This question already has answers here:
Nothing = String.Empty (Why are these equal?)
(4 answers)
Difference between C# and VB.Net string comparison
(2 answers)
Closed 7 years ago.
The code below returns TRUE when I expected it to return FALSE.
Why does it return TRUE? I Expect nothing to set the value of the string to null, not empty (According to msdn)
CodeingGround sample
Module VBModule
Sub Main()
dim x as String
x = nothing
console.writeline(x = string.Empty)
End Sub
End Module
Nothing (Visual Basic)
Represents the default value of any data type. For reference types,
the default value is the null reference.
***EDIT****
Nothing = String.Empty (Why are these equal?)
Nothing in VB.net is the default value for a type. The language spec says in section 2.4.7:
Nothing is a special literal; it does not have a type and is
convertible to all types in the type system, including type
parameters. When converted to a particular type, it is the equivalent
of the default value of that type.
So, when you test against String.Empty, Nothing is converted to a string, which has a length 0. The Is operator should be used for testing against Nothing, and String.Empty.Equals(Nothing) will also return false.
Per then comment,
when converted to a particular type, it is the equivalent of the
default value of that type.
The default value for a string is null. I dont understand why that answer was accepted.
Difference between C# and VB.Net string comparison
The above post explains the answer clearly, credit goes to Tim Schmelter in the comment section for finding the above post
Per Tim Schmeleters comments
it is called from the vb compiler as the documentation states in String.Equality Operator
Related
This question already has answers here:
Ternary operator VB vs C#: why resolves Nothing to zero? [duplicate]
(7 answers)
Closed 3 years ago.
I was surprised when I tried to assign null value to a variable using ternary expression in vb.net. When I assign through ternary expression, it doesn't works as expected.
Dim i As Integer? = Nothing
Dim j As Integer? = Nothing
i = If(True, j, 1)
j = If(True, Nothing, 1)
After execution of this code: i is nothing but j becomes 0 (zero). Why?
What is the explanation?
Why I can"t assign directly Nothing (Null) value?
I think the important thing to understand here is Nothing in VB.Net is not the same as null in other languages, because you can still assign Nothing to value types. In many other languages, null is a reference-type construct only. If you're familiar with C#, Nothing is closer to default(T) than to null.
With that in mind, take a fresh look at this expression:
If(True, Nothing, 1)
The compiler evaluates the entire expression on it's own merits, knowing nothing about i or j, even though j is the target of the assignment. The expression has to be able to stand alone.
VB.Net must also determine a type to use for the expression, and it must do this at compile-time. It is not able to infer anything from the use of Nothing about needing an Integer? rather than a basic non-nullable Integer for this type, because VB.Net is perfectly happy to assign Nothing to value types. Therefore the type of the conditional expression can only be inferred from the 1 literal in the final argument, which is a plain Integer, and not Integer?.
Given that resulting type, we now must evaluate Nothing as an integer, where the result is the 0 you observed. In theory, this part is done at runtime rather than compile time, but in practice I suspect the compiler or jitter recognizes the chance to optimize things and rewrites it all down to just j = 0.
If you want to be able to assign an Integer? with a value of Nothing, do it as you did in the first example and keep a variable handy with the correct type you can use for the assignment.
This question already has answers here:
Difference between Equals/equals and == operator?
(11 answers)
Closed 3 years ago.
If I declare a string but do not assign a value, the Equals function throws an exception but it doesn't throw an exception if it is compared to a value.
The error list warns about the problem:
Warning BC42104 Variable a is used before it has been assigned a
value. A null reference exception could result at runtime.
Dim a As String
Dim b as string = "bar"
a.Equals("foo") 'causes System.NullReferenceException
a = "foo" 'No exception although a is nothing
a = b 'No exception although a is nothing
I know the warning says it COULD cause an exception, but does anyone know why this is the happening?
