public static class Ext 
{
    public static IEnumerable<T> Where(this IEnumerable<T> source,Func<T,bool> predicate)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
        if (predicate == null)
        {
            throw new ArgumentNullException("predicate");
        }
        return WhereIterator(source,predicate);
    }

    private static IEnumerable<T> WhereIterator(IEnumerable<T> source,bool> predicate)
    {
        foreach (T current in source)
        {
            if (predicate(current))
            {
                yield return current;
            }
        }
    }
}

但是得到以下错误.我的System.dll已被默认包含在引用中.我可能做错了什么？

Error   1   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?) 

Error   2   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?) 

Error   3   The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?)

解决方法

public static IEnumerable<T> Where<T>(this IEnumerable<T> source,bool> predicate)

private static IEnumerable<T> WhereIterator<T>(IEnumerable<T> source,bool> predicate)

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