ODS List是一组抽象类,它们为分页/排序的对象数据源实现过滤的select方法。我已经定义了三个absract类,它们代表过滤器,返回的数据以及产生这些结果的方法:
[Serializable]
public abstract class ListFilter
{
// Define filter properties
}
[Serializable]
public abstract class ListData
{
// Define properties to be returned from the query
}
public abstract class ListMethods
{
public int ListCount(ListFilter filter)
{
var rows = listQuery();
rows = listFilter(rows, filter);
return rows.Count();
}
/// <summary>
/// List returns a page of data
/// </summary>
/// <param name="filter"></param>
/// <param name="sortType"></param>
/// <param name="startRowIndex"></param>
/// <param name="maximumRows"></param>
/// <returns></returns>
public IEnumerable<ListData> List(ListFilter filter, string sortType, int startRowIndex, int maximumRows)
{
var rows = listQuery();
rows = listFilter(rows, filter);
rows = listSort(rows, sortType);
return rows.Distinct().Skip(startRowIndex).Take(maximumRows).ToList();
}
public abstract IQueryable<ListData> listQuery();
public virtual IQueryable<ListData> listFilter(IQueryable<ListData> rows, ListFilter filter)
{
return rows;
}
public virtual IQueryable<ListData> listSort(IQueryable<ListData> rows, string sortType)
{
bool sortDescending = false;
if (!string.IsNullOrEmpty(sortType))
{
string[] values = sortType.Split(' ');
sortType = values[0];
if (values.Length > 1)
{
sortDescending = values[1] == "DESC";
}
}
if (!string.IsNullOrEmpty(sortType))
{
if (sortDescending)
{
rows = rows.OrderBy(sortType + " DESC");
}
else
{
rows = rows.OrderBy(sortType);
}
}
return rows;
}
}
当我尝试将ListData强制转换为显式返回的数据时,我的实现遇到了问题。
[Serializable]
public class EmployeeData : ODSList.ListData
{
public int EmployeeId { get; set; }
public int? ReportsToId { get; set; }...
}
public override IQueryable<ListData> listQuery()
{
var dc = new NorthwindDataContext();
var allrows = from emp in dc.Employees
select new EmployeeData()
{
EmployeeId = emp.EmployeeID,
ReportsToId = emp.ReportsTo, ...
};
return (IQueryable<ListData>)allrows; <-- PROBLEM ENCOUNTERED HERE
}
我点击的诊断是:
无法转换类型为'System.Data.Linq.DataQuery 1[BusinessLayer.EmployeeData]' to type 'System.Linq.IQueryable
1 [ODSList.ListData]'的对象。
答案 0 :(得分:0)
你可以尝试
return allrows.AsQueryable();
我不确定,但你可能需要做
return allrows.Cast<ListData>().AsQueryable(); // if EmployeeData inherits from ListData; it is not required.
// but i'm not sure if ListData is the same as ODSList.ListData
答案 1 :(得分:0)
您使用的.Net版本是什么?如果您使用的是早于4.0的版本,则通用接口中指定的类型是不变的,这意味着您无法从IQueryable转换为IQueryable。
查看MSDN文章here。
编辑:经过一些实验并找到this SO post后,我发现以下内容对您有用:
public override IQueryable<ListData> listQuery()
{
var dc = new NorthwindDataContext();
var allrows = from emp in dc.Employees
select new EmployeeData()
{
EmployeeId = emp.EmployeeID,
ReportsToId = emp.ReportsTo, ...
} as ListData;
return allrows.OfType<ListData>();
}