I wish to run a unit test on a particular dictionary in my code, trying to get a value I don't expect to be in the database (in this case, key=1).
I have written the following code:
Try
Dim s As String = myDict(1)
Catch ex As KeyNotFoundException
Assert.AreEqual("The given key was not present in the dictionary.", ex.Message)
Catch ex As Exception
Assert.Fail()
Throw
End Try
which works fine, but the code analysis is complaining about the "Dim s as String" declaration, as it says that s will never be used for anything. Well that's intentional, because I intend for this to throw an exception and s is irrelevant.
However, I can't seem to find a way to eliminate s from the code. Simply removing the assignment:
Try
myDict(1)
Catch ex As KeyNotFoundException
Assert.AreEqual("The given key was not present in the dictionary.", ex.Message)
Catch ex As Exception
Assert.Fail()
Throw
End Try
now fails to compile. Any suggestions on how to do this?
Unfortunately there is really no way to fix this in typed code. The call myDict(1) is an indexer and it's not legal as a statement (also illegal in C#). In order to test this you will need to use this expression as a part of legal statement.
One way to accomplish this is pass the value as a parameter to a method which doesn't use it
Sub Unused(ByVal o As Object)
End Sub
...
Unused(myDict(1))
if you are using NUnit Framework than
you could use the following code
Dim f As Func(Of Integer, String) = Function(i) myDict.Item(i)
Dim a As TestDelegate = Function() f(1)
Dim ex As KeyNotFoundException = Assert.Throws(Of KeyNotFoundException)(a)
Assert.AreEqual("The given key was not present in the dictionary.", ex.Message)
This is a similar solution which is proposed by JaredPar
Another option is to make the test returning a value and use the ExpectedException attribute so the code could look like this:
<TestCase(New Object(0 - 1) {}, Result:=Nothing), ExpectedException(GetType(KeyNotFoundException), ExpectedMessage:="The given key was not present in the dictionary."), Test> _
Public Function MyTest() As String
Return myDict.Item(1)
End Function
Looks like I can do this by putting a line after the dictionary call which uses the s variable:
Try
Dim s As String = theDocumentsWithUserNameDictDto.Dict(1)
Assert.Fail("Found unexpected value for dictionary key 1: " & s)
Catch ex As KeyNotFoundException
Assert.AreEqual("The given key was not present in the dictionary.", ex.Message)
End Try
I still don't expect the variable to be used (if the test passes), but this does have the benefit of providing extra clarity to the user if the test does fail for some reason.
Related
Using VS 2013 VB.net for my ClickOnce application. I've got a function which verifies database functionality and the guts are wrapped in a Try Catch. A portion of my Catch block looks like this:
Catch ex As Exception When Err.Number = "5"
My.Application.Log.WriteException(ex)
If My.Settings.g_blnDebugMode Then
MessageBox.Show(Err.Number & " " & ex.ToString, "Exception Error")
End If
If Err.Description.Contains("The specified table does not exist") Then
MessageBox.Show("Selected file is not a valid database.", "Exception Error")
ElseIf Err.Description.Contains("The specified password does not match the database password.") Then
MessageBox.Show("The specified password does not match the current database password.", "Exception Error")
End If
Return False
What I want to do is, clear two different fields based on the two custom error messages at the bottom. Something like TextBox1.Text = "" or TextBox2.Text = "" depending on which error is thrown (invalid password or invalid database). My problem is that I don't seem to be able to set them directly or set the value of a module or global variable from within the catch block.
Error is:
Cannot refer to an instance member of a class from within a shared method or shared member initializer without an explicit instance of the class.
If it's possible how can I work around this and set my TextBoxes based on the results in the Catch block?
