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
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.
When I get to the Read loop I get an index out of bounds error. I think its on the reader ordinal value, but I am not sure why I am getting it.
Private Function Create(Reader As SqlDataReader) As IEnumerable(Of MyObject)
SetOrdinals(MyObjectReader)
Dim MyObjects = New List(Of MyObject)
While MyObjectReader.Read()
Dim Temp = New MyObject() With {
.FirstValue = MyObjectReader.GetValue(Of Integer)(MyObjectReader(FirstValue_Ord)),
.SecondValue = If(MyObjectReader.GetValue(Of String)(MyObjectReader(SecondValue_Ord)), String.Empty).Trim(),
.ThirdValue = If(MyObjectReader.GetValue(Of String)(MyObjectReader(ThirdValue_Ord)), String.Empty).Trim(),
MyObjects.Add(Temp)
End While
Return MyObjects
End Function
Private Sub SetOrdinals(MyObjectReader As SqlDataReader)
FirstValueOrd = MyObjectReader.GetOrdinal("FirstValue")
SecondValue_Ord = MyObjectReader.GetOrdinal("SecondValue")
ThirdValue_Ord = MyObjectReader.GetOrdinal("ThirdValue")
End Sub
End Class
Public Module Extensions
<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
End Module
You should just be passing in the ordinal to the GetValue calls:
While MyObjectReader.Read()
Dim Temp = New MyObject() With {
.FirstValue = MyObjectReader.GetValue(Of Integer)(FirstValue_Ord),
.SecondValue = If(MyObjectReader.GetValue(Of String)(SecondValue_Ord), String.Empty).Trim(),
.ThirdValue = If(MyObjectReader.GetValue(Of String)(ThirdValue_Ord), String.Empty).Trim()
}
MyObjects.Add(Temp)
End While
Here my version :)
Private Function Create(reader As SqlDataReader) As IEnumerable(Of MyObject)
Dim objects As New List(Of MyObject)()
Dim ordinals As New Ordinals(reader)
While reader.Read()
Dim Temp As New MyObject With
{
.FirstValue = reader.GetValueOrDefault(Of Integer)(ordinals.FirstValue),
.SecondValue = reader.GetValueOrDefault(ordinals.SecondValue, "").Trim(),
.ThirdValue = reader.GetValueOrDefault(ordinals.ThirdValue, "").Trim()
}
objects.Add(Temp)
End While
Return MyObjects
End Function
Private Class Ordinals
Public Property FirstValue As Integer
Public Property SecondValue As Integer
Public Property ThirdValue As Integer
Public Sub New(reader As SqlDataReader)
FirstValue = reader.GetOrdinal(nameOf(FirstValue))
SecondValue = reader.GetOrdinal(nameOf(SecondValue))
ThirdValue = reader.GetOrdinal(nameOf(ThirdValue))
End Sub
End Class
Public Module Extensions
<Extension>
Function GetValueOrDefault(Of T)(reader As SqlDataReader, ordinal As Integer) As T
Return reader.GetValueOrDefault(Of T)(ordinal, Nothing)
End Function
<Extension>
Function GetValueOrDefault(Of T)(reader As SqlDataReader,
ordinal As Integer,
defaultValue As T) As T
Dim value = reader(ordinal)
If value = DbNull.Value Then
Return defaultValue
End If
Return DirectCast(value, T)
End Function
End Module
Because extension method execute checking for DbNull.Value against already extracted object, we get rid from reading same value twice from SqlDataReader.
SqlDataReader.IsDbNull(index) reads value before checking for DbNull.
Extension method have two overloads:
- One which return default value of given type, if value is DbNull.Value. Nothing in vb.net is default value of type.
- And one which takes parameter for default value you want return if value is DbNull.Value. Possibility pass default value makes lines where you create new object shorter and more readable. We get rid of inline if statement.
Your extension method with name GetValue have "side effects". By name consumer of this method expect to get value from SqlDataReader. So he can expect to get DbNull.Value if database query return NULL, but instead he get null for string or 0 for integer. Name GetValueOrDefault is little bid more informative, so you don't need to go inside method to check what is doing.
We created a new Drawing.Point dynamically at runtime and it works fine. Now we want to set the properties "X" and "Y" at runtime.
We tried to do it like this:
Public Function SetObjectProperty(propertyName As String, value As Integer, refObj As Object)
Dim propertyInfo As PropertyInfo = refObj.GetType().GetProperty(propertyName)
If propertyInfo IsNot Nothing Then
propertyInfo.SetValue(refObj, value, Nothing)
Return refObj
End If
Return Nothing
End Function
But it didn't work. The properties aren't set with the values.
Did we miss anything?
