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    <p><em>Edited to address the updated question...</em></p> <p>You can box and unbox objects if you want to use a struct as a reference.</p> <p>However, the <code>Nullable&lt;&gt;</code> type basically allows to enhance any value type with an additional state flag which tells whether the value shall be used as <code>null</code> or if the stuct is "valid".</p> <p>So to address your questions:</p> <ol> <li><p>This is an advantage when used in collections, or because of the different semantics (copying instead of referencing)</p></li> <li><p>No it doesn't. The CLR does respect this when boxing and unboxing, so that you actually never box a <code>Nullable&lt;&gt;</code> instance. Boxing a <code>Nullable&lt;&gt;</code> which "has" no value will return a <code>null</code> reference, and unboxing does the opposite.</p></li> <li><p>Nope.</p></li> <li><p>Again, this isn't the case. In fact generic constraints for a struct do not allow nullable structs to be used. This makes sense due to the special boxing/unboxing behavior. Therefore, if you have a <code>where T: struct</code> to constrain a generic type, nullable types will be disallowed. Since this constraint is defined on the <code>Nullable&lt;T&gt;</code> type as well, you cannot nest them, without any special treatment to prevent this.</p></li> </ol> <p>Why not using references? I already mentioned the important semantic differences. But apart from this, reference types use much more memory space: Each reference, especially in 64-bit environments, uses up not only heap memory for the instance, but also memory for the reference, the instance type information, locking bits etc.. So, apart from the semantics and performance differences (indirection via reference), you end up with using a multiple of the memory used for the entity itself for most common entities. And the GC gets more objects to handle, which will make the total performance compared to structs even worse.</p>
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    1. POWhy Nullable<T> is a struct?
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    1. COAd 2. `int? a = null; bool b = (a == null);` Since `a` is not real null reference, just `Nullable` holding `null` value, isn't it a special treatment? Ad 3. `int? a = 1; a++; // a is now 2` - isn't it mutating the state?
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    2. COIt depends, I'd have to check the IL generated by the different C# versions. If `== null` always reverts to a reference comparison, then no, this wouldn't be special treatment in the compiler since the boxing required to compare with null will actually yield a null reference. Note that when using an unconstrained generic and comparing with null, it *will* in fact do a boxing operation (and issue a warning, at least R# does).
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    3. COI have to disagree with what you say about memory use. The plain difference is the pointer-size (32 or 64 bit), and if null or aliased, a reference type will take less total memory than a value-type if the class/struct size is greater than that. Since you will mostly not be copying the value itself, there tends to be much aliasing (though without the risks as they are aliased from different stack frames) so references tend to have lower total memory pressure. There are trade-offs that mean that value-type still wins upto around 16bytes, but the trade-offs can go either way.
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