title: When are structs the answer?
body: <p>I'm doing a raytracer hobby project, and originally I was using structs for my Vector and Ray objects, and I thought a raytracer was the perfect situation to use them: you create millions of them, they don't live longer than a single method, they're lightweight. However, by simply changing 'struct' to 'class' on Vector and Ray, I got a very significant performance gain. </p>
  
  <p>What gives? They're both small (3 floats for Vector, 2 Vectors for a Ray), don't get copied around excessively. I do pass them to methods when needed of course, but that's inevitable. So what are the common pitfalls that kill performance when using structs? I've read <a href="http://msdn.microsoft.com/en-us/library/ms973839.aspx" rel="noreferrer">this</a> MSDN article that says the following:</p>
  
  <blockquote>
    <p>When you run this example, you'll see that the struct loop is orders of magnitude faster. However, it is important to beware of using ValueTypes when you treat them like objects. This adds extra boxing and unboxing overhead to your program, and can end up costing you more than it would if you had stuck with objects! To see this in action, modify the code above to use an array of foos and bars. You'll find that the performance is more or less equal.</p>
  </blockquote>
  
  <p>It's however quite old (2001) and the whole "putting them in an array causes boxing/unboxing" struck me as odd. Is that true? However, I did pre-calculate the primary rays and put them in an array, so I took up on this article and calculated the primary ray when I needed it and never added them to an array, but it didn't change anything: with classes, it was still 1.5x faster. </p>
  
  <p>I am running .NET 3.5 SP1 which I believe fixed an issue where struct methods weren't ever in-lined, so that can't be it either.</p>
  
  <p>So basically: any tips, things to consider and what to avoid?</p>
  
  <p>EDIT: As suggested in some answers, I've set up a test project where I've tried passing structs as ref. The methods for adding two Vectors:</p>
  
  <pre><code>public static VectorStruct Add(VectorStruct v1, VectorStruct v2)
  {
    return new VectorStruct(v1.X + v2.X, v1.Y + v2.Y, v1.Z + v2.Z);
  }
  
  public static VectorStruct Add(ref VectorStruct v1, ref VectorStruct v2)
  {
    return new VectorStruct(v1.X + v2.X, v1.Y + v2.Y, v1.Z + v2.Z);
  }
  
  public static void Add(ref VectorStruct v1, ref VectorStruct v2, out VectorStruct v3)
  {
    v3 = new VectorStruct(v1.X + v2.X, v1.Y + v2.Y, v1.Z + v2.Z);
  }
  </code></pre>
  
  <p>For each I got a variation of the following benchmark method:</p>
  
  <pre><code>VectorStruct StructTest()
  {
    Stopwatch sw = new Stopwatch();
    sw.Start();
    var v2 = new VectorStruct(0, 0, 0);
    for (int i = 0; i &lt; 100000000; i++)
    {
      var v0 = new VectorStruct(i, i, i);
      var v1 = new VectorStruct(i, i, i);
      v2 = VectorStruct.Add(ref v0, ref v1);
    }
    sw.Stop();
    Console.WriteLine(sw.Elapsed.ToString());
    return v2; // To make sure v2 doesn't get optimized away because it's unused. 
  }
  </code></pre>
  
  <p>All seem to perform pretty much identical. Is it possible that they get optimized by the JIT to whatever is the optimal way to pass this struct? </p>
  
  <p>EDIT2: I must note by the way that using structs in my test project <i>is</i> about 50% faster than using a class. Why this is different for my raytracer I don't know. </p>