Usually the method to achieve what you are trying is to use a second a catch block
Catch ex As Exception When Err.Number = ""
MessageBox.Show(ex.Message)
FlushTextBox1();
Catch ex As Exception When Err.Number = ""
MessageBox.Show(ex.Message)
FlushTextBox2();
The error is likely appearing because the try-catch block is inside a Shared method. What that means is, the change in the value of TextBox will be repeated for all instances of the class. If you want the TextBox to behave like this, add the Shared keyword to its own declaration and remove it from the Sub's declaration. If you don't want this behaviour at all, just remove the Shared keyword. For more information on the error check the MSDN article
Alternatively you can call a local function (as shown in code) FlushTextBox1() to change the value of the TextBox outside the Sub.
How do I handle this type of error or exception?
Try
If log.Trim = txtUSN.Text Then
MessageBox.Show("USN found: " & log)
Else
MessageBox.Show("USN not found: " & log)
End If
Catch ex as Exception
MessageBox.Show(ex.Message)
The message was "Object reference not set to an instance of an object."
This is the rest of the code:
Dim log As String
Dim sql As New SqlCommand
sql.Connection = MyConnection
sql.CommandText = "SELECT * FROM dbo.tblAcc WHERE USN = '" & txtUSN.Text & "' "
MyConnection.Open()
log = sql.ExecuteScalar
MyConnection.Close()
The simple answer is your trying to use an object that is nothing. If it's nothing you can't use it, hence "Object reference not set to an instance of an object."
As already mentioned in my comments above, the culprit is: log. I'm not sure where you have declared this or when your using it and how your using it, for all I know it's nothing. If you have more code that would be greatly appreciated as I can point out where it's nothing, until then here's how to get around your issue.
Try
If log IsNot Nothing Then
If log.Trim = txtUSN.Text Then
MessageBox.Show("USN found: " & log)
Else
MessageBox.Show("USN not found: " & log)
End If
Else
MessageBox.Show("Log is NOTHING!")
End If
Catch ex as Exception
MessageBox.Show(ex.Message)
End Try
**EDIT**
After you posted more code in a comment (please post code in the area, not in comment's) it seem's there are a few issue's. You have log defined as a string; when you do this set it to something like: String.Empty instead of nothing. Also you want to sse the ExecuteScalar method to retrieve a single value (for example, an aggregate value) from a database which could be an integer, long, single etc data types. In your query your selecting everything, you can't call ExecuteScalar to return that data... I would recommend looking up information about building queries and executing them, it's to long for me to get in depth with it here.
Happy Coding!
Make sure that (log) is not an empty string.
if not String.IsNullorEmpty(log) then
end if
I have a simple block of legacy code that sits inside of a loop that crawls through some potentially bad xml node by node and which needs to be refactored as it is not working as intended:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
writer.WriteRaw(strXMLfragment)
Catch ex As Exception
InvalidXML = True
End Try
What this block is meant to do is check for valid xml and then write the xml out. What it actually does is check for invalid xml and then write the xml out only if it is valid. So it needs to be fixed to work as intended.
My first attempt at a fix:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
'writer.WriteRaw(strXMLfragment)
Catch ex As Exception
InvalidXML = True
Finally
writer.WriteRaw(strXMLfragment)
End Try
This works on my test data but I am concerned that WriteRaw may throw an exception on other data. I haven't found a conclusive statement about what will cause WriteRaw to throw an exception and what will happen when code in a Finally block throws an exception.
So I tried rewriting it like this:
Try
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
Catch ex As Exception
InvalidXML = True
End Try
Try
writer.WriteRaw(strXMLfragment)
Catch
End Try
Frankly it looks ugly as hell. Is there a more elegant way to refactor this or will the first attempt be suitable?
When an excpetion is raised in a Finally block, nothing special happens: the exception is propagated out and up like any other exception, and code after the exception in this finally block will not be executed.
Your first attempt will fail if strXMLfragment is null or an empty string (or due to a already running asynchronous operation).
So if you really want to handle/swallow all exceptions, you'll have to use two Try blocks.
To make it cleaner, you might want to pull your first Try/Catch into it's own private function and make it reusable:
Private Function TryParseXml(ByVal xml as String) as Boolean
Try
XDocument.Parse(xml)
Return True
Catch ex As Exception
Return False
End Try
End Function
Then wrap your writer.WriteRaw call in it's own Try/Catch.
