title: Struct memory hack to overlap object reference - Is it possible?
body: <p>I'm guessing the answer to this is going to be "It's not possible, switch to C++". But I thought I'd throw it out there anyway.</p>
  
  <p>I'm dealing with a massive binary tree. I've got an array of structs to represent the branch nodes that I use to help with locality of memory when iterating through the tree.</p>
  
  <p>To save a bit of memory, and therefore improve cache locality I'm looking at overlapping an object reference for leaf nodes. That object reference will point to all the leaf data. Basically, something like this:</p>
  
  <pre><code>[StructLayout(LayoutKind.Explicit)]
  struct BranchData
  {
      [FieldOffset(0)] // 1 byte
      internal byte SplitIndex;
      [FieldOffset(1)] // 4 bytes
      internal float SplitValue;
      [FieldOffset(5)] // 4 bytes
      internal int LowIndex;
      [FieldOffset(9)] // 4 bytes
      internal int HighIndex;
      [FieldOffset(0)] // 8 bytes (We're working with x64 here)
      internal LeafData Node;
  }
  </code></pre>
  
  <p>The above gives the following runtime error</p>
  
  <blockquote>
    <p>Could not load type 'BranchData' from assembly
    'WindowsFormsApplication1, Version=1.0.0.0, Culture=neutral,
    PublicKeyToken=null' because it contains an object field at offset 0
    that is incorrectly aligned or overlapped by a non-object field.</p>
  </blockquote>
  
  <p>I could use a separate array to store the leaf data, and use the indexes to point to that array, but then I have 2 memory lookups (for what are certainly distant areas of memory). One for the location in the leaf array to get the reference, and one to get the leaf data. If I can achieve this overlap, I get rid of one of those lookups.</p>
  
  <p>I am able to pin objects and use unsafe code to solve this problem. Speed is the key element here.</p>