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    <p>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.</p> <p>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 <a href="http://en.wikipedia.org/wiki/Three-valued_logic" rel="noreferrer">here</a>.</p> <p>The C# team made different assumptions about what NULL means, resulting in the behaviour difference you show. <a href="http://blogs.msdn.com/b/ericlippert/archive/2012/03/26/null-is-not-false.aspx" rel="noreferrer">Eric Lippert wrote a blog about the meaning of NULL in C#</a>. Per Eric Lippert: "I also wrote about the semantics of nulls in VB / VBScript and JScript <a href="http://blogs.msdn.com/b/ericlippert/archive/2003/09/30/53120.aspx" rel="noreferrer">here</a> and <a href="http://blogs.msdn.com/b/ericlippert/archive/2003/10/01/53128.aspx" rel="noreferrer">here</a>".</p> <p>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.</p> <p><strong>Update:</strong></p> <p>A <code>bool</code> (as opposed to a <code>bool?</code>) 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 <code>5=null</code> and <code>5&lt;&gt;null</code> BOTH return false. In C#, of the comparable expressions <code>5==null</code> and <code>5!=null</code> only the <s>second</s> first <em>[updated 2014-03-02 - PG]</em> 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.</p>
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    1. POWhy is there a difference in checking null against a value in VB.NET and C#?
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    1. COThanks for the link. I also wrote about the semantics of nulls in VB / VBScript and JScript here: http://blogs.msdn.com/b/ericlippert/archive/2003/09/30/53120.aspx and here: http://blogs.msdn.com/b/ericlippert/archive/2003/10/01/53128.aspx
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    2. COAnd FYI the decision to make C# incompatible with VB in this way was a controversial one. I was not on the language design team at the time but the amount of debate that went into this decision was considerable.
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    3. CO@EricLippert: Thank you; I wil check out those additional links.
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