Nullable DateTimePicker?
May 6, 2009
I have a Nullable DateTimePicker but it lacks a vital property, that is to be able to change its ForeColor, anyone who has some suggestion on how can this be done? Or perhaps a free nullable DateTimePicker out there that supports changing of backcolor and forecolor?This is C# but it should not be different with .Net, and I prefer posting it here since there is more traffic here over the C# section.
using System;
using System.Collections.Generic;
using System.ComponentModel;[code].....
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Apr 7, 2010
I have a date time picker bould to a Nullable Datetime field in a sql server datatable. The ShowCheckBox option is set to true. If the user unchecks the checkbox on the datetimepicker I would like to the field to be set back to null in the DataTable. The default behaveour does not seem to do this and there is no specific event for the uncheck.
Private Sub dtpCertDate_ValueChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles dtpCertDate.ValueChanged
If dtpCertDate.Focused Then
[code].....
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Sep 17, 2009
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?
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Dec 5, 2011
I have a bigger code block which I have recreated in this simpler example:
[Code]...
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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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Sep 15, 2011
I have the following line of code:
CType(IIf(CBool(product.IsDiscontinued Is Nothing Or product.IsDiscontinued = True), False, True), Boolean?)
What does the Boolean? mean at the end. I have seen it used on other data types as well.
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Jan 31, 2012
Okay, none of the previous questions I have seen with this error seem to apply in this situation.tEmp is a class that contains (among other things) two nullable date fields.[code]During a routine on my form, I am attempting to see if the following conditions are true[code]I get the above error (the title of question), but when I hover over the debugger, NextReview and InsuranceEligibleDate are both Nothing (which is correct for the employee I am looking at).I am using VB2010, and the Properties in the Employee class are using the new way of declaring Properties (i.e. no set/get)
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Dec 15, 2010
dim str as nullable(of string) is this syntax incorrect?
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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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Nov 17, 2010
I am a c#/asp.net developer and I am having to work on a VB/asp.net. I have two variables
Dim originalDate as DateTime?
Dim newDate as DateTime?
Both nullable datetimes, originalDate is a nullable date I am getting from the database and newDate time is set in code, I need to compare them, they can either both have dates, neither have dates or one have and one not.
I have a bit of code as follows:
if origEndDate = origEndDate then
When both origEndDate and origEndDate are "nothing" this statement is false (well when I run it in the watch window it comes back as nothing)!I don't understand why this is the case because I was under the impression doing an "=" compares the two values and as they are the same surely it should be true? What syntax should I be using as in C# I can do the above as so:
if (origEndDate == origEndDate) { }
and it will come back as true.
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May 8, 2011
assuming v is a nullable, I'm wondering what are the implications / differences between these usages:
VB:
If v Is Nothing Then
If v.HasValue Then
C#:
if (v == null)
if (!v.HasValue)
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Dec 22, 2009
I've picked up some code from a colleague, and in one of his methods he has a boolean parameter: ByVal showProactiveChases As Boolean?. I had to look up the ? operator yesterday to see that it denotes a Nullable type. My question is, if I change it to: ByVal showProactiveChases As Nullable(Of Boolean), does the meaning remain the same? I think provided it doesn't change the meaning the second way is much more readable.
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Apr 7, 2009
I am creating a setup project for my VB application. Everything seems to look fine. However when I tell it to build the solution I get the following error:Property 'keycode' is non-nullable. I have never seen this before in other setup projects and I can't find any help from MS.
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Jan 17, 2012
In the below sample code, I have tried to use Nullable(of T) for DateTime and I want to use it same of the Dimensions property which is List(of Dimension).
CODE:
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May 20, 2009
I am getting an unusual error "Nullable object must have a value" when running the code below.
[Code].....
Interestingly the Stored proc used by the table adapter still runs and does the insert but the function returns the error.
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Jun 3, 2010
For a project I need to convert a date field in my SQL Server 2008 Database into a VB.net ListOf. Most of the records in StartDate field are NULL. VB.net doesn't recognize null values for DateTime.I need the NULL values to be dismissed, as they will be returned in a ListOf for a web service that I'm creating.
Here is my syntax and a sample of the current returned values in the XML message of the ListOf:
Class Property StartDate() As System.Nullable(Of DateTime) = Nothing
asmx.vb myUserRoleAppInfo.StartDate = myPermissionUser.StartDate
XML Message (Listof)
[Code]...
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Aug 11, 2009
In VB.NET, why can't I see a member of a structure when I make it a nullable type?
Example:
Public Structure myNullable
Dim myNullVar As Integer
End Structure
Sub Main()
[Code]...
