为什么在 VB.NET 和 C # 中检查 null 与值有区别？

在 VB.NET中会发生这种情况:

Dim x As System.Nullable(Of Decimal) = Nothing
Dim y As System.Nullable(Of Decimal) = Nothing


y = 5
If x <> y Then
Console.WriteLine("true")
Else
Console.WriteLine("false") '' <-- I got this. Why?
End If

但是在 C # 中会发生这种情况:

decimal? x = default(decimal?);
decimal? y = default(decimal?);


y = 5;
if (x != y)
{
Debug.WriteLine("true"); // <-- I got this -- I'm with you, C# :)
}
else
{
Debug.WriteLine("false");
}

有什么区别吗？

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May be this post well help you:

If I remember correctly, 'Nothing' in VB means "the default value". For a value type, that's the default value, for a reference type, that would be null. Thus, assigning nothing to a struct, is no problem at all.

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Because x <> y returns Nothing instead of true. It is simply not defined since x is not defined. (similar to SQL null).

Note: VB.NET Nothing <> C# null.

You also have to compare the value of a Nullable(Of Decimal) only if it has a value.

So the VB.NET above compares similar to this(which looks less incorrect):

If x.HasValue AndAlso y.HasValue AndAlso x <> y Then
Console.WriteLine("true")
Else
Console.WriteLine("false")
End If

The VB.NET language specification:

7.1.1 Nullable Value Types ... A nullable value type can contain the same values as the non-nullable version of the type as well as the null value. Thus, for a nullable value type, assigning Nothing to a variable of the type sets the value of the variable to the null value, not the zero value of the value type.

For example:

Dim x As Integer = Nothing
Dim y As Integer? = Nothing


Console.WriteLine(x) ' Prints zero '
Console.WriteLine(y) ' Prints nothing (because the value of y is the null value) '
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最佳答案

VB.NET and C#.NET are different languages, built by different teams who have made different assumptions about usage; in this case the semantics of a NULL comparison.

My personal preference is for the VB.NET semantics, which in essence gives NULL the semantics "I don't know yet". Then the comparison of 5 to "I don't know yet". is naturally "I don't know yet"; ie NULL. This has the additional advantage of mirroring the behaviour of NULL in (most if not all) SQL databases. This is also a more standard (than C#'s) interpretation of three-valued logic, as explained here.

The C# team made different assumptions about what NULL means, resulting in the behaviour difference you show. Eric Lippert wrote a blog about the meaning of NULL in C#. Per Eric Lippert: "I also wrote about the semantics of nulls in VB / VBScript and JScript here and here".

In any environment in which NULL values are possible, it is imprtant to recognize that the Law of the Excluded Middle (ie that A or ~A is tautologically true) no longer can be relied on.

Update:

A bool (as opposed to a bool?) can only take the values TRUE and FALSE. However a language implementation of NULL must decide on how NULL propagates through expressions. In VB the expressions 5=null and 5<>null BOTH return false. In C#, of the comparable expressions 5==null and 5!=null only the second first [updated 2014-03-02 - PG] returns false. However, in ANY environment that supports null, it is incumbent on the programmer to know the truth tables and null-propagation used by that language.

Update

Eric Lippert's blog articles (mentioned in his comments below) on semantics are now at:

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Look at the generated CIL (I've converted both to C#):

C#:

private static void Main(string[] args)
{
decimal? x = null;
decimal? y = null;
y = 5M;
decimal? CS$0$0000 = x;
decimal? CS$0$0001 = y;
if ((CS$0$0000.GetValueOrDefault() != CS$0$0001.GetValueOrDefault()) ||
(CS$0$0000.HasValue != CS$0$0001.HasValue))
{
Console.WriteLine("true");
}
else
{
Console.WriteLine("false");
}
}

Visual Basic:

[STAThread]
public static void Main()
{
decimal? x = null;
decimal? y = null;
y = 5M;
bool? VB$LW$t_struct$S3 = new bool?(decimal.Compare(x.GetValueOrDefault(), y.GetValueOrDefault()) != 0);
bool? VB$LW$t_struct$S1 = (x.HasValue & y.HasValue) ? VB$LW$t_struct$S3 : null;
if (VB$LW$t_struct$S1.GetValueOrDefault())
{
Console.WriteLine("true");
}
else
{
Console.WriteLine("false");
}
}

You'll see that the comparison in Visual Basic returns Nullable<bool> (not bool, false or true!). And undefined converted to bool is false.

Nothing compared to whatever is always Nothing, not false in Visual Basic (it is the same as in SQL).

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This is a definite weirdness of VB.

In VB, if you want to compare two nullable types, you should use Nullable.Equals().

In your example, it should be:

Dim x As System.Nullable(Of Decimal) = Nothing
Dim y As System.Nullable(Of Decimal) = Nothing


y = 5
If Not Nullable.Equals(x, y) Then
Console.WriteLine("true")
Else
Console.WriteLine("false")
End If
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Your VB code is simply incorrect - if you change the "x <> y" to "x = y" you will still have "false" as the result. The most common way of expression this for nullable instances is "Not x.Equals(y)", and this will yield the same behavior as "x != y" in C#.

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The problem that's observed here is a special case of a more general problem, which is that the number of different definitions of equality that may be useful in at least some circumstances exceeds the number of commonly-available means to express them. This problem is in some cases made worse by an unfortunate belief that it is confusing to have different means of testing equality yield different results, and such confusion might be avoided by having the different forms of equality yield the same results whenever possible.

In reality, the fundamental cause of confusion is a misguided belief that the different forms of equality and inequality testing should be expected to yield the same result, notwithstanding the fact that different semantics are useful in different circumstances. For example, from an arithmetic standpoint, it's useful to be able to have Decimal which differ only in the number of trailing zeroes compare as equal. Likewise for double values like positive zero and negative zero. On the other hand, from a caching or interning standpoint, such semantics can be deadly. Suppose, for example, one had a Dictionary<Decimal, String> such that myDict[someDecimal] should equal someDecimal.ToString(). Such an object would seem reasonable if one had many Decimal values that one wanted to convert to string and expected there to be many duplicates. Unfortunately, if used such caching to convert 12.3 m and 12.40 m, followed by 12.30 m and 12.4 m, the latter values would yield "12.3", and "12.40" instead of "12.30" and "12.4".

Returning to the matter at hand, there is more than one sensible way of comparing nullable objects for equality. C# takes the standpoint that its == operator should mirror the behavior of Equals. VB.NET takes the standpoint that its behavior should mirror that of some other languages, since anyone who wants the Equals behavior could use Equals. In some sense, the right solution would be to have a three-way "if" construct, and require that if the conditional expression returns a three-valued result, code must specify what should happen in the null case. Since that is not an option with languages as they are, the next best alternative is to simply learn how different languages work and recognize that they are not the same.

Incidentally, Visual Basic's "Is" operator, which is lacking in C, can be used to test for whether a nullable object is, in fact, null. While one might reasonably question whether an if test should accept a Boolean?, having the normal comparison operators return Boolean? rather than Boolean when invoked on nullable types is a useful feature. Incidentally, in VB.NET, if one attempts to use the equality operator rather than Is, one will get a warning that the result of the comparison will always be Nothing, and one should use Is if one wants to test if something is null.


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