C# - Get A Technical Reason For No Implicit Conversion From DBNull To Nullable Types?
Jan 10, 2012
Is there a technical reason why there is no implicit conversion from DBNull to the various nullable and/or sql types? I understand why the conversions don't currently happen, but don't understand why an implicit conversion wasn't created at the time or added in subsequent versions of the framework.Just to be clear, I'm looking for technical reasons, not "because that's they way they did it" or "I like it that way".
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Feb 19, 2009
I do the following:
Dim rw() as exportDataset.vendorPricingRow = ExportDataset.vendorPricing.Select("field1 = 1")
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Aug 27, 2010
I have: Dim nVar1 As Long?Dim nVar2 As Long?Dim nVarSum As Long?nVar1 = Nothing nVar2 = 5 nVarSum = nVar1 + nVar2 I would prefer the result to end with nVarSum being 5, instead of Nothing. I understand if you add something to an unknown value, you will end up with "somthing + unknown" or x+5 will always equal "x+5" not "5" because you are still carrying around that unknown "x". However, how can I effectively treat an unknown or Nothing as a zero for the purposes of addition in this case?(What is basically happening is that the end user is sending us a data file, this code parses that file and then sums together about 15 fields. If the user leaves those fields blank instead of assigning a zero to them, I need to treat it as if it was a zero for this one addition operation, but all the rest of the code needs to continue seeing it as a Nothing value since the user did not ACTUALLY submit zero... they submitted blank or nothing)
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Jun 27, 2012
Can Nullable Types be used in VB.NET? If so, is it possible to have a Nullable Integer that I can use with a field that accepts NULL in SQL Server?
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Aug 25, 2010
I'm converting an Access VBA app to VB.NET. It has dates defined as variants or objects to handle null values. I thought this would be a great chance to use the Nullable type. But I can't get it to work. Either it is inappropriate for what I am trying to use it for, or I am doing it wrong. Anyone know how to use it? Reader is a SqlDataReader.
[Code]...
I was hoping I didn't have to put a lot of If .. Null statements throughtout the code, but that is my only option unless someone has any other idea.
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Oct 26, 2010
VB 2008 Express edition. I'm pretty new to VB2008, a little over halfway through my first classi am writing a console program to ask the user for the price of gas for 7 days, then to get the average of that price. here is my code so far:
Option Strict On
Module Module1
Sub Main()
[code].....
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Jul 21, 2011
I have 2 methods with the same name, with diferente signatures.
Public Sub Method1(ByVal Parameter1 as String, ByVal Parameter2 as Boolean)
Public Sub Method1(ByVal Parameter1 as Integer, Optional ByVal Parameter2 as Boolean = False, Optional ByVal Parameter3 as Boolean = False)
If I call Method1("My String Parameter", False) it will not give and error, it will try to convert the parameter to integer and use the overload.
Is there a way to forbid the implicit conversion in this specific method or in this specific parameter. For instance, I changed the second method name, but its not an elegant solution.
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Jun 13, 2011
[Code]...
The code compiles and seems to run correctly, but is underlined with the message "Implicit Conversion from 'Double' to 'Single'" Why would this be an implicit conversion if each of the variables is cast as a Single? Is this something I should worry about or change?
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Nov 10, 2010
I encountered due to a coding error a behaviour I can't explain.In one line I have an implcit conversion from a string to a boolean value:
Class myClass
Public Property myString() as Boolean
Return "123" '<= Implicit Conversion occurs here
End Property
End class
[Code]...
Are there any differences in the behaviour between the .NET Versions?The code is compiled against .NET 2.0 with implicit conversion on.
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Feb 15, 2011
C#'s 'as' keyword will let you do this: int? input = value as int? Here's what I would assume the vb.net equivalent would be: Dim input As Integer? = TryCast(value, Integer?) There's an intellisense error in the TryCast stating the operand must be a reference type but Integer? is a value type.
Intellisense on Nullable(Of Integer) says 'Represents an object whose underlying type is a value type that can also be assigned null like a reference type.' It seems C#'s 'as' handles this like a reference type where TryCast doesn't have this built in. In VB10 I was able to take advantage of the new CTypeDynamic function to do the casting. Conversion.CTypeDynamic Method
Dim input As Integer? = CTypeDynamic(Of Integer?)(value)
Or:
Dim input As Integer? = CTypeDynamic(Value, GetType(Integer?))
