Nothing)
End If
Next
MyBase.SubmitChanges(failureMode)
End Sub
February 9, 2010, 9:23 pm
The great thing about fetching data via a LINQ to SQL query is that you get a nice formatted result and you can easily save back any changes you make with SubmitChanges(). Unfortunately, we all inevitably fall into scenarios where we have to make use of stored procedures for performance or other reasons. If you have a stored procedure whose result set contains columns from just a single table then you can easily map the stored procedure to that table, but in most cases the result set involves multiple tables making things a little more tricky. It’s easy to execute a stored procedure from LINQ to SQL (just drag the SP from the server explorer into the designer and then execute it like a function on the data context) but you lose some of the benefits of LINQ to SQL. First of all, you just get a flat result set instead of a hierarchical result set using the auto generated entity classes. Second, you can’t just make changes to the result and call SubmitChanges. Luckily, with a little extra work, the flat, detached result set can be converted into a hierarchical, attached result set where changes can easily be saved.
If you don’t want to bother reading the whole article and all of the code, here’s the short answer: use the Attach() method.
Below is an example that runs a stored procedure to return all users in the database joined with their articles. The results are converted into an attached list of users, each containing a collection of articles. Notice that not all of the columns need to be known, just the primary key and timestamp are required. For more info on the timestamp, check out this article.
Module Module1
Sub Main()
Using testContext As New TestDataContext
' Print SQL queries to the console for testing purposes
testContext.Log = Console.Out
' Get attached entities
Dim users = GetAttachedUsersWithGroups(testContext)
' Make some random changes to prove the concept
users.First.userName = "foo"
users.First.Articles.First.text = "bar"
' Submit the changes to see what SQL gets executed
testContext.SubmitChanges()
End Using
Console.ReadKey()
End Sub
Public Function GetAttachedUsersWithGroups(ByVal context As TestDataContext) As IEnumerable(Of User)
' Get some data from a stored procedure
Dim result = context.GetAllUsersWithArticles
' Convert flat result set to groups of articles by user
Dim userGroups = From row In result _
Group row By row.userID, row.userTimestamp _
Into articles = Group _
Select userID, userTimestamp, articles
Dim users As New List(Of User)
' Create LINQ to SQL entities
For Each userGroup In userGroups
Dim user As New User With {.userID = userGroup.userID, _
.ts = userGroup.userTimestamp}
For Each article In userGroup.articles
user.Articles.Add(New Article With {.articleID = article.articleID, _
.title = article.title, _
.ts = article.articleTimestamp})
Next
users.Add(user)
Next
' Attach the users to the data context. This will also attach the articles
' because they have been added to each user's Articles collection.
context.Users.AttachAll(users)
Return users
End Function
End Module
February 5, 2010, 7:26 pm
In LINQ to SQL you can chain multiple where clauses like this:
Module Module1
Sub Main()
Using context As New TestDataContext
context.Log = Console.Out
Dim articles = context.Articles.Where(Function(a) a.articleID > 10) _
.Where(Function(a) a.articleID Mod 2 = 0) _
.ToList()
End Using
Console.ReadKey()
End Sub
End Module
This will generate SQL that looks roughly like this:
SELECT ... FROM Articles WHERE articleID > 10 AND articleID % 2 = 0
Since chained where clauses are equivalent to ANDing multiple expressions in a single WHERE, the above SQL is exactly what you would expect to see. Unfortunately, things get more complicated when one of the expressions cannot be converted to SQL, like in this case:
Module Module1
Sub Main()
Using context As New TestDataContext
context.Log = Console.Out
Dim articles = context.Articles.Where(AddressOf FilterArticle).ToList()
End Using
Console.ReadKey()
End Sub
Function FilterArticle(ByVal a As Article) As Boolean
Return a.articleID Mod 2 = 0
End Function
End Module
The above code generates SQL that looks like this:
The query has no where clause, it just loads all the articles and then filters them on the client side. It’s usually optimal to do the filtering on the SQL side, but the behaviour is reasonable. I wouldn’t expect the ORM to be capable of magically converting the contents of the FilterArticle function into SQL (it sure would be nice though). This is still expected behaviour, but here’s an example where things get weird:
Module Module1
