I have this method:
Private Sub SetIfNotNull(ByVal input As Object, ByRef destination As Object, ByVal ConversionType As ConversionType)
If input IsNot Nothing AndAlso input <> "" Then
Select Case ConversionType
Case DealerTrackConnection.ConversionType._String
destination = input
Case DealerTrackConnection.ConversionType._Integer
destination = Convert.ToInt32(input)
Case DealerTrackConnection.ConversionType._Double
destination = Convert.ToDouble(input)
Case DealerTrackConnection.ConversionType._Date
destination = Convert.ToDateTime(input)
Case DealerTrackConnection.ConversionType._Decimal
destination = Convert.ToDecimal(input)
End Select
End If
End Sub
And here is one call in which it fails:
SetIfNotNull(ApplicantElement.Element("suffix").Value, NewApplicant.Suffix, ConversionType._String)
If the element from the XML file is nothing (there is no tag), the method call fails. but I am checking for nothing. Why is it doing this and how would I modify the code to fix it this time and everytime.
The problem isn't in your SetIfNotNull method, rather it is in this piece of code: ApplicantElement.Element("suffix").Value
The element is null, so the Value call throws a NullReferenceException. Try this instead:
CType(ApplicantElement.Element("suffix"), String)
Also, you can consolidate the checks in this line:
If input IsNot Nothing AndAlso input <> "" Then
into this:
If Not String.IsNullOrEmpty(input) Then
It seems that ApplicantElement.Element("suffix") is nothing and therefor the exception occurs before your method is called, isn't it?
If Not ApplicantElement.Element("suffix") Is Nothing Then
SetIfNotNull(ApplicantElement.Element("suffix").Value, NewApplicant.Suffix, ConversionType._String)
End If
Related
So I'm reading through my source code looking for places to improve the code when I come across this unholy chunk of code.
Public Function ReadPDFFile(filePath As String,
Optional maxLength As Integer = 0) As List(Of String)
Dim sbContents As New Text.StringBuilder
Dim cArrayType As Type = GetType(PdfSharp.Pdf.Content.Objects.CArray)
Dim cCommentType As Type = GetType(PdfSharp.Pdf.Content.Objects.CComment)
Dim cIntegerType As Type = GetType(PdfSharp.Pdf.Content.Objects.CInteger)
Dim cNameType As Type = GetType(PdfSharp.Pdf.Content.Objects.CName)
Dim cNumberType As Type = GetType(PdfSharp.Pdf.Content.Objects.CNumber)
Dim cOperatorType As Type = GetType(PdfSharp.Pdf.Content.Objects.COperator)
Dim cRealType As Type = GetType(PdfSharp.Pdf.Content.Objects.CReal)
Dim cSequenceType As Type = GetType(PdfSharp.Pdf.Content.Objects.CSequence)
Dim cStringType As Type = GetType(PdfSharp.Pdf.Content.Objects.CString)
Dim opCodeNameType As Type = GetType(PdfSharp.Pdf.Content.Objects.OpCodeName)
Dim ReadObject As Action(Of PdfSharp.Pdf.Content.Objects.CObject) = Sub(obj As PdfSharp.Pdf.Content.Objects.CObject)
Dim objType As Type = obj.GetType
Select Case objType
Case cArrayType
Dim arrObj As PdfSharp.Pdf.Content.Objects.CArray = DirectCast(obj, PdfSharp.Pdf.Content.Objects.CArray)
For Each member As PdfSharp.Pdf.Content.Objects.CObject In arrObj
ReadObject(member)
Next
Case cOperatorType
Dim opObj As PdfSharp.Pdf.Content.Objects.COperator = DirectCast(obj, PdfSharp.Pdf.Content.Objects.COperator)
Select Case System.Enum.GetName(opCodeNameType, opObj.OpCode.OpCodeName)
Case "ET", "Tx"
sbContents.Append(vbNewLine)
Case "Tj", "TJ"
For Each operand As PdfSharp.Pdf.Content.Objects.CObject In opObj.Operands
ReadObject(operand)
Next
Case "QuoteSingle", "QuoteDbl"
sbContents.Append(vbNewLine)
For Each operand As PdfSharp.Pdf.Content.Objects.CObject In opObj.Operands
ReadObject(operand)
Next
Case Else