There's a cost here as CTypeDynamic examines the type at runtime.
My question is what is the elegant way to handle this without CTypeDynamic?
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Nov 15, 2011
Dim input As Integer? = TryCast(value, Integer?)There's an intellisense error in the TryCast stating the operand must be a reference type but Integer? is a value type.Intellisense on Nullable(Of Integer) says 'Represents an object whose underlying type is a value type that can also be assigned null like a reference type.'It seems C#'s 'as' handles this like a reference type where TryCast doesn't have this built in.
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Dec 1, 2009
VB.Net just showing its true colors again? When I turn Option Strict On, I am still able to do implicit narrowing conversions in a for each loop.For example, this code actually compiles;
Private Sub foobar()
Dim foo() As foo = {New foo}
For Each bar As bar In foo[code].....
From my understanding of Option Strict, this should not work.See here "Restricts implicit data type conversions to only widening conversions".My code above is a narrowing conversion, and so should be illegal.It will fail at runtime with an InvalidCastException.
C# does the same thing. Below is another implicit narrowing conversion.
private void foobar()
{
foo[] foo={new foo()};[code]......
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Aug 16, 2005
would like to define some custom Implicit conversion function within my VB class. How can i do that. I have seen samples in C# but not in VB
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Aug 21, 2009
In C# you can use the implicit keyword to define an implicit user-defined type conversion operator.
In VB.NET you can define a CType conversion operator that will explicitly convert a user-defined type into another type.
Is there a way to declare an implicit conversion operator in VB.NET?
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Jan 11, 2011
Dim
Dr As
DataRow
Dr = DataSet11.Jobdata.NewRow
[code]....
With Option Strict On, I am getting an error with the above code, which is part of a procedure to add a new record in an 2003 access database table named "Jobdata". The items in Combobox3 is populated by another table in the same database, which has a Status field and a Key field. What I'm trying to do is to insert in the Status field in Jobdata table the correspoding integer value in Key to which ever
status is selected from the combobox.
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Aug 24, 2011
I have recently been working on a number of sections of code that deal with the insertion of Nullable types into a database. As I'm sure anyone who has dealt with similar code will be aware of the annoyance of constantly writing conditional logic to deal with the insertion of nulls into a database. I.e.:
MyValue.HasValue ? MyValue.Value : DBNull.Value;
If(MyValue.HasValue, MyValue.Value, DBNull.Value)
Basically I am just wondering why DBNull.Value exists and why Null simply couldn't be used?
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Jan 17, 2010
It seems like overkill to set the value of a nullable type and implement iNotifyPropertyChanged. Is there a better way of doing this?
[Code]...
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Mar 20, 2011
I am a novice programmer to VB 2008 and I am trying to develop the following code for a game. When I build the following code, an error message appears:
Option Strict On disallows implicit conversion from integer to string.
Another error message
appears: Name FuchsiaTurnScoreLabel is not declared.
Option
Strict
On
Public
Class PlayForm
'Declare integer and decimal variables for calculations
[Code] .....
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Jul 15, 2011
I am getting this warning in my code, and for the most part I have ignored it.
Friend Sub StartTablet()
Dim success As Boolean
Dim eventObject As IntPtr = IntPtr.Zero
[code]....
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Feb 5, 2011
Option strict on disallows implicit conversion from 'Integer?' to 'Integer', 'Integer' to 'String'
Here's my code which is just a stub. I am working off of some example code and applying it to my own project:
<System.ComponentModel.DataObjectMethodAttribute _
(System.ComponentModel.DataObjectMethodType.Select, False)> _
Public Function GetTrackActivitiesByParams(ByVal ActivityKey As Nullable(Of
[Code]....
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Jan 18, 2012
In my ASP.Net Web-Application, I'm getting this error: Conversion from type 'DBNull' to type 'Boolean' is not valid.
[Code]...
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May 4, 2012
I am trying to query for records where that column IS NULL:
Dim UnassignedSubSvc =
From c In CurrentContext.SubService
Where c.Product.ProductSubServiceId **is null**
Select c).ToList()
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Mar 9, 2010
Is there any difference between the 2 methods below for calculating c ... specifically boxing/unboxing issues?
Dim a As Integer? = 10
Dim b As Integer? = Nothing
Dim c As Integer
' Method 1
[code]....
