I have following code in VB.NET:
Public Sub Test(ByRef clientId As Nullable(Of Integer))
Dim t As Object = IIf(clientId.HasValue, clientId.Value, DBNull.Value)
End Sub
The calling side pass in Nothing as clientId value, but when I run this statement I got exception.
Nullable object must have a value
Is my statement wrong?
Change it to use "If":
Dim t As Object = If(clientId.HasValue, clientId.Value, DBNull.Value)
The problem with "IIf" is that it's just a function call - all arguments are always evaluated, while the VB 'If' operator behaves like the '?' operator in C#/Java - it only evaluates what it needs to evaluate.
Dim cp As Single
' Try
If tt.getsumpaid4cors(CInt(sidtxt.Text), CInt(coidtxt.Text), CInt(tidtxt.Text), CInt(stidtxt.Text)).HasValue Then
cp = tt.getsumpaid4cors(CInt(sidtxt.Text), CInt(coidtxt.Text), CInt(tidtxt.Text), CInt(stidtxt.Text)).Value
Else
cp = 0
End If
Related
While CommitReader.Read()
Yield New Commit() With {
.FirstValue = CommitReader.GetInt32(CommitReader.GetOrdinal("FirstValue")),
.SecondValue = CommitReader.GetString(CommitReader.GetOrdinal("SecondValue")).Trim(),
'Lots of values
End While
I know I can do something like this; however there are 24 properties and I would like to make this part as clean as possible
While CommitReader.Read()
new Commit (){
Dim index As Integer = reader.GetOrdinal("FirstValue")
If reader.IsDBNull(index) Then
FirstValue = String.Empty
Else
FirstValue = reader(index)
End If
index = reader.GetOrdinal("SecondValue")
If reader.IsDBNull(index) Then
SecondValue = String.Empty
Else
SecondValue = reader(index)
End If
}
End While
Is there a better way to handle this type of thing? I am mainly a C# developer so if the syntax is off a little sorry, I am winging it in VB.
It's a shame that SqlDataReader doesn't have the generic Field extension method like DataRow does, but you could define your own extension method (has to be in a module in VB.NET) to help with the null checks, perhaps something like this:
<Extension>
Function GetValue(Of T)(rdr As SqlDataReader, i As Integer) As T
If rdr.IsDBNull(i) Then
Return Nothing
End If
Return DirectCast(rdr.GetValue(i), T)
End Function
And use it something like this:
While CommitReader.Read()
Yield New Commit() With {
.FirstValue = CommitReader.GetValue(Of Integer?)(CommitReader.GetOrdinal("FirstValue")),
.SecondValue = CommitReader.GetValue(Of String)(CommitReader.GetOrdinal("SecondValue")),
'Lots of values
End While
I haven't tested this fully to make sure it handles all data types appropriately (may be worth looking at DataRowExtensions.Field to see how it does it).
Note that you are using String.Empty as the "null" value for strings, while this will use Nothing/null (I also had to remove the .Trim call to avoid NREs). If you want empty string instead, you could use (adding the Trim back in):
.SecondValue = If(CommitReader.GetValue(Of String)(CommitReader.GetOrdinal("SecondValue")), String.Empty).Trim()
You may also want to move the GetOrdinal calls out of the loop to improve performance.
Obviously you have repetition in your code if ... else ... condition.
So you can extract it in another method.
For your case generic extension method seems good candidate.
Public Module Extensions
<Extension>
Public Function GetValueOrDefault(Of T)(originalValue As object,
defaultValue As T) As T
If originalValue = DbNull.Value Then
Return defaultValue
End If
return DirectCast(originalValue, T)
End Function
End Module
Then use it:
While CommitReader.Read() = True
Dim temp = new Commit With
{
Dim index As Integer = reader.GetOrdinal("FirstValue")
FirstValue = reader(index).GetValueOrDefault(String.Empty)
Dim index As Integer = reader.GetOrdinal("SecondValue")
FirstValue = reader(index).GetValueOrDefault(String.Empty)
}
End While
You can create another overload which return "default" value for given type if it is DbNull
<Extension>
Public Function GetValueOrDefault(Of T)(originalValue As object) As T
Return originalValue.GetValueOrDefault(Nothing)
End Function
Nothing in vb.net is default value, for reference types it is null for Integer it is 0 for example.
