EFore 分页查询链式封装

此文提供一个初步的封装思路，简化代码编写。

1. 先封装一个 ResultSet 存放查询结果：

    public class ResultSet<T>
    {
        /// <summary>
        /// 总记录数
        /// </summary>
        public int Total { get; set; }

        public List<T> Value { get; set; }
    }

2. 封装三个扩展方法： WhereIf, IfNotnull,PageQuery

public static class ExtendMethods
{
    public static IQueryable<T> WhereIf<T>(this IQueryable<T> source,bool condition, Expression<Func<T, bool>> predicate) 
    {
        if (condition)
        {
            source = source.Where(predicate);
        }
        return source;
    }
    public static IQueryable<T> IfNotNull<T>(this IQueryable<T> source, string condition, Expression<Func<T, bool>> predicate)
    {
        if (!string.IsNullOrEmpty(condition))
        {
            source = source.Where(predicate);
        }
        return source;
    }
    public static ResultSet<T> PageQuery<T>(this IQueryable<T> source, int pageSize,int page)
    {
        var count  =source.Count();
        var data  = source.Skip((page - 1) * pageSize).Take(pageSize).ToList();
        return new ResultSet<T>()
        {
            Total = count,
            Value = data
        };
    }
}

　　3. 使用示例：

public ResultSet<Users> GetData(int pageSize, int page, string userName, string keyWords = "")
      {
          return _context.Set<Users>
              .IfNotNull(userName, u => u.UserName == userName)
              .WhereIf(!string.IsNullOrWhiteSpace(keyWords), u => u.Name.Contains(keyWords))
              .PageQuery(pageSize, page);
      }