Sub Main()
Using context As New TestDataContext
context.Log = Console.Out
Dim articles = context.Articles.Where(AddressOf FilterArticle) _
.Where(Function(a) a.articleID > 10) _
.ToList()
End Using
Console.ReadKey()
End Sub
Function FilterArticle(ByVal a As Article) As Boolean
Return a.articleID Mod 2 = 0
End Function
End Module
This code generates the same SQL as last time:
It is filtering both where clauses on the client side even though the second one could have been converted to SQL. If you flip the where clauses like this:
Module Module1
Sub Main()
Using context As New TestDataContext
context.Log = Console.Out
Dim articles = context.Articles.Where(Function(a) a.articleID > 10) _
.Where(AddressOf FilterArticle) _
.ToList()
End Using
Console.ReadKey()
End Sub
Function FilterArticle(ByVal a As Article) As Boolean
Return a.articleID Mod 2 = 0
End Function
End Module
then you will still get the expected SQL:
SELECT ... FROM Articles WHERE articleID > 10
The where clause that can be converted to SQL is filtered in the SELECT statement, but the clause that cannot be converted is filtered on the client side. I would have hoped that the order of the where clauses would not matter since they are just being ANDed, but that is not the case.
The lesson is that if you need to chain a where clause that cannot be converted to SQL, try to put it at the end of the chain. This can be a real issue if you are using a data access layer that automatically filters queries (eg: for security) with a function that cannot convert to SQL. If all of your LINQ to SQL queries have this built in filter then none of them will ever generate WHERE clauses in the SQL, it will just load the entire table every time.
February 4, 2010, 8:57 pm
Download the code here: WpfLoadingOverlay.zip
A common issue in MDI or TDI style user interfaces is that it can take a long time to render new forms when they are opened. Even a one or two second delay is enough to make an application seem very unresponsive. If there’s nothing you can do to improve the actual performance (ie: the total time to load the form) you can at least improve the perceived performance. This code sample creates a simple form with a tab control and a button. Every time you click the button it adds a new tab with 4000 text boxes. Depending on the speed of your computer the form will probably take around five seconds to load. As soon as you click the button, the new tab appears with a loading animation that continues until the contents have been rendered. This is not a trivial task because we want to render a loading animation at the same time that we are waiting for another long rendering operation to complete. Basically, we need two rendering threads. You can’t have multiple rendering threads in a single window, but you can put your loading animation in a new window with its own rendering thread and make it look like it’s not a separate window.
The loading overlay is a separate, chromeless window that does not appear on the taskbar and disappears as soon as loading is complete. The window is also semi-transparent and automatically positioned exactly over top of the form that is loading, so it looks like it is a part of the existing window.
Download the sample project and try it out.
January 31, 2010, 8:45 pm
If you attach an entity with a required association that is nulled out, you will be unable to call GetChangeSet(). In my opinion, the expected behaviour is that the entity should show up in the change set as though it is valid, but an exception should be thrown when you attempt to call SubmitChanges() because a foreign key constraint has been violated. In fact, with code like this we will get exactly that result (an exception is thrown on SubmitChanges()):
Using testData As New TestDataContext
Dim newArticle As New Article With {.title = "Foobar", _
.text = "blah blah blah"}
testData.Articles.InsertOnSubmit(newArticle)
Dim changes = testData.GetChangeSet()
testData.SubmitChanges()
End Using
There is a required association to the Users table that has not been set at all. Using the following snippet, with the User property explicitly set to Nothing an exception will be thrown on GetChangeSet() instead of SubmitChanges():
Using testData As New TestDataContext
Dim newArticle As New Article With {.title = "Foobar", _
.text = "blah blah blah", _
.User = Nothing}
testData.Articles.InsertOnSubmit(newArticle)
Dim changes = testData.GetChangeSet()
testData.SubmitChanges()
End Using
It gives this error on GetChangeSet():
An attempt was made to remove a relationship between a User and a Article. However, one of the relationship’s foreign keys (Article.userID) cannot be set to null.