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Dec 7, 2010
I am trying to understand why the two code samples behave differently. I always believed the If() function to mimic the If language feature. Or am I looking at a behavior of Nullable(Of Integer) that is causing this?
Sample #1:
If Not String.IsNullOrWhiteSpace(PC.SelectedValue) Then
Dim pcFilter1 As Integer? = CInt(PC.SelectedValue)
Else
[Code]....
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Sep 19, 2011
I'm struggling to implement a maybe monad - which I've called Nullable in this example.
The Nullable Class is implemented as follows: Public NotInheritable Class Nullable(Of TClass)
Private _value As TClass
Private _hasValue As Boolean
Public Shared Function Create(ByVal value As TClass) As Nullable(Of TClass)
Return New Nullable(Of TClass)(value)
End Function
[Code]...
The way the monad is implemented at the moment I would need to issue the following to access a property on the nested child class dim id = MyParentClass.ChildClass.Value.ID
but ideally what I'd like to be able to do is to have the following statement dim id = MyParentClass.ChildClass.Id and if the ChildClass is null then just return a default value for the property type.
I tried implementing this using Default Properties and setting the Value as default via an attribute but it wouldn't compile. Is that going to be possible or perhaps there is a better way of architecting it - or maybe I just haven't 'got' the maybe monad?
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Dec 1, 2009
I would expect the following vb.net function to return a value of Nothing, but instead its returning a value of 0.
[Code]...
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May 15, 2011
I have the following POCO:
Public Class T1
<Required()>
<MaxLength(128)>
<Key(), Column(Order:=0)>
Property K1 As String
[Code]...
I would expect C2 to be created as Nullable, but both C1 and C2 are Not Null. Adding
<Required(AllowEmptyStrings:=True)>
Does not make a difference, as it seems the decoration is aimed at data validation, not DB creation.
So how do i get a nullable column with Code First?
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Jan 26, 2011
I'm trying to assign a date object to nullable but my code error on CBDate1 = nullDate. The message indicates that I need to assign a value to a nullable. This is my first time using nullable[code\]...
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Feb 10, 2011
I'm trying to assign a date object to nullable but my code error on CBDate1 = nullDate. The message indicates that I need to assign a value to a nullable. This is my first time using nullable.
[Code]....
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May 8, 2009
I have a Search Form that can search by a few different fields. The problem field is birthDate. On the form it is a string. In the SQL 2005 db it is a DateTime that can be null.
The code below is sequential as far as declaring the variable on the form and then setting it. Then the call to the BLL and then the call to the DAL.
On this line -->
dgvSearchResults.DataSource =ConnectBLL.BLL.Person.Search(_firstName,_middleName,_lastName,_sSN, (DateTime)_birthDate,_applicationID,_applicationPersonID,_fuzzy);
[Code]....
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Sep 23, 2010
Any variable of type Nullable<T> can be assigned to null. For instance:
int? i = null;
At first I couldn't see how this would be possible without somehow defining an implicit conversion from object to Nullable<T>:
public static implicit operator Nullable<T>(object box);
But the above operator clearly does not exist, as if it did then the following would also have to be legal, at least at compile-time (which it isn't):
int? i = new object();
Then I realized that perhaps the Nullable<T> type could define an implicit conversion to some arbitrary reference type that can never be instantiated, like this:
public abstract class DummyBox
{
private DummyBox()
{ }
[code]....
But this seems to imply that there is a custom implicit conversion from Nullable<T> to object:
public static implicit operator object(Nullable<T> value);
This is clearly not the case as object is a base class for all types, and user-defined implicit conversions to/from base types are illegal (as well they should be).It seems that object x = i; should box i like any other value type, so that x.GetType() would yield the same result as typeof(int?) (rather than throw a NullReferenceException).
So I dug around a bit and, sure enough, it turns out this behavior is specific to the Nullable<T> type, specially defined in both the C# and VB.NET specifications, and not reproducible in any user-defined struct (C#) or Structure (VB.NET).This particular boxing and unboxing behavior appears to be impossible to implement by hand. It only works because both C# and VB.NET give special treatment to the Nullable<T> type.
Isn't it theoretically possible that a different CLI-based language could exist where Nullable<T> weren't given this special treatment? And wouldn't the Nullable<T> type therefore exhibit different behavior in different languages?How do C# and VB.NET achieve this behavior? Is it supported by the CLR? (That is, does the CLR allow a type to somehow "override" the manner in which it is boxed, even though C# and VB.NET themselves prohibit it?)Is it even possible (in C# or VB.NET) to box a Nullable<T> as object?
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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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