问题
I just shoot myself in the foot and would like to know whether there were actual reasons to make this situation possible.
And anyway, this question can stay for the convenience of the future foot shooters.
Suppose we have a nullable value in vb.net:
Dim i as Integer?
We want to assign a value to it, basing on a condition, and using a ternary operator, because it's so neat and stuff:
i = If(condition(), Nothing, 42)
That is, if a condition is true, employ the nullability, otherwise the value.
At which point the shooting occurs. For no apparent reason VB compiler decides that the common base type for Nothing and Integer is Integer, at which point it silently translates the statement to:
i = If(condition(), 0, 42)
Now, if you were to do this in C#:
i = (condition()) ? null : 42;
You would immediately get a compiler error saying that <null> doesn't mix well with int. Which is great, as my foot would have been healthier had I went the C# way this time. And for this to compile, you have to explicitly write:
i = (condition()) ? null : (int?)42;
Now, you can do the same in VB and get the correct null-ness you would expect:
i = If(condition(), Nothing, CType(42, Integer?))
But that requires having your foot shot in the first place. There's no compiler error and there's no warning. That's with Explicit On and Strict On.
So my question is, why?
Should I take this as a compiler bug?
Or can someone explain why the compiler behaves this way?
回答1:
This is because VB's Nothing is not a direct equivalent to C#'s null.
For example, in C# this code will not compile:
int i = null;
But this VB.Net code works just fine:
Dim i As Integer = Nothing
VB.Net's Nothing is actually a closer match for C#'s default(T) expression.
回答2:
The ternary operator can only return one type.
In C#, it tries to choose a type based on null and 42. Well, null doesn't have a type, so it decides that the return type of the ternary operator is that of 42; a plain old int. Then it complains because you can't return null as a plain old int. When you coerce 42 as a int?, the ternary operator is going to return an int?, so null's valid.
Now, I don't know about VB, but quoting from the MSDN,Assigning Nothing to a variable sets it to the default value for its declared type.
Which, since VB determines that the ternary operator will return an int (using the same process C# did), Nothing is 0. Again, coercing the 42 to be an int? turns Nothing into the default value of int?, which is null, as you expected.
回答3:
I am thinking this has something more to do with IF than with Nothing. Consider this code:
''# This raises an exception
Dim x As Integer?
x = If(True, Nothing, Nothing)
MessageBox.Show(x.Value)
''# As does
Dim x As Integer?
x = Nothing
MessageBox.Show(x.Value)
''# Changing one of the truthpart arguments of If is what seems to return the zero.
Dim x As Integer?
x = If(True, Nothing, 5)
MessageBox.Show(x.Value)
Why it is doing this I still don't know, probably a question for the VB team. I don't think it has to do with the Nothing keyword or Nullable.
回答4:
Nothing and null are not the same thing... from the MSDN:
Assigning Nothing to a variable sets it to the default value for its declared type.
Also
If you supply a value type in Expression, IsNothing always returns False.
Keep in mind that int? is a nullable type, but it's still a value type, not a reference type.
Try setting it to DbNull.Value instead of Nothing...
回答5:
In a number of cases Nothing will get converted to the default value. To use Nothing the same way you would use null you need to cast it to the correct nullable type.
Dim str As String
Dim int As Nullable(Of Integer) ' or use As Integer?
Dim reader As SqlDataReader
Dim colA As Integer = reader.GetOrdinal("colA")
Dim colB As Integer = reader.GetOrdinal("colB")
str = If(reader.IsDBNull(colA), DirectCast(Nothing, String), reader.GetString(colA))
int = If(reader.IsDBNull(colB), DirectCast(Nothing, Nullable(Of Integer)), reader.GetInt32(colB))
回答6:
This happens because Integer is not a reference type. 'Nothing' is only supposed to work for reference types. For value types assigning Nothing is automatically converted to the default value, which is in the case of an Integer 0.
回答7:
This is actually now possible in VS2015 (at the very least) by using New Integer?
Ex.:
If(testInt> 0, testInt, New Integer?), where testInt is of type Integer?
来源：https://stackoverflow.com/questions/4147277/ternary-operator-vb-vs-c-why-resolves-nothing-to-zero