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copied!<p>The important part is not what happens IN this class, but what happens, when another class uses (and links to) it. Let me explain with another example:</p> <p>Assume you have a Assembly1.dll containing a class declaring</p> <pre><code>public static const int SOME_ERROR_CODE=0x10; public static readonly int SOME_OTHER_ERROR_CODE=0x20; </code></pre> <p>and another class consuming this e.g.</p> <pre><code>public int TryFoo() { try {foo();} catch (InvalidParameterException) {return SOME_ERROR_CODE;} catch (Exception) { return SOME_OTHER_ERROR_CODE;} return 0x00; } </code></pre> <p>You compile your class into Assembly2.dll and link it against Assembly1.dll, as expected, your method will return 0x10 on invalid parameters, 0x20 on other errors, 0x00 on success.</p> <p>Especially, if you create Assembly3.exe containing something like</p> <pre><code>int errorcode=TryFoo(); if (errorcode==SOME_ERROR_CODE) bar(); else if (errorcode==SOME_OTHER_ERROR_CODE) baz(); </code></pre> <p>It will work as expected (After being linked against Assembly1.dll and Assembly2.dll)</p> <p>Now if you get a new version of Assembly1.dll, that has</p> <pre><code>public const int SOME_ERROR_CODE=0x11; public readonly int SOME_OTHER_ERROR_CODE=0x21; </code></pre> <p>If you recompile Assembly3.exe and link the last fragment against new Assembly1.dll and unchanged Assembly2.dll, it will stop working as expected:</p> <p>bar() will NOT be called correctly: Assembly2.dll remembers the LITERAL 0x20, which is not the same literal 0x21 that Assembly3.exe reads out of Assembly1.dll</p> <p>baz() will be called correctly: Both Assembly2.dll and Assembly3.exe refer to the SYMBOL REFERENCE called SOME_OTHER_ERROR_CODE, which is in both cases resolved by the current version of Assembly1.dll, thus in both cases is 0x21.</p> <p>In Short: a <code>const</code> creates a <code>LITERAL</code>, a <code>readonly</code> creates a <code>SYMBOL REFERENCE</code>.</p> <p><code>LITERALS</code> are internal to the framework and can not be marshalled and thus used by native code. </p> <p>So </p> <pre><code>public static readonly String Empty = ""; </code></pre> <p>creates a <code>symbol reference</code> (resovled at time of first use by a call to the String cosntuctor), that can be marshalled an thus used from native, while </p> <pre><code>public static const String Empty = ""; </code></pre> <p>would create a literal, that can't.</p>

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