The problem is that System.Drawing.Point is a value type. When you pass this value into SetValue, it is boxed. The value is changed on the boxed object but the original value is not changed. Here is a modification that does the boxing before changing the value. You will also need the ByRef parameter modifier:
Public Function SetObjectProperty(propertyName As String, value As Integer, ByRef refObj As Object)
Dim type = refObj.GetType()
Dim propertyInfo As PropertyInfo = type.GetProperty(propertyName)
If propertyInfo IsNot Nothing Then
If type.IsValueType Then
Dim boxedObj As ValueType = refObj
propertyInfo.SetValue(boxedObj, 25)
refObj = boxedObj
Else
propertyInfo.SetValue(refObj, value)
End If
Return refObj
End If
Return Nothing
End Function
You can use it as before:
Dim p As Point
SetObjectProperty("X", 25, p)
Btw, think about if you really need the return value. It does not seem to be necessary.
The VALUE must be a Drawing.Point() Type, not an Integer.
You may utilize something like
Public Function SetObjectProperty(propertyName As String, value As Point, refObj As Object)
Dim propertyInfo As PropertyInfo = refObj.GetType().GetProperty(propertyName)
If propertyInfo IsNot Nothing Then
propertyInfo.SetValue(refObj, value.X, Nothing)
Return refObj
End If
Return Nothing
Above, you can utilize value.X or even only value to get both coordinates.
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
What I want to do is, based on the type of T do different opperations. Below is a simple example of my problem.
Public Shared Function Example(Of T)() As T
Dim retval As T
If TypeOf retval Is String Then
Dim myString As String = "Hello"
retval = myString
ElseIf TypeOf retval Is Integer Then
Dim myInt As Integer = 101
retval = myInt
End If
Return retval
End Function
I get the error "Value of Type 'String' Cannot be converted to 'T'" Same with the integer part. If I cast either to an object before asigning them to retval it works but I think that would defeat my purpose and be less efficient. Any Ideas? Thanks!
It's probably a bit late, but try this:
Public Shared Function CAnyType(Of T)(ByRef UTO As Object) As T
Return CType(UTO, T)
End Function
Public Shared Function ExecuteSQLstmtScalar(Of T)(ByVal strSQL As String) As T
Dim T_ReturnValue As T
' Here we have the result of a DB query '
Dim obj As Object = "Value from DB query cmd.ExecuteScalar"
Dim strReturnValue As Object = obj.ToString();
Try
Dim tReturnType As Type = GetType(T)
If tReturnType Is GetType(String) Then
Return CAnyType(Of T)(strReturnValue)
ElseIf tReturnType Is GetType(Boolean) Then
Dim bReturnValue As Boolean = Boolean.Parse(strReturnValue)
Return CAnyType(Of T)(bReturnValue)
ElseIf tReturnType Is GetType(Integer) Then
Dim iReturnValue As Integer = Integer.Parse(strReturnValue)
Return CAnyType(Of T)(iReturnValue)
ElseIf tReturnType Is GetType(Long) Then
Dim lngReturnValue As Long = Long.Parse(strReturnValue)
Return CAnyType(Of T)(lngReturnValue)
Else
MsgBox("ExecuteSQLstmtScalar(Of T): This type is not yet defined.")
End If
Catch ex As Exception
End Try
Return Nothing
End Function
(the secrect is casting your generic result to object, then casting from type Object to template type T).
PS:
You are responsible to ensure that your code works correctly with nullable types and NOT nullable types, as well as System.DbNull.Value. For example when string is NULL and return value type is Boolean (not nullable). On a sidenote, please also note that VB Nothing is NOT equal NULL, it's equal to C#'s default(T) (e.g. System.Guid.Empty for Guid)
With a generic method, T will be of exactly one type each time. Let's say that you have code calling Example(Of Integer). Now, in your mind, replace T with Integer. The resulting method will contain these lines (amongst others).
Dim retval As Integer
If TypeOf retval Is String Then
Dim myString As String = "Hello"
retval = myString
' more code follows '
Assigning a String to an integer like that will never work. Sure, that code will also never execute, since the If-block prevents that, but the code will still not compile. (As a side not, the above code will fail to compile because the TypeOf keyword is restricted to use with reference types, but that is another story)
Typically when creating generic methods, you will want to do the same thing with whatever input you get, but in a type safe manner. If you want to have different behavior for different types of input, you are usually better off by overloading the methods instead.
retVal = (T) "Hello World!"
Do retval = Ctype(Mystring, T) or retVal = Ctype(MyInt, T)
An alternative solution is encapsulate this kind of logic in a class and use VB CallByName function:
Class Aux(Of T)
Public Value As T
Private dicc As Dictionary(Of String, Object)
Sub New()
dicc = New Dictionary(Of String, Object)
dicc.Add("system.string", "hola")
dicc.Add("system.int32", 15)
dicc.Add("system.double", 15.0)
End Sub
Public Function Test() As T
Dim typeName As String = GetType(T).ToString.ToLower
If dicc.ContainsKey(typeName) Then
CallByName(Me, "Value", CallType.Set, dicc(typeName))
End If
Return Value
End Function
Protected Overrides Sub Finalize()
MyBase.Finalize()
If Not (dicc Is Nothing) Then dicc.Clear()
dicc = Nothing
End Sub
End Class