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Why does (int)(object)10m throw "Specified cast is not valid" exception?
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Closed 2 years ago.
Okay, so I'm no stranger to coding and this strikes me as absolutely silly. Maybe someone can shed some light on why this is failing as usually VB.Net is intuitive and flexible with this type (pun intended) of thing?
I have a simple example, where I want to pass an object set with an integer value of lets say 6 to a procedure with a nullable Int64 parameter. I'd like to do so as I can't change the incoming value from object, it is a property of a class and may very well be nothing, hence the nullable Int64 being used.
Private Sub NullableParamTest(ByVal ID As Int64?)
MsgBox(ID)
End Sub
Now a simple sample like this will cause the exception:
Dim objTest As Object = 6
NullableParamTest(objTest)
Why is this not being boxed properly when the object is being set to an integer and passing to a procedure with an Int64? type? If I wrap the objTest with CInt(objTest) and cast it first it's fine, but I shouldn't need to do that at least in VB. I have logging methods that optionally take in various IDs as Int64? but the source is this Object property causing it to fail...even though they have perfectly valid Int64s set.
I wanted to avoid the whole Optional...Int64 = Nothing as that's not really setting it to nothing, granted it works as my IDs would never be zero but it's not true to what is really going on.
Thanks to the comments from #GSerg, it appears the following is the answer to this question found here:
A boxed value can only be unboxed to a variable of the exact same type.
It links to another answer describing this behavior as being performance related. In short you have to convert/cast the value within the object first before passing it.
In my case my passed value of an ID would never be 0 so using a regular Int64 and checking that it's nothing would work, otherwise using Object for a type is the alternative as wrapping all of the method calls with over half a dozen converts would be hideous.
The other comment by #GSerg mentions the notation of adding an & after the value to make it an explicit Int64, also allowing it to work in this case. Note that casting the value in this way as an Int32, which in theory would work (as it fits within an Int64 space), in fact does not due to the first part of my comment that it has to be unboxed as the exact same value type for the parameter.
Related
Suppose I have an object which has a type like NullableArray{Int64} -- how do I know that the elements have type Int64 (well, Nullable{Int64}) without actually accessing any element (ie, it can be done when the object is empty). Is there a general way to figure out what SubType is from an object of type Type{SubType}?
Edit: Whoops, to expand on the accepted answer below I realized I was doing eltype(x) but needed to do eltype(eltype(x)) for the example of Nullables.
you can use eltype
assert(eltype(collect(1:3)) == Int)
I'm using Authorize.net API and they require card expiration field to be formated as "yyyy-mm". We did that with this simple line of code:
expirationDate = model.Year.ToString("D4") & "-" & model.Month.ToString("D2")
and this absolutelly worked. I still have cards stored in the system that were saved using this method! But today I was testing something completelly unrelated, and wanted to add another card, and bam, this code exploded with this exception:
System.InvalidCastException: 'Conversion from string "D4" to type 'Integer' is not valid.'
Inner exception to that one is:
Input string was not in a correct format.
This just... doesn't make sense to me. Why in the world is it trying to convert format specifier (D4) into an integer? What input string? What in the world changed in two days?
The problem is that your are using a Nullable(Of Integer). This is a different structure that does not support the overloads of the ToString method a normal Integer has.
You can view the overloads of the Nullable structure here.
I suggest you use the GetValueOrDefault() method to get the proper Integer and also apply the value you expect in case the value is Nothing.
If it is impossible that a instance with a Nothing set for the year reaches this method you can simply use the Value property.
I still do not fully understand why you get this strange error message. Maybe you could check out what the actual method that is called is? Pointing at the method should give you that information. It can't be Nullable(Of Integer).ToString
Well, I found a workable solution and something of an answer thanks to #Nitram's comment. The type of Year/Month property has been changed from Integer to Integer?. Obviously, this isn't a very satisfying answer because I still don't understand why the nullable int can't be formatted, and yet the code compiles perfectly. The working solution for me has been using static format method on String as so:
expirationDate = String.Format("{0:D4}-{1:D2}", model.Year, model.Month)
This works fine even with nullable types.
Compare weirdness when working with LINQ and Entity Framework.
I want to retrieve an ID from my DB and I get this weird message.
I could simply fix it as you can see but I want to understand why this happens.
Question:
Why do I get this error message even if I check with "HasValue" or I use "FirstOrDefault"? It can't be null in my opinion but I obviously miss something.
Add .Value if you are 100% sure the Integer? has a value.