It appears that the internal implementation of LINQ to SQL distinguishes between an unset relationship, and one that has specifically been set to Nothing. The awkward thing here is that it is not always easy to avoid this issue since you don’t even have to call InsertOnSubmit. Attaching an entity by setting an association to an already attached object gives the same result.
Using testData As New TestDataContext
Dim existingUser = testData.Users.First
Dim newUserGroup As New UserGroup With {.User = existingUser, .Group = Nothing}
Dim changes = testData.GetChangeSet()
testData.SubmitChanges()
End Using
In this snippet there are two required associations: User and Group. As soon as User is set, the UserGroup entity is attached to the DataContext. However, since Group is Nothing the ChangeSet is now corrupt.
This bug is described in this forum thread where a Microsoft employee called it a bug and recommended that he post it on Connect (Microsoft’s bug tracking site). The bug report on Connect can be found here. One hour after it was posted Microsoft replied saying this:
We are currently investigating. The investigation process normally takes 7-14 days.
They then went silent for 9 months before posting this:
Hi,
Thank you for taking the time to send this feedback and bug report. We have reviewed the issue and confirmed the behavior, but we will not be fixing this in the next release of LINQ to SQL.
LINQ to SQL Team
That’s Microsoft for ya.
January 31, 2010, 4:48 pm
By default, LINQ to SQL uses deferred loading. When you want to eager load an entity’s associated data you need to set DataLoadOptions using the LoadOptions property on the DataContext. If you have a one-to-many relationship between Users and Articles you can force LINQ to SQL to eager load Articles with Users like this:
Using testData As New TestDataContext
' Log SQL queries to the console
testData.Log = Console.Out
' Set LoadOptions
Dim options As New DataLoadOptions
options.LoadWith(Function(user As User) user.Articles)
testData.LoadOptions = options
' Load users with their articles
Dim users = testData.Users.ToList
For Each user In users
Dim articles = user.Articles.ToList
Next
End Using
This will generate a single SELECT statement with a JOIN on the Articles table. The same goes for for one-to-one relationships. You can also use LoadWith as many times as you want. For one-to-one relationships and no more than a single one-to-many relationship this will still generate one query with JOINs to all the LoadWith tables. However, if you want to eager load multiple one-to-many relationships you will get into a select N + 1 situation (or worse). For example, this code eager loads Articles and UserGroups with each User entity:
Using testData As New TestDataContext
' Log SQL queries to the console
testData.Log = Console.Out
' Set LoadOptions
Dim options As New DataLoadOptions
options.LoadWith(Function(user As User) user.Articles)
options.LoadWith(Function(user As User) user.UserGroups)
testData.LoadOptions = options
' Load users with their articles
Dim users = testData.Users.ToList
For Each user In users
Dim articles = user.Articles.ToList
Dim userGroups = user.UserGroups.ToList
Next
End Using
Technically, the behaviour here is correct. It will successfully eager load both the Articles and UserGroups collections for each User, but it will not do it in a single query. When I ran this I got one query that fetched the Users and Articles like last time, but then a separate SELECT for each UserGroup rather than another JOIN. Even though this won’t alter the behaviour of the code, it will definitely make a major impact on performance, especially if there are a lot of users in the database.
Scott Guthrie confirmed this behaviour in a post on David Hayden’s blog. This is what he said:
In the case of a 1:n associations, LINQ to SQL only supports joining-in one 1:n association per query.
Lame.
January 23, 2010, 9:15 am
A couple months ago I wrote this article explaining why I think it is reasonable for unit tests to hit a real database. Subsequently, I wrote a follow up article describing some techniques for rolling back your database to its original state after each test. In that article I found that just using simple transactions did not solve the problem because you need access to all database connections being used, and they all have to be rolled back. I have since found a way around this problem using distributed transactions.
With the Microsoft Distributed Transaction Coordinator (MSDTC) the activity over multiple connections can be lumped into a single transaction using the TransactionScope class. MSDTC needs to be running for this to work, but since this is just for unit tests it doesn’t need to be enabled on your production environment.