For using this overload you need provide type parameter explicitly
While CommitReader.Read() = True
Dim temp = new Commit With
{
Dim index As Integer = reader.GetOrdinal("FirstValue")
FirstValue = reader(index).GetValueOrDefault(Of String)()
Dim index As Integer = reader.GetOrdinal("SecondValue")
FirstValue = reader(index).GetValueOrDefault(Of String)()
}
End While
Notice that your solution executing reader twice, for checking is it null and for reading value. This can cause "tiny" performance issue.
So in extension method above we read value only once and then check value for DbNull.
If you concatenate a string with a Null you get the string:
FirstValue = reader(index) & ""
Kind of "unprofessional" but saves a lot of coding time if all you are doing is converting a possible Null to an empty string. Easy to forget however, so later data dependent errors may pop up.
I am trying to find the length of a control's tag to determine a Boolean's value. I've tried a couple methods to get the text length of a Tag in a control and determine if it has a length of 1 or higher, but none of them seem to be working. They all end up with a System.NullReferenceException error.
Boolean = Control.Tag.ToString.Length > 1
Boolean = Control.Tag.ToString.Count > 1
Boolean = Not Control.Tag.Equals("")
Boolean = Not Control.Tag.ToString.Equals("")
Thats because your Tag is Null (or as it's called in VB Nothing).
So before you check the length of the Tag, you need to make sure it's not Nothing. e.g with:
If Control.Tag Is Nothing Then ...
Before accessing a method or property of the tag, you must make sure that the tag is not Nothing. You can do this in a single expression by using shortcut evaluation:
Dim isDefined As Boolean = Control.Tag IsNot Nothing AndAlso Control.Tag.ToString.Length > 1
Since VB 14.0 / Visual Studio 2015 you can use a Null-conditional operator
Dim isDefined As Boolean = If(Control.Tag?.ToString.Length, 0) > 1
In VB.NET you can use VB.NET specific way as the VB Runtime evaluates Nothing as an empty string which is represented by String.Empty.
In VB.NET you can assign Nothing to any variable regardless it is a value type or a reference type.
Its C# equivalent is default(T) which for reference types returns null and for value types returns the value represented by a state where all bits are zero. E.g. default(bool) returns false
So these ways are also working:
' Let's assume you set the Control.Tag property value to this variable
Dim controlTag As Object = Nothing
' Len() method can accept any Object
Dim controlTagLength As Integer = Len(controlTag)
Dim hasValueByLength As Boolean = controlTagLength > 0
' Always call Equals() method on a constant
' or on a well defined non-null value e.g. String.Empty
' to avoid NullReferenceException
Dim hasValueByInstanceEquals As Boolean = String.Empty.Equals(controlTag)
' Or you can use the static Equals() method that accepts Object
Dim hasValueByStaticEquals As Boolean = String.Equals(controlTag, String.Empty)
I have a below line of code like this.
Private Sub SomeFunction(ByRef SomeShortVariable As Nullable(Of Short))
Dim SomeStringVariable As String = "" 'Let's assume it is "", that's how I am getting it in real time code
SomeShortVariable = IIf(SomeStringVariable = "", Nothing, SomeStringVariable) 'I want to set SomeShortVariable to Nothing but I am getting 0
End Sub
The variable SomeShortVariable is always sets to 0 even though I want it to be Nothing.
I know Short by default will set the variable to 0.
But how can I make it Nothing. I am using .NET 2.0.
Make SomeShortVariable a Nullable(Of Short) variable.