Why do I get this error message even if I check with "HasValue"
Entity Framework just uses the objects you give it. It can't create a new object where OPX_ isn't nullable.
the setOpxRights function presumably takes an Integer as a parameter and Option Strict On won't allow an Integer? to be implicitly converted to an Integer. If you are sure that it will always have a value, pass in cctUser.OPX_Rechte.Value
The compiler is not perfect, we can see that OPX_Rechte will have a value because of the where statment, but for the compiler you are just using a the object cctUser that have an Integer? and it needs an Integer.
I'm trying to use the following code to check for a DBNull and set the variable to nothing if it is, or a short if it isn't. The problem is it is failing to set the variable to Nothing and sets it to a 0 instead. Anybody know why?
variable = If(currentRow.Item("variable") Is DBNull.Value,
Nothing, CShort(currentRow.Item("variable")))
If variable is declared As Short? then the code works with a slight tweak: you need to cast either operand of If to the target type first:
variable = If(condition, CType(Nothing, Short?), CShort(…))
(You could also have cast the third operand instead, or both.)
This cast is necessary because of how If deduces types: if the two result types mismatch, a common type is deduced which is the closest parent type, i.e. a type from which both inherit. However, with Nothing, new rules come into play because as far as VB is concerned, Nothing is already a valid Short – a default-initialised one (see old answer below for explanation). So VB doesn’t try any type coercion, it simply uses Short as the return value.
Old answer below, assuming that OP had declared variable As Short:
You cannot set value types to Nothing. If you assign Nothing to a value type then it will be set to its type’s default value instead – which is 0 for Short.
You can test this easily:
Dim s as Short = Nothing
Console.WriteLine(s)
Setting a value type to Nothing is the same as invoking its default constructor (New Short()) or declaring a new variable of that type without initialising it. The corresponding operation in C# would be to assign default(T) (short s = default(short)).
If you want to represent null value types, you have to use nullable types:
Dim s as Short? = Nothing
Now s is of type Nullable<Short> (Short? is a shortcut of that) and can be assigned a proper Nothing.
I'm attempting to create a dataset based on the properties of an object. For example, I have an instance of a Person class with properties including ID, Forename, Surname, DOB etc. Using reflection, I'm adding columns to a new dataset based on the object properties:
For Each pi As PropertyInfo In person.GetType().GetProperties()
Dim column As New DataColumn(pi.Name, pi.PropertyType)
table.Columns.Add(column)
Next
My problem is that some of those properies are nullable types which aren't supported by datasets. Is there any way to extract the underlying system type from a nullable type?
Thanks.
Here's your answer, in VB. This may be overkill for your purposes, but it also might be useful to some other folks.
First off, here's the code to find out if you're dealing with a Nullable type:
Private Function IsNullableType(ByVal myType As Type) As Boolean
Return (myType.IsGenericType) AndAlso (myType.GetGenericTypeDefinition() Is GetType(Nullable(Of )))
End Function
Note the unusual syntax in the GetType. It's necessary. Just doing GetType(Nullable) as one of the commentors suggested did not work for me.
So, armed with that, you can do something like this... Here, in an ORM tool, I am trying to get values into a generic type that may or not be Nullable:
If (Not value Is Nothing) AndAlso IsNullableType(GetType(T)) Then
Dim UnderlyingType As Type = Nullable.GetUnderlyingType(GetType(T))
Me.InnerValue = Convert.ChangeType(value, UnderlyingType)
Else
Me.InnerValue = value
End If
Note that I check for Nothing in the first line because Convert.ChangeType will choke on it... You may not have that problem, but my situation is extremely open-ended.
Hopefully if I didn't answer your question directly, you can cannibalize this and get you where you need to go - but I just implemented this moments ago, and my tests are all passing.
Nullable.GetUnderylingType(myType)
will return the underlying type or null if it's not a nullable type.
I'm guessing that the problem is recognizing whether the property is nullable or not. In C# you do this with this code:
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
... but I'm not sure what the equivalent of that last clause is in VB.NET.
You can also use the GetGenericParameters() method on that type. myNullableObject.GetType().GetGenericParameters()[0] should give you the type of nullable it is (so Guid, Int32, etc.)
#Mendelt Siebenga: You can only call GetType on the value property if the variable is not set to null; otherwise, you'll get an exception.
What you want to do is use the "GetValueOrDefault" property and call GetType on that, since you are guaranteed it will not be null. Example:
Dim i As Nullable(Of Integer) = Nothing
Dim t As Type = i.GetValueOrDefault().GetType()