linq-Cast

源码：

public static IEnumerable<TResult> Cast<TResult>(this IEnumerable source) {
            IEnumerable<TResult> typedSource = source as IEnumerable<TResult>;
            if (typedSource != null) return typedSource;
            if (source == null) throw Error.ArgumentNull("source");
            return CastIterator<TResult>(source);
        }
 
static IEnumerable<TResult> CastIterator<TResult>(IEnumerable source) {
            foreach (object obj in source) yield return (TResult)obj;
        }

看到yield，这是一个转换成延时加载的Enumerable,return (TResult)obj;具体实现用的是强制类型转换。

,使用

// Custom class.
class Clump<T> : List<T>
{
　　// Custom implementation of Where().
　　public IEnumerable<T> Where(Func<T, bool> predicate)
　　{
　　　　Console.WriteLine("In Clump's implementation of Where().");
　　　　return Enumerable.Where(this, predicate);
　　}
}

 

Clump<string> fruitClump =
      new Clump<string> { "apple", "passionfruit", "banana",
            "mango", "orange", "blueberry", "grape", "strawberry" };
           //var t1 =  fruitClump.Where(f => f.Contains("o"));
           //var t2 = fruitClump.AsQueryable().Where(f => f.Contains("o"));
           //var t3 = fruitClump.AsEnumerable().Where(f => f.Contains("o"));

            for (int i = 0; i < fruitClump.Cast<string>().Count(); i++)
            {
                Console.WriteLine(fruitClump.Cast<string>().ToList()[i]);
            }