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copied!<p>The reason you didn't find references to <code>GraphicsDevice.Begin/EndScene()</code> is because those methods don't exist. The <code>GraphicsDevice</code> class is a managed class used to encapsulate the graphics functionality required by a user of the XNA framework; it is not a one to one mapping of the native <code>IDirect3DDevice9</code> interface.</p> <p>I assume by looking at the source code you mean using something like Reflector, and I'm also assuming that you're looking at the code as C# code. One thing that is important to keep in mind is that large portions of the XNA framework are written in C++/CLI and consequently when reflected to C# the results can be somewhat unpredictable.</p> <p>For the specific case of calls to the native <code>IDirect3DDevice9::BeginScene()</code> and <code>IDirect3DDevice9::EndScene()</code>, you need to dig a little.</p> <p><strong>All the following assumes XNA 4.0. 3.1 should still be similar</strong>. In Reflector, you'll notice the <code>GraphicsDevice</code> class has a private <code>bool</code> <code>_insideScene</code>. If you analyse this, you'll see it set in various locations, such as the <code>GraphicsDevice.DrawPrimitives()</code> call. The C# reflection of this method should set this member with the following code:</p> <pre><code>if (!this._insideScene) { **(((int*) this.pComPtr))[0xa4](this.pComPtr); this._insideScene = true; } </code></pre> <p>The key line here is <code>**(((int*) this.pComPtr))[0xa4](this.pComPtr);</code>. If you were to write a C++/CLI assembly, include the appropriate DirectX headers (<code>d3d9.h</code>) and call <code>BeginScene()</code> on an instance of <code>IDirect3DDevice9</code> (i.e. <code>pDevice->BeginScene()</code>) and then disassemble your assembly in C#, you would have a line identical to the above. This is how the IL for the native call maps back to C#, <strong>but it is not possible to write that line in C# and compile it</strong>.</p> <p>The line of code is essentially a raw virtual method call. You can read more information about virtual method tables <a href="http://en.wikipedia.org/wiki/Virtual_method_table" rel="noreferrer">here</a>, but the bare-bones of it for this situation is as follows: an object's virtual table pointer is (compiler dependent, but true in this situation) the first element in a native object. <code>pComPtr</code> is an unsafe pointer to the native COM <code>IDirect3DDevice9</code> object. Therefore, dereferencing <code>pComPtr</code> (<code>*pComPtr</code>) gives you the pointer to the virtual method table. The dissassembly is a bit ambiguous here, but the IL is fairly clear. What happens next is <code>0xa4</code> is added to the virtual method table pointer. To understand this, remember that native arrays are just a pointer to the first element of the array. An object's virtual method table is an array of function pointers. <code>0xa4</code> is 164 in decimal, meaning the pointer is moved forward 164 bytes. As the library is an x86 library, pointers are 4 bytes long. This means that we have moved to the 41st method in the virtual method table (<code>164 / 4 = 41</code>). If you have a look at the <code>IDirect3DDevice9</code> interface declaration in <code>d3d9.h</code> (part of the DirectX SDK), you'll see that the 41st method is <code>BeginScene</code>. This pointer is then dereferenced to give the actual <code>BeginScene</code> method, and then called passing in the (normally hidden) <code>this</code> pointer as the first (and only) parameter (where <code>this</code> for the native call is the <code>this.pComPtr</code> pointer).</p> <p>You can see similar code in the private unsafe <code>GraphicsDevice.Present()</code> method, this time offsetting to <code>0xa8</code>, or function 42, which is <code>IDirect3DDevice9::EndScene()</code>.</p> <p>Therefore, you can see the <code>GraphicsDevice</code> correctly ensures that <code>BeginScene</code> has been called before any drawing is done, and <code>EndScene</code> is called before the native <code>IDirect3DDevice9::Present()</code> method is called, but it deliberately hides this functionality from users of the XNA framework.</p>

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