public class QueryableSample : IQueryable
    {
        public Expression Expression { get; }
        public Type ElementType => typeof(ModelItem);
        public IQueryProvider Provider { get; }

        public IEnumerator GetEnumerator()
        {
            return Provider.Execute<IEnumerable>(Expression).GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public QueryableSample(IQueryProvider provider, Expression expression)
        {
            if (provider == null)
                throw new ArgumentNullException("provider");
            if (expression == null)
                throw new ArgumentNullException("expression");

            Provider = provider;
            Expression = expression;
        }
    }

    public class QueryProvider : IQueryProvider
    {
        public IQueryable CreateQuery(Expression expression)
        {
            return new QueryableSample(this, expression);
        }
        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            return (IQueryable<TElement>) new QueryableSample(this, expression);
        }
        public object Execute(Expression expression)
        {
            return QueryResult.Execute(expression, false);
        }
        public TResult Execute<TResult>(Expression expression)
        {
            bool IsEnumerable = (typeof(TResult).Name == "IEnumerable`1");
            return (TResult)QueryResult.Execute(expression, IsEnumerable);
        }
    }
    public sealed class QueryResult
    {
        public static object Execute(Expression expression, bool isEnumerable)
        { // 利用expression得到数据结果，设其为records
            QueryableSample records = null;
            if (isEnumerable)
                return records.Provider.CreateQuery(expression);
            else
                return records.Provider.Execute(expression);
        }
    }