Dim myXml = "<temproot>" & strXMLfragment & "</temproot>"
If TryParseXml(myXml) Then
xmlFrag.LoadXml(myXml)
Else
Try
writer.WriteRaw(strXMLfragment)
Catch ex as Exception
' handle exception
End Try
End If
Yes, ultimately this is using two Try/Catch blocks. There is no real way around this as the only real way to determine if Xml is valid is to attempt to parse it and wait for it to blow up.
In the end I came up with this profoundly simple and elegant refactoring:
Try
writer.WriteRaw(strXMLfragment)
xmlFrag.LoadXml("<temproot>" & strXMLfragment & "</temproot>")
Catch ex As Exception
InvalidXML = True
End Try
With the WriteRaw line executing first it will always write out the XML unless it throws an exception. Then the LoadXml line can test for validity without interfering with writing the xml out. This way the InvalidXML flag is set as designed and there won't be any unexpected exceptions.
Here's the code example :
Try
Throw New FirstException()
Finally
Throw New SecondException()
End Try
I figured out it only throws SecondException out and FirstException just vanishes.
I thought FirstException would be inside InnerException property of SecondException but it appears it is not.
I'm not blocked on anything as I don't really need the FirstException to show up, I'm just rather intrigued about this behaviour.
Is there a way to know SecondException did get thrown first when
catching it all at upper level ?
If the first exception really is overriden by the second, what is the
reason ?
Does it happen in every other language ? Is it logical ?
I guess the primary explanation for why this works this way is that you are never catching your first exception and passing it along the chain. If you have a situation like the above where you may be throwing several exceptions on the way back to the original caller then you have to either catch them as they are thrown (and include them as an inner exception when creating the next one) :
Dim ex1 As Exception = Nothing
Try
Throw New Exception("first exception")
Catch ex As Exception
ex1 = ex
Finally
Throw New Exception("second exception", ex1)
End Try
Or, probably better - just don't throw until you have all of the exceptions figured out:
Dim ex1 As Exception = Nothing
Try
ex1 = New Exception("first exception")
Finally
Throw New Exception("second exception", ex1)
End Try
Throwing and catching exceptions is expensive, so it's probably best to not throw until you're ready to return and just log along the way.
One of the limitations of exception handling in .net is that there is no nice way for code in a Finally block to know what exception, if any, caused the code in the Try block to exit, nor is there any normal way for code in a finally block which does have such information to make it available to code which might throw an exception.
In vb.net, it's possible to kludge things in a manner that works pretty well, even though it looks a bit ugly.
Module ExceptionDemo
Function CopySecondArgToFirstAndReturnFalse(Of T)(ByRef dest As T, src As T) As Boolean
dest = src
Return False
End Function
Function AnnotateExceptionAndReturnFalse(ex As Exception, TryBlockException As Exception) As Boolean
If ex Is Nothing Then Return False ' Should never occur
If TryBlockException Is Nothing Then Return False ' No annotation is required
ex.Data("TryBlockException") = TryBlockException
Return False
End Function
Sub ExceptionTest(MainAction As Action, CleanupAction As Action)
Dim TryBlockException As Exception = Nothing
Try
MainAction()
Catch ex As Exception When CopySecondArgToFirstAndReturnFalse(TryBlockException, ex)
' This block never executes, but above grabs a ref to any exception that occurs
Finally
Try
CleanupAction()
Catch ex As Exception When AnnotateExceptionAndReturnFalse(ex, TryBlockException)
' This block never executes, but above performs necessary annotations
End Try
End Try
End Sub
Sub ExceptionTest2(Message As String, MainAction As Action, CleanupAction As Action)
Debug.Print("Exception test: {0}", Message)
Try
ExceptionTest(MainAction, CleanupAction)
Catch ex As Exception
Dim TryBlockException As Exception = Nothing
Debug.Print("Exception occurred:{0}", ex.ToString)
If ex.Data.Contains("TryBlockException") Then TryBlockException = TryCast(ex.Data("TryBlockException"), Exception)