EDIT:
Also your statement should look like this:
SomeShortVariable = If(String.IsNullOrEmpty(SomeStringVariable), Nothing, New Nullable(Of Short)(Short.Parse(SomeStringVariable)))
SECOND EDIT:
If you're using Visual Studio 2005 the above won't work, because the If operator was only introduced in VS2008. So what you'll have to do is this:
If String.IsNullOrEmpty(SomeStringVariable) Then
SomeShortVariable = Nothing
Else
SomeShortVariable = Short.Parse(SomeStringVariable)
End If
Of course you will want to validate that SomeStringVariable is a numeric string first. :)
Regarding your update, well, that's cause it's Short, and not Nullable(Of Short) in the parameter. Make it nullable of Short and you are done. Although I would refactor to avoid ByRef, have a string parameter SomeStringVariable and return a Nullable(Of Short). Then things would start to make more sense.
Private Shared Function SomeFunction(SomeStringVariable As String) _
As Nullable(Of Short)
If String.IsNullOrEmpty(SomeStringVariable) Then
Return Nothing
Else
Return Convert.ToInt16(SomeStringVariable)
End If
End Function
EDIT: Actually shorthand syntax won't work in this case, for the reasons I outlined in my comment regarding change to If. Just don't use shorthand.
Watch out for the recommendations to use Convert or Parse on cases where the input string could be something other than an empty string but not a number (any user supplied input). It is typically better to use TryParse unless you are absolutely sure someone hasn't passed something in that you aren't expecting. Consider the following:
Dim someString = "a"
Dim someShort as new Nullable(Of Short)
Dim tempShort as Short
Console.WriteLine(someShort)
If Integer.TryParse(someString, tempShort) then
someShort = tempShort
end if
console.WriteLine(someShort)
if Not String.IsNullOrEmpty(someString) then
someShort = Short.Parse(someString) ' Throws FormatException
end if
Console.WriteLine(someShort)
I'm using an SQLDataReader to retrieve values from a database which may be null. I've worked out how to handle Null string values but can't get the same trick to work with integers or booleans:
Using cmd As DbCommand = store.GetStoredProcCommand("RetrievePOCO")
store.AddInParameter(cmd, "ID", DbType.Int32, ID)
Using reader As IDataReader = store.ExecuteReader(cmd)
If reader.Read() = True Then
Dim newPOCO As New POCO()
With newPOCO
'If the source column is null TryCast will return nothing without throwing an error
.StatusXML = TryCast(reader.GetString(reader.GetOrdinal("StatusXML")), String)
'How can a null integer or boolean be set elegantly?
.AppType = TryCast(reader.GetInt32(reader.GetOrdinal("AppType")), System.Nullable(Of Integer))
.Archived = TryCast(reader.GetBoolean(reader.GetOrdinal("Archived")), Boolean)
So how can a null integer or boolean be set elegantly? I've seen suggestions in C# but they don't translate correctly to VB giving a 'TryCast operand must be reference type, but integer? is a value type' compiler errors.
I use the following function in this scenario:
Public Shared Function NoNull(ByVal checkValue As Object, ByVal returnIfNull As Object) As Object
If checkValue Is DBNull.Value Then
Return returnIfNull
Else
Return checkValue
End If
End Function
Your code would look something like this:
With newPOCO
.StatusXML = NoNull(reader("StatusXML"), "")
.AppType = NoNull(reader("AppType"), -1)
.Archived = NoNull(reader("Archived"), False)
End With
Note that this function requires you to pass the value which should be used if the value is DbNUll as the second Parameter.
You can take advantage of the IsDBNull method of the SqlDataReader and use the VB.NET ternary operator to assign a default value to your poco object
.StatusXML = If(reader.IsDBNull(reader.GetOrdinal("StatusXML")), _
"",reader.GetString(reader.GetOrdinal("StatusXML")))
It is just one line, not very elegant because you need to call two times the GetOrdinal method.
Public Function NotNull(Of T)(ByVal Value As T, ByVal DefaultValue As T) As T
If Value Is Nothing OrElse IsDBNull(Value) Then
Return DefaultValue
Else
Return Value
End If
End Function
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