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Mar 1, 2012
I have the following NHibernate Linq query:
From eachLine In myNhSession(Of SamplePoco)()
Where eachLine.SampleIntField = 1234
The property SamplePoco.SampleIntField is type Nullable(Of Int32)
When I run the query, I get the following exception:
System.InvalidCastException: Unable to cast object of type 'NHibernate.Hql.Ast.HqlCoalesce' to type 'NHibernate.Hql.Ast.HqlBooleanExpression'
If I change the property type to Int32, it works. It seems that Nullable types are automatically converted into a coalesce expression by the Linq compiler.
Debugging the NHibernate, I just found out that this Where clause was converted into: {where ((eachLine.SampleIntField == 1234) ?? False)}. As I can understand, the whole condition comparison was translated to be coalesced instead of just the Nullable property.
If I put this way eachLine.SampleIntField.Equals(1234) it doesn't work as well ('Equals not implemented' exception)
If I change the query to the following code, it works:
From eachLine In myNhSession(Of SamplePoco)()
Where {1234}.Contains(eachLine.SampleIntField)
Another code that works as well (coalescing the field properly as I was expecting by the first query):
From eachLine In myNhSession(Of SamplePoco)()
Where If(eachLine.SampleIntField,0) = 1234
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Jan 9, 2011
I have a class that implements a range of numbers....call it NumericRange(Of T). Internally, NumericRange stores T as a Nullable, T?. I have another class that wraps this class as NumericRange(Of UInt16). Call this class MyNumRange (I'm being simplistic here). So In MyNumRange, I have a few constructors defined:
[Code]....
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Dec 8, 2011
I'm trying to do something similar to what's described here, but with nullable types.
[URL]
int availableUnits = unitsInStock ?? 0;
In VB, it would be this:
Dim availableUnits As Int32 = If(unitsInStock, 0)
However I'm working with db columns, which could be DbNull, and nullable types, which can be Nothing (which is different to DbNull). If a column is DbNull, I want to return Nothing, otherwise return the value. For eg:
Dim availableUnits As Int32? = If(myDataReader("UnitsInStock").Value, Nothing)
The error I'm getting is "Specified cast is not valid" but I'm not sure why. I've tried this as well:
Dim availableUnits As Int32? = If(isDbNull(myDataReader("UnitsInStock").Value), myDataReader("UnitsInStock").Value, Nothing)
Which is messy and just results in the same error. The only thing that works is this:
Dim availableUnits As Int32?
If isDbNull(myDataReader("UnitsInStock").Value) Then
availableUnits = myDataReader("UnitsInStock").Value
Else
availableUnits = Nothing
End If
Which is just silly. Is there a better way of getting nullable db values into nullable variables that I'm not aware of?
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Jul 21, 2011
I'm using my data objects in a crystal report. The thing is, when I select my object, it gets added to the field explorer table. My properties are listed except for the Nullable Type properties. Why would this happen. I've had quite a few issues yesterday as this is my first attempt at a crystal report in .Net. I thought I worked around them but now I've hit this snag.
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Nov 20, 2011
[code] I get the error message of "Option Strict disallows implicit conversion from Long to UInteger." [code] I dimensioned each variable as a UInteger and not a Long, the values for both X & Y are well below the maximum values of a UInteger as far as I can tell (Data Types Summary). [code]
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Nov 9, 2011
I am having problems with my code, where I get the "Object cannot be cast from DBNull to other types" message when Insert(). I get the message on every Insert to this table, even though the data is successfully inserted. If I Update the same data after Insert(), it does so without error. I have checked my values and none of them are DBNull. Even when the same values are passed on Insert that are passed on Update.
My code is :
Private Sub Save()
Dim bo As tbl_GL
bo = New tbl_GL()
bo.ConnectionString = AppConfig.ConnectString
[Code] .....
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Jul 6, 2011
I have written an application which manages some bookings, and want to use Crystal Reports to provide the reporting engine. I have used it before with datasets with no problems.
I am using EF4, and have a LINQ query and report datasource assignment as below:
Code:
Dim ActiveCustomers = From Customer In FMEntities.Customers
Where Customer.Status = 1
Select Customer
[Code].....
I have read up that I may need to convert the incoming data, but I am confused as to what it needs to be converted to. Should it be a datatable?
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