C# Difference between First() and Find()

So I know that Find() is only a List<T> method, whereas First() is an extension for any IEnumerable<T>. I also know that First() will return the first element if no parameter is passed, whereas Find() will throw an exception. Lastly, I know that First() will throw an exception if the element is not found, whereas Find() will return the type's default value.

I hope that clears up confusion about what I'm actually asking. This is a computer science question and deals with these methods at the computational level. I've come to understand that IEnumerable<T> extensions do not always operate as one would expect under the hood. So here's the Q, and I mean from a "close to the metal" standpoint: What is the difference between Find() and First()?

Here's some code to provide basic assumptions to operate under for this question.

var l = new List<int> { 1, 2, 3, 4, 5 };
var x = l.First(i => i == 3);
var y = l.Find(i => i == 3);

Is there any actual computational difference between how First() and Find() discover their values in the code above?

Note: Let us ignore things like AsParallel() and AsQueryable() for now.

Here's the code for List<T>.Find (from Reflector):

public T Find(Predicate<T> match)
{
    if (match == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    for (int i = 0; i < this._size; i++)
    {
        if (match(this._items[i]))
        {
            return this._items[i];
        }
    }
    return default(T);
}

And here's Enumerable.First:

public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    foreach (TSource local in source)
    {
        if (predicate(local))
        {
            return local;
        }
    }
    throw Error.NoMatch();
}

So both methods work roughly the same way: they iterate all items until they find one that matches the predicate. The only noticeable difference is that Find uses a for loop because it already knows the number of elements, and First uses a foreach loop because it doesn't know it.

First will throw an exception when it finds nothing, FirstOrDefault however does exactly the same as Find (apart from how it iterates through the elements).

BTW Find is rather equal to FirstOrDefault() than to First(). Because if predicate of First() is not satisfied with any list elements you will get an exception. Here what returns a dotpeek, another great free reflector replacement with some of ReSharper features

Here for Enumerable.First(...) and Enumerable.FirstOrDefault(...) extension methods:

    public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
        if (source == null) throw Error.ArgumentNull("source");
        if (predicate == null) throw Error.ArgumentNull("predicate");
        foreach (TSource element in source) { 
            if (predicate(element)) return element;
        } 
        return default(TSource); 
    }


    public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
        if (source == null) throw Error.ArgumentNull("source"); 
        if (predicate == null) throw Error.ArgumentNull("predicate"); 
        foreach (TSource element in source) {
            if (predicate(element)) return element; 
        }
        throw Error.NoMatch();
    }

and here is for List<>.Find:

/// <summary>
/// Searches for an element that matches the conditions defined by the specified predicate, and returns the first occurrence within the entire <see cref="T:System.Collections.Generic.List`1"/>.
/// </summary>
/// 
/// <returns>
/// The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type <paramref name="T"/>.
/// </returns>
/// <param name="match">The <see cref="T:System.Predicate`1"/> delegate that defines the conditions of the element to search for.</param><exception cref="T:System.ArgumentNullException"><paramref name="match"/> is null.</exception>
[__DynamicallyInvokable]
public T Find(Predicate<T> match)
{
  if (match == null)
    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
  for (int index = 0; index < this._size; ++index)
  {
    if (match(this._items[index]))
      return this._items[index];
  }
  return default (T);
}

1- Find() returns Null if the entity is not in the context but First() will throw an exception
2- Find() returns entities that have been added to the context but have not yet been saved to the database

Since List<> is not indexed in any way, it has to go through all values to find a specific value. Therefore it doesn't make much of a difference compared to traversing the list via an enumerable (apart from the creation of a enumerable helper object instance).

That said, keep in mind that the Find function was created way earlier than the First extension method (Framework V2.0 vs. V3.5), and I doubt that they would have implemented Find if the List<> class had been implemented at the same time as the extension methods.