Why does this Linq Cast Fail when using ToList?

Okay, this really depends on a few oddities combined:

• Even though in C# you can’t cast a byte[] to an sbyte[] directly, the CLR allows it:

  var foo = new byte[] {246, 127};
  // This produces a warning at compile-time, and the C# compiler "optimizes"
  // to the constant "false"
  Console.WriteLine(foo is sbyte[]);
  
  object x = foo;
  // Using object fools the C# compiler into really consulting the CLR... which
  // allows the conversion, so this prints True
  Console.WriteLine(x is sbyte[]);
  

• Cast<T>() optimizes such that if it thinks it doesn’t need to do anything (via an is check like the above) it returns the original reference – so that’s happening here.

• ToList() delegates to the constructor of List<T> taking an IEnumerable<T>

• That constructor is optimized for ICollection<T> to use CopyTo… and that’s what’s failing. Here’s a version which has no method calls other than CopyTo:

  object bytes = new byte[] { 246, 127 };
  
  // This succeeds...
  ICollection<sbyte> list = (ICollection<sbyte>) bytes;
  
  sbyte[] array = new sbyte[2];
  
  list.CopyTo(array, 0);
  

Now if you use a Select at any point, you don’t end up with an ICollection<T>, so it goes through the legitimate (for the CLR) byte/sbyte conversion for each element, rather than trying to use the array implementation of CopyTo.