Why is there a difference in checking null against a value in VB.NET and C#?

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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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