'Do Nothing
End Select
Case cSequenceType
Dim seqObj As PdfSharp.Pdf.Content.Objects.CSequence = DirectCast(obj, PdfSharp.Pdf.Content.Objects.CSequence)
For Each member As PdfSharp.Pdf.Content.Objects.CObject In seqObj
ReadObject(member)
Next
Case cStringType
sbContents.Append(DirectCast(obj, PdfSharp.Pdf.Content.Objects.CString).Value)
Case cCommentType, cIntegerType, cNameType, cNumberType, cRealType
'Do Nothing
Case Else
Throw New NotImplementedException(obj.GetType().AssemblyQualifiedName)
End Select
End Sub
Using pd As PdfSharp.Pdf.PdfDocument = PdfSharp.Pdf.IO.PdfReader.Open(filePath, PdfSharp.Pdf.IO.PdfDocumentOpenMode.ReadOnly)
For Each page As PdfSharp.Pdf.PdfPage In pd.Pages
ReadObject(PdfSharp.Pdf.Content.ContentReader.ReadContent(page))
If maxLength > 0 And sbContents.Length >= maxLength Then
If sbContents.Length > maxLength Then
sbContents.Remove(maxLength - 1, sbContents.Length - maxLength)
End If
Exit For
End If
sbContents.Append(vbNewLine)
Next
End Using
'Return sbContents.ToString
Dim ReturnList As New List(Of String)
For Each Line In sbContents.ToString.Split(vbNewLine)
If String.IsNullOrWhiteSpace(Line.Trim) Then
Else
ReturnList.Add(Line.Trim)
End If
Next
Return ReturnList
End Function
All this does is read the text parts of a PDF using PDFSharp. What caught my eye however was line 17. Is that a Sub inside of the function?
So, what exactly is this Sub inside of a function? I didn't write this code so I've never seen anything like this before.
How does this work exactly and why wouldn't I use a function to do the processing and then return the results?
In short, my question is, what is this, how does it work, and why would I want to use something like this?
That's a so-called Lambda expression. They're used to create inline (or more correctly: in-method) methods, which makes them more dynamic than normal methods.
In your example a lambda expression is not necessary and only makes the code harder to understand. I suppose the author of that code wrote a lambda expression instead of a separate method in order to not expose ReadObject to any outside code.
One of the best uses for a lambda expression IMO is when you want to make thread-safe calls to the UI thread, for instance:
If Me.InvokeRequired = True Then
Me.Invoke(Sub() TextBox1.Text = "Process complete!")
Else
TextBox1.Text = "Process complete!"
End If
...where the same code without a lambda would look like this:
Delegate Sub UpdateStatusTextDelegate(ByVal Text As String)
...somewhere else...
If Me.InvokeRequired = True Then
Me.Invoke(New UpdateStatusTextDelegate(AddressOf UpdateStatusText), "Process complete!")
Else
UpdateStatusText("Process complete!")
End If
...end of somewhere else...
Private Sub UpdateStatusText(ByVal Text As String)
TextBox1.Text = Text
End Sub
There are also other examples where lambda expressions are useful, for instance if you want to initialize a variable but do some processing at first:
Public Class Globals
Public Shared ReadOnly Value As Integer = _
Function()
DoSomething()
Dim i As Double = CalculateSomething(3)
Return Math.Floor(3.45 * i)
End Function.Invoke()
...
End Class
Yet another usage example is for creating partially dynamic event handlers, like this answer of mine.
So I am trying to make a code where the user inputs an ID number and receives the line of text that the ID corresponds to. I am having trouble with the code as I am unable to display the result (or correct result) in a label box.
For example:
If I type in ID 1, it displays the data that corresponds to ID 2, ID 2 corresponds to ID 3 and ID 3 ends the loop (there are only 3 records in the data currently).