If TryBlockException IsNot Nothing Then Debug.Print("TryBlockException was:{0}", TryBlockException.ToString)
End Try
Debug.Print("End test: {0}", Message)
End Sub
Sub ExceptionDemo()
Dim SuccessfulAction As Action = Sub()
Debug.Print("Successful action")
End Sub
Dim SuccessfulCleanup As Action = Sub()
Debug.Print("Cleanup is successful")
End Sub
Dim ThrowingAction As Action = Sub()
Debug.Print("Throwing in action")
Throw New InvalidOperationException("Can't make two plus two equal seven")
End Sub
Dim ThrowingCleanup As Action = Sub()
Debug.Print("Throwing in cleanup")
Throw New ArgumentException("That's not an argument--that's just contradiction")
End Sub
ExceptionTest2("Non-exception case", SuccessfulAction, SuccessfulCleanup)
ExceptionTest2("Exception in main; none in cleanup", ThrowingAction, SuccessfulCleanup)
ExceptionTest2("Exception in cleanup only", SuccessfulAction, ThrowingCleanup)
ExceptionTest2("Exception in main and cleanup", ThrowingAction, ThrowingCleanup)
End Sub
End Module
The module above starts with a couple helper modules which should probably be in their own "Exception helpers" module. The ExceptionTest method shows the pattern for code which might throw an exception in both the Try and Finally block. The ExceptionTest2 method calls ExceptionTest and reports what exception if any comes back from it. ExceptionDemo calls ExceptionTest2 in such a way as to cause exceptions in different combinations of the Try and Finally blocks.
As shown, if an exception occurs during cleanup, that exception will be returned to the caller, with the original exception being an item in its Data dictionary. An alternative pattern would be to catch the exception that occurs on cleanup and include it in the data of the original exception (which would be left uncaught). My general inclination is that it's probably better in many cases to propagate the exception that occurs during cleanup, since any code which was planning to deal with the original exception will probably expect that cleanup succeeded; if such an expectation cannot be met, the exception that escapes should probably not be the one the caller was expecting. Note also that the latter approach would require a slightly different method of adding information to the original exception, since an exception which is thrown in a nested Try block might need to hold information about multiple exceptions that were thrown in nested Finally blocks.
I ran into an interesting dilemma today. I have a function that handles information and checks for duplicate values, then returns the next number that is not a duplicate. So, I have something like this:
Public Function GetNextNonDuplicateNumber(NumberToCheck as Long) as Long
//the non-duplicate the function will return
Dim NonDuplicate as Long
If CheckForDuplicate(NumberToCheck) = True Then
Throw New DuplicateException()
Else
NonDuplicate = NumberToCheck
End If
End Function
Then at the bottom of the function I have a catch block that handles the duplicate by incrementing until I don't have a duplicate any more, like this:
Catch ex as DuplicateException
NonDuplicate = IncrementToNonDuplicateValue(NumberToCheck)
Throw ex
Return NonDuplicate
End Function
As you can see, I want to handle the exception specifically, but I also want to throw it when I'm done because I want to alert other code outside the function.
The problem is that simply throwing it exits out of the function with a null value. Am I thinking about a try/catch the wrong way, or is there a way around this?
If you caught an exception and recovered from it (with your IncrementToNonDuplicate...) then there is no reason to throw an exception anymore. Code between catch and end try should just clean the resources like closing a file or datareader if you will rethrow it.
You could rather return a structure that contains NonDuplicate value and required information about errors in function.
Other way would be to throw a custom exception that will contain information like "Invalid number: it should be...)
You can return a boolean indicating if a duplicate is found, and change the parameter to be passed in by reference so you can update the value.
Public Function GetNextNonDuplicateNumber(ByRef NonDupeNumber as Long) as Boolean