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?
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)
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.
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?
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.
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
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:
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?
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.
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".
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?
I have a generic Class I'm using to hold information loaded from a database.I have a method which takes a DataRow as an argument, uses the object's known column name and extracts the data from the DataRow, such that:Dim loadData As T = CType(myDataRow("myColumnName"), T))works as my default assignment in most cases.Unfortunately, due to some horrifying design constraints, some of my columns may be null, and may also be taken from enumerations.This means that when <T> is Nullable(Of SomeEnumeration) the above code does not work because I can't cast 0 directly to SomeEnumeration.Zero.Is there some way to check whether <T> is Nullable(Of [Enum])? Or some way to write a method which allows Integers to be cast to Nullable(Of [Enum])?I feel like I'm forgetting something that would allow me to write one of the other of these, but my weak google-fu is turning up nothing.
EDIT: Okay, thanks to dasblinkenlight's answer below, I can detect when this circumstance is occurring, but what I need to do now is to take a type <T> which I know is Nullable(Of SomeClass), get a type reference to SomeClass and then create a new object of type Nullable(Of SomeClass) and assign that to LoadData.My problem was that I had a lot of difficulty in finding any function which would accept baseType as an actual Type.Parse accepted baseType as a parameter, I knew baseType was an [Enum] type because of dasblinkenlight's code, so I was, in this instance, able to code a solution. It's a solution which is very specific to my problem (i.e., T is Nullable(of SomeEnumeration)), but it's a solution nonetheless.
Im working on my first n-tier application. I am trying to serialize a structure and Im getting an error"Soap Serializer does not support serializing Generic Types : System.Nullable`1[System.DateTime]."Here is the structure that is being serialized Namespace Structures
<Serializable()> _ Public Structure structAllergy Public AllergyID As String Public ProfileID As String
[code]....
The bold line is the line that is throwing the error.
Actually in our project we create different database using vb form code..so this database is a chance of transferring to other users to use others database. so when we create the database it will insert the database name into the main database. Problem: One user who is accessing MS 2010 ACCESS has created a database and transferred into other PC where MS 2003 access available..when he is going to insert database name into main database which is transferred it is not inserting into main database exception is occured..
I am attaching DBs using following code. Dim conn As New SqlConnection("Server=MyHomeServerSQLExpress;Database=master;Integrated Security=SSPI") Dim cmd As New SqlCommand("", conn) cmd.CommandText = "CREATE DATABASE dbNoPWD ON ( FILENAME = 'd:dbNoPWD.mdf' ), ( FILENAME = 'd:dbNoPWD_log.ldf' ) FOR ATTACH" conn.Open() cmd.ExecuteNonQuery() cmd.Dispose() conn.Dispose()
I do not want to give any username and password. When I ran the above code and then checked SSMS, I found my attach DB was not checked in Roles (pls. see pic.) The problem with this is my network computers can not access this DB. I want to run some code like above (without sa password) and want that all my network computers can access the DB without my user get involved in setting up SSMS.
I have a class with a Property called 'Value' which is of type Object.Value can be of any type, a structure, a class, an array, IList etc.My problem is with the setter and determining whether the value has changed or not.This is simple enough for value types, but reference types and lists present a problem.For a class, would you assume that the Equals method has been implemented correctly, or just assume that the value has changed every time the setter is called?If I did assume it's changed, then perhaps I should assume it for value types as well, so that the behaviour is consistent.
I accidently wrote some code today that was like this[code]...
I immediately noticed the issue, but I had already hit the run button. It compiled successfully, I ran it through to the section and it threw an exception.
You can't do this in C#, it gives a compile error "cannot convert from 'int?' to 'int'".
Is there an 'Option Explicit' type switch that I can turn on to ensure that this sort of error does not occur again?
Been looking at this for a few hours now, is it possible to insert into the FIRST line in a rich text box? I have a simple app that regularly copies a reference file to several PCs but I need to have the success/failure and timestamp visible, so I want to insert into the top line each time - ie. in descending order.
Im confused on how to write a code on checking duplicates before insertion on vb.net. I know the sequence on how to do it but its kinda confusing when writing a code.
1.) Create a query that will check if the mobile number already exists on the database
2.) If not, insert the new data
3.) Else, a message will prompt that "You are already registered".
writing this in PHP is kinda easy for me but in vb.net.. , unfortunately, not easy.. Im still a beginner on vb.net.
vb.net Using connection As New SqlConnection(strConnString) Using command As New SqlCommand(strQuery, connection) command.Parameters.AddWithValue("@CuserID", intUserID) command.Parameters.AddWithValue("@CstartTime", dtStartTime)
I've been trying to learn enough to develop the family tree diagram and making progress.[URL]..It illustrates many of the drawing, saving, re-opening, editing, drag/drop features that the family tree diagram would need, and is a good starting point.
To adapt this for the project, after searching for snippets online, I've tried to add dynamic insertion of a label as a start (eventually, it should be a textbox that the user, in runtime, can enter text into, save, re-open, edit). But, I can't get this to work. No errors display, but I must be doing something wrong.
Here's the label code which I've so far appended at the end:
I know you can do it with an WMI event or overriding WndProc and looking for the right messages, but i was wondering if there wasn't something hidden in the net framework that makes this task easyer.
