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    <p>It depends on the type.</p> <ul> <li><p>For <strong>value types</strong>, <code>Is</code> <em>doesn’t work</em>, only <code>=</code>, and <code>Nothing</code> refers to the default instance of that type (i.e. the instance that you get by calling <code>New T()</code> for a given type <code>T</code>).</p></li> <li><p>For <strong>reference types</strong>, <code>Is</code> performs a reference comparison (identical to <code>object.ReferenceEquals(a, Nothing)</code>). <code>a = Nothing</code> usually <em>does not work</em>, <strong>unless</strong> <code>Operator =</code> has explicitly been defined for that class.</p> <p>If, furthermore, <code>Operator =</code> has been implemented correctly, then <code>foo = Nothing</code> and <code>foo Is Nothing</code> should yield the same result (but the same isn’t true for any other value instead of <code>Nothing</code>) but <code>foo Is Nothing</code> will be more efficient since it’s a compiler intrinsic while <code>Operator =</code> will call a method.</p></li> <li><p>For <strong>nullable value types</strong> (i.e. instances of <code>Nullable(Of T)</code>), special rules apply: like all other operators, <code>=</code> is <a href="http://blogs.msdn.com/b/ericlippert/archive/2007/06/27/what-exactly-does-lifted-mean.aspx" rel="nofollow noreferrer"><em>lifted</em></a> (notice the error in that blog post …) by the compiler to the underlying type. The result of comparing two <code>Nullable</code>s is thus not <code>Boolean</code> but <code>Boolean?</code> (note the <code>?</code>). However, because of so-called “null propagation” for lifted operators, this will <em>always</em> return <code>Nothing</code>, no matter the value of <code>foo</code>. Quoting the <a href="http://download.microsoft.com/download/D/B/1/DB1E9BCC-A878-4BCC-880F-71387EAAF1E1/Visual%20Basic%20Language%20Specification%2010.0.doc" rel="nofollow noreferrer">Visual Basic 10 language specification</a> (§1.86.3):</p> <blockquote> <p>If ether (sic!) operand is <code>Nothing</code>, the result of the expression is a value of <code>Nothing</code> typed as the nullable version of the result type.</p> </blockquote> <p>So if the users want to compare a <code>Nullable</code> variable to <code>Nothing</code>, they must use the <code>foo Is Nothing</code> syntax for which, once again, the compiler generates special code to make it work (§1.79.3 of the Visual Basic 10 language specification). <sub>Hat tip to Jonathan Allen for (correctly) persisting that I was wrong; hat tip to Jared Parsons for passing me a link to the Visual Basic 10 specification.</sub></p></li> </ul> <p>(The above assumes that <code>Option Strict On</code> is used, as you <em>always</em> should. In case that isn’t the case, the results will differ slightly since calling <code>foo = Nothing</code> may perform a late-bound call.)</p>
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    1. POWhat is the difference between 'foo = Nothing' and 'foo is Nothing' in VB.NET?
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    1. USPeter Mortensen
    1. USKonrad Rudolph
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    1. POWhat is the difference between 'foo = Nothing' and 'foo is Nothing' in VB.NET?
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    1. COFor strings = and Is produce different results, I daresay = is implemented with principle of least surprise for strings so I deem that correct.
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      1. USLasse Vågsæther Karlsen
    2. CO@Lasse: Yes, strings are a special case in VB since `=` will call a special method instead of `Equals`, that will treat `Nothing` equal to the empty string.
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      1. USKonrad Rudolph
    3. COYou forgot Nullable(Of T). For that `foo = Nothing` will compile, but it gives the wrong answer.
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      1. USJonathan Allen
 

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