I have included my code below
Private Sub btnSearch_Click(sender As Object, e As EventArgs) Handles btnSearch.Click
Filename = "NamesAndAges.txt"
FileOpen(1, Filename, OpenMode.Input,,,)
Dim ID As Integer
ID = txtSearch.Text
Dim Found As Boolean
Found = False
Do While Not EOF(1) And Found = False
If LineInput(1).Contains(ID) Then
lblDisplaySearch.Text = LineInput(1)
Found = True
Else MsgBox("Not Found")
End If
Loop
FileClose(1)
End Sub
Thanks in advance, would also really appreciate if anyone could explain the code they use as I am still a visual basic beginner.
Every time you call LineInput(1), it reads a line, so you're reading a line and checking if it contains ID then reading another line and setting lblDisplaySearch.Text to that value.
Try something like this:
Dim line As String
Do While Not EOF(1) And Found = False
line = LineInput(1)
If line.Contains(ID) Then
lblDisplaySearch.Text = line
Found = True
Else MsgBox("Not Found")
End If
Loop
I would strongly suggest simplifying this code, by using File.ReadLines:
Private Sub btnSearch_Click(sender As Object, e As EventArgs) Handles btnSearch.Click
Filename = "NamesAndAges.txt"
Dim line = File.ReadLines(Filename).FirstOrDefault(Function(x) x.Contains(txtSearch.Text))
If line Is Nothing Then
MsgBox("Not Found")
Exit Sub
End If
lblDisplaySearch.Text = line
End Sub
The primary advantage is that you don't need to manage the Do While loop. Instead, you are passing the match condition (Function(x) x.Contains(txtSearch.Text)) to the FirstOrDefault method, which internally will find the first line matching the condition and return it, or return Nothing if it's not found.
For Each and IEnumerable
VB.NET allows you to loop over a set of items without knowing or caring about the index within the set. For example:
For Each x As String In File.ReadLines(Filename)
'do something with the line, which is now in x
Next
In order to use For Each with an object, that object has to implement a specific interface — the IEnumerable interface.
Interfaces
Interfaces guarantee that a given object has, or implements, specific members. In this case, if an object implements the IEnumerable interface, that means it has a GetEnumerator method, which the For Each uses under the hood.
IEnumerable(Of T)
The object returned from File.ReadLines implements another more advanced generic interface called IEnumerable(Of T). With this interface, the compiler can figure out automatically that each step of the For Each will be parsing a string, and we don't need to specify it as a string:
For Each x In File.ReadLines(Filename)
'x is known to be a String here
Next
Lambda expressions
The condition "the line which contains the search text" is written as a lambda expression, which (in this case) is made into a method without an explicit name or definition. This is convenient, because we don't have to write this:
Function ContainsCondition(line As String, toFind As String) As Boolean
Return line.Contains(toFind)
End Function`
every time we want to express a condition this way.
Type of x in the condition
Because File.ReadLines returns an IEnumerable(Of T), in this case an IEnumerable(Of String), the compiler can figure out that the condition is working on strings, so we don't have to specify that x is a string within the condition.
I'm getting this error and I don't know how to fix it. I know i'm supposed to write what I've tried, but i have no idea what to try, even after looking for a few hours about how to deal with the error.
If more information is needed please let me know. Thanks
The highlighted part of the code is:
Dim cdtrabajador As String = dgvr.Cells(0).Value.ToString
Object reference not set to an instance of an object.
NullReference Exception was unhandled by user code
Private Sub PeopleDataGridView_CellFormatting(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellFormattingEventArgs) Handles DataGridView1.CellFormatting
If DataGridView1.RowCount > 0 AndAlso e.RowIndex > -1 Then
If e.RowIndex > -1 Then
Dim dgvr As DataGridViewRow = DataGridView1.Rows(e.RowIndex)
Dim cdtrabajador As String = dgvr.Cells(0).Value.ToString
Dim qry = From dr As PersonalObraDataSet.PersonalObRow In PersonalObraDataSet.PersonalOb Where dr.cdTrabajador = cdtrabajador
If qry.Count > 0 Then
Dim Nombre As String = qry.First.Nombre1
dgvr.Cells(1).Value = Nombre
End If
End If
End If
End Sub
Hard to say for sure what the root of the issue is, but you may be safe to just null check the cell before you attempt to perform any action on it, and return in the case where the cell is null. Something like:
Dim cellObj as Object = dgvr.Cells(0).Value
if Not cellObj = Nothing Then
Dim cdtrabajador As String = cellObj.ToString
'...