This is because Dim a As String declares the type of variable a but doesn't assign anything to it. This is basically saying : "This variable was made to hold a String object, but it doesn't hold any right now". On the other hand, Dim b As String = "bar" declares the variable and its type, but also assigns a String object to it ("bar"). The reason a.Equals("foo") returns an exception is because you only declared it without assigning anything to it (so you are trying to access an object that isn't there). a = "foo" works because you are assigning a String object of value "foo" to the variable a. It's like saying : "This variable now holds a String object with value 'foo'".
Edit :
While your code points to the assignment of the variable a, I was made aware that you wanted to know why the = operator, as a comparison operator, works. This is because what I said earlier isn't entirely true when I said it didn't hold an object. It actually is of Nothing value (which sets it as a null reference) if no String object was assigned to it (String is a nullable object).
See : https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/nothing
Hope this helps.
I'm trying to update my SmartSheet API from v1 to v2 and am having some difficulty on the code below.
The code returns rows for the selected sheet, however the "ColumnType" property of all the Cell's within the rows are NULL.
I know to return this you have to specify it as an inclusion - which I believe I have.
Dim sheet As Sheet = smartsheet.SheetResources.GetSheet(curSheet.Id, New RowInclusion() {RowInclusion.COLUMN_TYPE, RowInclusion.COLUMNS}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing)
For Each Row As Row In sheet.Rows
If Row.ParentRowNumber Is Nothing Then
Dim i As Integer = 0
Dim colType As ColumnType
If Not Row.Cells(i).ColumnType = ColumnType.TEXT_NUMBER Then
'Do some stuff here...
End if
Next
Any help would be great.
Thanks,
Steve
The short answer is just get the latest SDK from https://github.com/smartsheet-platform/smartsheet-csharp-sdk/pull/60 and update your GetSheet to the following:
Dim sheet As Sheet = client.SheetResources.GetSheet(SHEETID, New SheetLevelInclusion() {SheetLevelInclusion.COLUMN_TYPE}, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing)
Notice the use of SheetLevelInclusion rather than RowInclusion. You should be all set.
If you care about the details, the longer answer is... The GetSheet method doesn't accept an array/IEnumerable of RowInclusion as the second argument. It expects an array/IEnumerable of SheetLevelExclusion. In C#, the same invocation call would fail as C# imposes stricter type checking on the generic type parameter of IEnumerable. However, due to Visual Basic's leniency around implicit conversions between Enum types and its lenient conversions for arrays (and similar types like IEnumerable) it is possible to invoke a function with the "wrong" type of argument when the argument is an array/IEnumerable and the elements are Enums. In this case, Visual Basic is actually converting the RowInclusion values to their underlying numeric value (Enum is always implicitly or explicitly backed by an underlying numeric type) and converting those values to the SheetLevelExclusion value corresponding to the same underlying numeric value so that it can invoke the GetSheet method.
The other complication here is that the SDK didn't have COLUMN_TYPE as an available SheetLevelExclusion value. So the pull request/branch I linked to above adds that. In my simple test here that made it work.
today I stumbled upon a strange behaviour of the VB.net If() statement. Maybe you can explain why it is working like it does, or maybe you can confirm that it is a bug.
So, I have a SQL database with a table "TestTable" with an int column "NullableColumn" that can contain NULL. I'd like to read out the content of this column.
So I declare a variable of type Nullable(Of Integer) for that matter, open a SqlClient.SqlDataReader for "SELECT NullableColumn FROM TestTable" and use the following code to get the content of this column:
Dim content as Nullable(Of Integer)
...
Using reader as SqlClient.SqlDataReader = ...
content = If(reader.IsDBNull(reader.GetOrdinal("NullableColumn")), Nothing, reader.GetInt32(reader.GetOrdinal("NullableColumn")))
End Using
But after that my variable content has the value 0, not Nothing as I would have expected.
When debugging everything looks alright, so
reader.GetOrdinal("NullableColumn") delivers the correct ordinal position of this column (which is 0)
reader.IsDBNull(0) and reader.IsDBNull(reader.GetOrdinal("NullableColumn")) deliver True, since the content of this column indeed is NULL
If(1=2, Nothing, "Not Nothing") delivers the String "Not Nothing"
If(1=1, Nothing, "Not Nothing") delivers Nothing
reader.GetInt32(reader.GetOrdinal("NullableColumn")) throws an error, since NULL can't be converted to Integer
So, why does my variable has the value 0?
