/// <summary>
/// 返回序列的唯一元素；如果该序列并非恰好包含一个元素，则会引发异常。
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <returns></returns>
public static T Single <T>(this Queryable <T> queryable)
{
if (queryable.OrderBy.IsNullOrEmpty())
{
queryable.OrderBy = "GETDATE()";
}
queryable.Skip(0);
queryable.Take(1);
return(queryable.ToList().Single());
}
/// <summary>
/// 返回序列中的第一个元素。
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <returns></returns>
public static T First <T>(this Queryable <T> queryable)
{
if (queryable.OrderBy.IsNullOrEmpty())
{
queryable.OrderBy = "GETDATE()";
}
queryable.Skip(0);
queryable.Take(1);
var reval = queryable.ToList();
return(reval.First());
}
/// <summary>
/// 返回序列的唯一元素；如果该序列并非恰好包含一个元素，否则返回null。
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <param name="expression"></param>
/// <returns></returns>
public static T SingleOrDefault <T>(this Queryable <T> queryable, Expression <Func <T, bool> > expression)
{
var type = queryable.Type;
queryable.WhereIndex = queryable.WhereIndex + 100;
ResolveExpress re = new ResolveExpress(queryable.WhereIndex);
re.ResolveExpression(re, expression);
queryable.Where.Add(re.SqlWhere);
queryable.Params.AddRange(re.Paras);
return(queryable.ToList().SingleOrDefault());
}
/// <summary>
/// 将Queryable转换为分页后的List《T》集合
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <param name="pageIndex">当前页码</param>
/// <param name="pageSize">每页显示数量</param>
/// <returns></returns>
public static List <T> ToPageList <T>(this Queryable <T> queryable, int pageIndex, int pageSize)
{
if (queryable.OrderBy.IsNullOrEmpty())
{
throw new Exception("分页必需使用.Order排序");
}
if (pageIndex == 0)
{
pageIndex = 1;
}
queryable.Skip = (pageIndex - 1) * pageSize;
queryable.Take = pageSize;
return(queryable.ToList());
}
/// <summary>
/// 返回序列中的第一个元素,如果序列为NULL返回default(T)
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="queryable"></param>
/// <returns></returns>
public static T FirstOrDefault <T>(this Queryable <T> queryable)
{
if (queryable.OrderBy.IsNullOrEmpty())
{
queryable.OrderBy = "GETDATE()";
}
queryable.Skip(0);
queryable.Take(1);
var reval = queryable.ToList();
if (reval == null || reval.Count == 0)
{
return(default(T));
}
return(reval.First());
}