How FirstOrDefault extension method works?

问题

I was wondering on how FirstOrDefault extension method works? Which one of the following algorithms does it follows?

Use:

var arr = new[] {1, 2, 3, 4, 5, 6, 7};
return arr.FirstOrDefault(x => x%2 == 0);

Algorithm 1:

for(int i = 0; i < arr.Length; i++)
{
   if(arr[i] % 2 == 0)
     return arr[i];
}
return 0;

Algorithm 2:

var list = new List<int>();
for(int i = 0; i < arr.Length; i++)
{
   if(arr[i] % 2 == 0)
     list.Add(arr[i]);
}
return list.Count == 0 ? 0 : list[0];

Does the FirstOrDefault algorithm is smart enough to choose the optimal one or it strictly follow any one of these algorithms?

回答1:

I looked in Reflector:

public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
    {
        if (list.Count > 0)
        {
            return list[0];
        }
    }
    else
    {
        using (IEnumerator<TSource> enumerator = source.GetEnumerator())
        {
            if (enumerator.MoveNext())
            {
                return enumerator.Current;
            }
        }
    }
    return default(TSource);
}

It tries to do it with a List if the collection can be cast as IList (and implements the Count property). Otherwise it uses the Enumerator.

EDIT: The other method with the predicate (which I now see you are talking about) is not as optimised and relies on the IEnumerable interface to perform a foreach rather than IList.

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 local in source)
    {
        if (predicate(local))
        {
            return local;
        }
    }
    return default(TSource);
}

回答2:

Neither, it uses an enumerator to read only the very first value. When there is no first value, it returns null (or rather, the default value for the current <T>).

回答3:

First/FirstOrDefault with pick the first element in the sequence, nothing clever.

FirstOrDefault
First

来源：https://stackoverflow.com/questions/3700131/how-firstordefault-extension-method-works

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