In order to use the TransactionScope class your project will need a reference to System.Transactions. Here’s a sample unit test using MSTest and Entity Framework where the database is altered with multiple connections within a transaction and then the changes are rolled back:
Imports System.Transactions
Imports System
Imports System.Text
Imports System.Collections.Generic
Imports Microsoft.VisualStudio.TestTools.UnitTesting
<TestClass()> _
Public Class UnitTestSample
<TestMethod()> _
Public Sub ProofOfConceptTest()
Using New TransactionScope
Dim conn1 As New DataTestEntities
Dim conn2 As New DataTestEntities
Dim row1 As New Users With {.userName = "user1", .password = "pass"}
Dim row2 As New Users With {.userName = "user2", .password = "pass"}
conn1.AddToUsers(row1)
conn2.AddToUsers(row2)
conn1.SaveChanges()
conn2.SaveChanges()
Dim conn3 As New DataTestEntities
Assert.AreEqual(conn3.Users.Count, 6)
End Using
End Sub
End Class
Alternatively, if you want every test method inside a test class to be within its own TransactionScope without adding a Using block to every single test, you can use the initialization and cleanup methods like this:
Imports System.Transactions
Imports System
Imports System.Text
Imports System.Collections.Generic
Imports Microsoft.VisualStudio.TestTools.UnitTesting
<TestClass()> _
Public Class UnitTestSample
Private _transaction As TransactionScope
<TestInitialize()> _
Public Sub Setup()
_transaction = New TransactionScope
End Sub
<TestCleanup()> _
Public Sub TearDown()
_transaction.Dispose()
End Sub
<TestMethod()> _
Public Sub ProofOfConceptTest()
Dim conn1 As New DataTestEntities
Dim conn2 As New DataTestEntities
Dim row1 As New Users With {.userName = "user1", .password = "pass"}
Dim row2 As New Users With {.userName = "user2", .password = "pass"}
conn1.AddToUsers(row1)
conn2.AddToUsers(row2)
conn1.SaveChanges()
conn2.SaveChanges()
Dim conn3 As New DataTestEntities
Assert.AreEqual(conn3.Users.Count, 6)
End Sub
End Class
As long as the use of MSDTC is an option, I have found this method to be far better than any of those described in the last article. It guarantees that the state or your database is maintained and is extremely fast (at least on small amounts of data).
October 27, 2009, 7:37 pm
WPF data binding has built in support for IDataErrorInfo, so it is easy to display a validation error when a property has invalid data. However, sometimes we need to manually find all the validation errors on an object. A perfect example of this is when trying to save data. Often you will want to verify that there are no validation errors before allowing the save operation to proceed. Using the MVVM pattern it would be ideal to determine whether there are any errors purely within your model view. The default behavior of IDataErrorInfo does not give a collection of all current errors. Instead, it will just tell you if a given property has an error (ie: using IDataErrorInfo.Item), so all we have to do to find all the errors is enumerate through each property on the class and call the Item property with that property name as the argument. The GetValidationErrors function below does exactly that:
Option Strict On
Imports System.ComponentModel
Public Class ValidationHelper
''' <summary>
''' Checks for errors in <c>validatable</c> and returns all the errors found.
''' </summary>
Public Shared Function GetValidationErrors(ByVal validatable As IDataErrorInfo) As IEnumerable(Of DataError)
Dim errors As New List(Of DataError)
' Iterate through every property in the class
For Each prop In validatable.GetType.GetProperties
' If the property has an error, then add it to the list
Dim errorMessage = validatable(prop.Name)
If errorMessage IsNot Nothing Then
errors.Add(New DataError(prop.Name, errorMessage))
End If
Next
Return errors
End Function
''' <summary>
''' Represents a single error. It's a propertyName, errorMessage pair
''' </summary>
Public Class DataError
Private _propertyName As String
Private _errorMessage As String
Public Sub New(ByVal propertyName As String, ByVal errorMessage As String)
_propertyName = propertyName
_errorMessage = errorMessage
End Sub
''' <summary>
''' The name of the property that has the error.
''' </summary>
Public ReadOnly Property PropertyName() As String
Get
Return _propertyName
End Get
End Property
''' <summary>
''' A description of the property's error.
''' </summary>
Public ReadOnly Property ErrorMessage() As String
Get
Return _errorMessage
End Get
End Property
End Class
End Class