In VB Nothing is not the same as null. The If operator must determine the type of its result based on the arguments passed to it. Nothing, of course, has no type so the only type that the If operator can return in your code is Int32. If the IsDBNull method returns true, then the If operator returns Nothing cast as Int32. In VB, Nothing returns the default value for a type. For an Int32, the default value is 0.
From MSDN on the Nothing keyword:
Nothing represents the default value of a data type. The default value depends
on whether the variable is of a value type or of a reference type.
For non-nullable value types, Nothing in Visual Basic differs from null in C#.
In Visual Basic, if you set a variable of a non-nullable value type to Nothing,
the variable is set to the default value for its declared type. In C#, if you
assign a variable of a non-nullable value type to null, a compile-time error
occurs.
I think just a regular If would work best:
If Not reader.IsDBNull(reader.GetOrdinal("NullableColumn")) Then
content = reader.GetInt32(reader.GetOrdinal("NullableColumn"))
End If
Or to keep it shorter
If Not reader.IsDBNull(reader.GetOrdinal("NullableColumn")) Then content = reader.GetInt32(reader.GetOrdinal("NullableColumn"))
But after that my variable content has the value 0, not Nothing as I would have expected.
How do you check the content value?
First of all, you should start with content.HasValue property. It should be False for your case of Nothing and True when correct value was fetched from database.
You should also get InvalidOperationException while accessing content.Value when it hasn't got value.
Chris has given the explanation but I dislike the style of assignment he’s chosen because it splits assignment from variable declaration.
By contrast, I recommend initialising variables upon declaration. In this case it’s admittedly slightly convoluted since you need cast either operator of If to the correct type first.
Dim content = If(reader.IsDBNull(reader.GetOrdinal("NullableColumn")),
DirectCast(Nothing, Integer?),
reader.GetInt32(reader.GetOrdinal("NullableColumn")))
Actually you can also use the slightly shorter New Integer?() instead of the DirectCast.
Of course now content is declared inside the Using block – this might not be what you want but you should try to make the declaration as local as possible.
Furthermore, this code is complex and will probably be reused. I suggest creating a separate (extension) method to convert database NULL values to nullables:
<Extension> _
Public Shared Function GetNullable(Of T)(SqlClient.SqlDataReader this, String fieldName) As T?
Dim i = this.GetOrdinal(fieldName)
Return If(this.IsDBNull(i), New T?(), this.GetFieldValue(Of T)(i))
End Function
Now you can use it as follows:
Dim content = reader.GetNullable(Of Integer)("NullableColumn")
This question already has an answer here:
Closed 10 years ago.
Possible Duplicate:
Bug?? If you assign a value to a nullable integer via a ternary operator, it can't become null
While this question may seem like a duplicate of many, it is actually being asked for a specific reason. Take this code, for example:
Dim n As Integer? = If(True, Nothing, 1)
In that code, the ternary expression should be returning Nothing, but it's setting n to 0. If this were C#, I could say default(int?) and it would work perfectly. Now it looks like I am going to have to ditch the ternary and use a regular If block, but I really want to use the ternary.
If Nothing were truly VB.NET's equivalent to C#'s default, how can you explain this behavior?
The VB.NET equivalent to C#'s default is the keyword Nothing. The code you wrote should compile just fine so long as Id.Value returns an Integer value.
The reason your updated sample is going wrong is because of the nature of Nothing. In VB.NET Nothing is the empty value and it's convertible to any type. Now for an If expression, the compiler has to infer what the type of the return should be, and it does this by looking at the two value arguments.
The value Nothing has no type, but the literal 1 has the type Integer. Nothing is convertible to Integer so the compiler determines Integer is the best type here. This means when Nothing is chosen as the value, it will be interpreted as Integer and not Integer?.
The easiest way to fix this is to explicitly tell the compiler that you want 1 to be treated as an Integer?.
Dim n As Integer? = If(True, Nothing, CType(1, Integer?))