'Perform the rest of your code
Else
Return
Also note that if Cells(0) is actually null, invoking Value on it may also be causing the error. If you use your debugger you should be able to pinpoint which value is null and either handle it, or look into reasons why it is null if you expect a value to be present and it isn't.
I am trying to run through all the controls in a panel and find which properties the user has changed for each control.
So I have this code:
Private Sub WriteProperties(ByVal cntrl As Control)
Try
Dim oType As Type = cntrl.GetType
'Create a new control the same type as cntrl to use it as the default control
Dim newCnt As New Control
newCnt = Activator.CreateInstance(oType)
For Each prop As PropertyInfo In newCnt.GetType().GetProperties
Dim val = cntrl.GetType().GetProperty(prop.Name).GetValue(cntrl, Nothing)
Dim defVal = newCnt.GetType().GetProperty(prop.Name).GetValue(newCnt, Nothing)
If val.Equals(defVal) = False Then
'So if something is different....
End If
Next
Catch ex As Exception
MsgBox("WriteProperties : " & ex.Message)
End Try
End Sub
Now I face three problems:
When the property refers to image (BackGround Image) I have an error :
ImageObject reference not set to an instance of an object.
The second problem is that the code:
If val.Equals(defVal) = False Then
'So if something is different....
End If
is executes sometimes when the val and defVal are the same.
This is happening in cases that the property is a "parentProperty" like FlatAppearance (which has more child properties)
My loop doesn't look into basic properties like Size, or Location which I want
Re: Not set to an instance of an object, do something like ...
If val IsNot Nothing AndAlso defVal IsNot Nothing AndAlso Not val.Equals(defVal) Then
Which will only do the comparison if neither value is Nothing (aka Null).
Unfortunately, #2 is a fundamental problem - .Equals by default checks if the 2 object references point at the same object in memory - eg if You did
Dim A As New SomeClass
Dim B As New SomeClass
If A.Equals(B) Then
...
End If
Would return False unless SomeClass has an overridden equality comparer, which many classes do not.
You could check if the value in question is a type you know you can compare (Integer, String, Double, etc). If not, you could iterate through its properties and perform the same check again. This would allow you to compare the public properties of any type for equality but wouldn't guarantee the internal state of the classes is the same.
Something Like (Untested/Pseudo)...
Function Compare (PropA, PropB) As Boolean
Dim Match = True
If PropA.Value Is Nothing Or PropB.Value Is Nothing
Match = False
Else
If PropA.Value.GetType.IsAssignableFrom(GetType(String)) Or
PropA.Value.GetType.IsAssignableFrom(GetType(Integer)) Or ... Then
Match = PropB.Value.Equals(PropB.Value)
Else
For Each Prop In PropA.Value.GetType.GetProperties()
Match = Compare(Prop, PropB.Value.GetType.GetProperty(Prop.Name))
If Not Match Then Exit For
Next
End If
End If
Return Match
End Function
This is still not ideal as the internal states of the values may differ.
I am coding in vb.net.
At times the data is empty/null this is due to user's input into the db.
i will like to bypass it, however i get no luck.
here is snippet of my code:
If hct.mydbvalue.name IsNot Nothing Then
dr("mydbvalue") = hct.mydbvalue.name
End If
I still get an error:
System.NullReferenceException: Object reference not set to an instance of an object.
is there a way if it is a null value to not do anything?
Both #FishBasketGordo and #Yuck are correct, you need to check the full object path for nullability:
If (hct IsNot Nothing) AndAlso (hct.mydbvalue IsNot Nothing) AndAlso (hct.mydbvalue.name IsNot Nothing) Then
dr("mydbvalue") = hct.mydbvalue.name
End If
You won't get a NullReferenceException from data in the database that's null when using ADO.NET; ADO.NET uses DBNull.Value to represent null, so your null reference is coming from somewhere else. In the code above, your exception could occur if any of the following were null:
hct
hct.mydbvalue
dr
Make sure that the whole chain is not null. If hct or mydbvalue is null, you'll get the exception.
