Question

Does anyone happen to know if, in .NET (4.0, if it matters), the Length property of an array, or the Count property of a List<T> is stored after it’s been calculated, until the array/list is changed?

I ask because a record linkage program I’m working on is already horrifically complex enough as it is, and I’d rather not add another O(n) on top of things if I can help it.

Along similar lines, is the hashcode of an instantiated System.String memoized? Looking at it through the debugger, I can see that List<T> has a private _size member that could be where Count gets its value, but I see nothing for int[] or string indicating that they store it anywhere.

I can see the size vs. speed tradeoff, but can anyone tell me for sure if there are backing fields that hold these? For example, since in C#, strings are immutable, wouldn’t it make sense to calculate the hashcode the first time GetHashCode is called, and just store it for every later use?

Answer 1

Using reflector the implementation of List<T>.Count is:

public int Count
{
  get
  {
    return this._size;
  }
}

I’m not sure about Array.Length. The code for it is:

public int Length 
{ 
    [ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success), 
    SecuritySafeCritical, MethodImpl(MethodImplOptions.InternalCall)] 
    get; 
}