以下两个代码片段之间的差异（Lambda表达式）

Question

Can anyone please tell me the difference between the following two lambda expressions:

    1. TabView mytab = TabCollection.Where(s => s.TabHeader == h).FirstOrDefault();

    2. TabView mytab = TabCollection.FirstOrDefault(s => s.TabHeader == h);

TabCollection is an ObservableCollection of type TabView.

Answer 1

The two statements will provide the same results.

The difference is in how the result is achieved. The second is slightly more efficient, as it does not need to generate an iterator for the Where method, and then get it's enumerator, and can instead directly enumerate the collection until a match is found.

Answer 2

TabCollection.Where(s => s.TabHeader == h).FirstOrDefault()

This creates WhereIterator and returns it. Then you starting iteration and return first element of it. That looks like

var iterator = new WhereEnumerableIterator<TSource>(TabCollection, predicate);

using (IEnumerator<TSource> enumerator = iterator.GetEnumerator())
{
   if (enumerator.MoveNext())
       return enumerator.Current;
}

return default(TSource);

Second one does not create iterator - it simply enumerates over source:

TabCollection.FirstOrDefault(s => s.TabHeader == h);

Same as

foreach (TSource local in TabCollection)
{
    if (predicate(local))        
        return local;        
}

return default(TSource);

So, second option is slightly more efficient.