To me this looks like hct.mydbvalue is null, and therefore you can't call "name" on it.
Private Function NullCheck(ByVal inObject As Object, Optional ByVal defaultValue As Object = "") As Object
Dim out As Object
If Not IsDBNull(inObject) Then
out = inObject ' This returns the value that was passed in when it is not null
Else
out = defaultValue ' This ensures that out is something and defaults to ""
End If
Return out
End Function
You should be checking whether hct is Nothing, as well as mydbvalue. If you look at the exception message property, it will tell you which is causing the error.
I'm also solving this problem, but in C#.
On my project we've complex object paths like "RootObject.childObject.LitleObject.TinyObject.StringName"
when any of these objects in the path is null, you'll get a null reference when you try something easy like
if(RootObject.childObject.LitleObject.TinyObject.StringName == "a")
I would be okay if it just works as whole rest of the path will be null.
eg. when childObject = null, then I want also RootObject.childObject.LitleObject.TinyObject.StringName to be null, not null reference exception.
However I've found no solution yet, but there is one new operator which can slightly help you in some null tasks.
a = object.object ?? defaultValue;
operator ?? is something like ISNULL in SQL server. If object on left is null, it returns the object from right.
It also replaces whole function NullCheck posted by Michael above.
Hope this will help a bit.
more info on operators
http://msdn.microsoft.com/en-us/library/ms173224(v=vs.80).aspx
http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.80).aspx
you're talking about diferent things.
It doesn't matter if you use ISDBNull(x.y), String.IsNullOrEmpty(x.y) or (x.y=null)
The problem is far sooner than your selected function is called.
when X is null, it cannot have a property Y. so when you call
AnyOfYourPrefferedFunctions(x.y);
the error raises during evaluation of x.y (null. doesn't exist), so it stops before the machine actually knows what is the function you want to call.
Probably only way to check this, would be using reflection. But you would need to send string with path and reference to root. so something like:
var v = GetValueThroughReflection(rootObject, "rootObject.path.to.the.last.object");
Then you'll be able to write a function which will go through the object path and find which one is null and handle it accordingly. e.g. returns null.
But when you'll heavy use that function, it can make your application run slower. as you'll use reflection for such simple task as is getting value out of variable.
from VS14+ you can use
If hct?.mydbvalue?.name IsNot Nothing Then
dr("mydbvalue") = hct.mydbvalue.name
End If
Try inserting a IF NOT isdbnull(hct.mydbvalue.name)
The following code checks all values.
If hct IsNot Nothing AndAlso
hct.mydbvalue IsNot Nothing AndAlso
Not String.IsNullOrWhitespace(hct.mydbvalue.name) Then
dr("mydbvalue") = hct.mydbvalue.name
End If
Note that the last test used String.IsNullOrWhitespace(). I'm assuming name is a string and you don't want to save empty strings.
Update 1
The following code is a simple console application to prove that using IsDbNull() or Micheal's NullCheck() will throw NullReferenceException when hct.mydbvalue is Nothing.
Module Module1
Sub Main()
Dim hct = New hct
Dim dr = New Dictionary(Of String, String)
Dim errorCount = 0
Try
Dim thisCallWillFail = IsDBNull(hct.mydbvalue.name)
Catch ex As NullReferenceException
Console.WriteLine(
"Using IsDBNull() threw NullReferenceException as expected."
)
errorCount += 1
End Try
Try
Dim thisCallWillFail = NullCheck(hct.mydbvalue.name)
Catch ex As NullReferenceException
Console.WriteLine(
"Using NullCheck() threw NullReferenceException as expected."
)
errorCount += 1
End Try
Console.WriteLine("errorCount = 2? {0}", errorCount = 2)
End Sub
Private Function NullCheck(ByVal inObject As Object,
Optional ByVal defaultValue As Object = "") As Object
Dim out As Object
If Not IsDBNull(inObject) Then
' This returns the value that was passed in when it is not null
out = inObject
Else
' This ensures that out is something and defaults to ""
out = defaultValue
End If
Return out
End Function
End Module
Public Class hct
Property mydbvalue As mydbvalue
End Class
Public Class mydbvalue
Property name As String
End Class