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copied!<p>The reason value types can't support inheritance is because of arrays.</p> <p>The problem is that, for performance and GC reasons, arrays of value types are stored "inline". For example, given <code>new FooType[10] {...}</code>, if <code>FooType</code> is a reference type, 11 objects will be created on the managed heap (one for the array, and 10 for each type instance). If <code>FooType</code> is instead a value type, only one instance will be created on the managed heap -- for the array itself (as each array value will be stored "inline" with the array).</p> <p>Now, suppose we had inheritance with value types. When combined with the above "inline storage" behavior of arrays, Bad Things happen, as can be seen <a href="https://isocpp.org/wiki/faq/proper-inheritance#array-derived-vs-base" rel="noreferrer">in C++</a>.</p> <p>Consider this pseudo-C# code:</p> <pre><code>struct Base { public int A; } struct Derived : Base { public int B; } void Square(Base[] values) { for (int i = 0; i < values.Length; ++i) values [i].A *= 2; } Derived[] v = new Derived[2]; Square (v); </code></pre> <p>By normal conversion rules, a <code>Derived[]</code> is convertible to a <code>Base[]</code> (for better or worse), so if you s/struct/class/g for the above example, it'll compile and run as expected, with no problems. But if <code>Base</code> and <code>Derived</code> are value types, and arrays store values inline, then we have a problem.</p> <p>We have a problem because <code>Square()</code> doesn't know anything about <code>Derived</code>, it'll use only pointer arithmetic to access each element of the array, incrementing by a constant amount (<code>sizeof(A)</code>). The assembly would be vaguely like:</p> <pre><code>for (int i = 0; i < values.Length; ++i) { A* value = (A*) (((char*) values) + i * sizeof(A)); value->A *= 2; } </code></pre> <p>(Yes, that's abominable assembly, but the point is that we'll increment through the array at known compile-time constants, without any knowledge that a derived type is being used.)</p> <p>So, if this actually happened, we'd have memory corruption issues. Specifically, within <code>Square()</code>, <code>values[1].A*=2</code> would <i>actually</i> be modifying <code>values[0].B</code>!</p> <p>Try to debug <em>THAT</em>!</p>

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