Направление вида Linq от строки
Я реализовал некоторую очень похожую вещь. Реализация итераторов является самой сложной. Я использовал повышение:: iterator_facade для реализации итераторов. Используя boost::iterator_facade Вы можете легкий адаптироваться , любой кэшировался на дисковые структуры данных для имения интерфейса контейнера STL.
3 ответа
Ideally, you want to use OrderByDescending - you could of course cheat:
public static class MyExtensionMethods
{
public static IOrderedQueryable<TSource> OrderBy<TSource,TValue>(
this IQueryable<TSource> source,
Expression<Func<TSource,TValue>> selector,
bool asc)
{
return asc ? source.OrderBy(selector) : source.OrderByDescending(selector);
}
}
And use OrderBy passing in the selector and a bool?
If you don't need the static typing, you can also build the expressions dynamically from the ground up, of course - like this short sample (similar in nature to the dynamic LINQ library).
It'd be an if statement I think, no other simple way to do it, i.e.:
query = (SelectArgs.SortDirection == "asc") ? query.OrderBy(p => p.LastName)
: query.OrderByDescending(p => p.LastName);
Have a look at this as well: Sorting a list using Lambda/Linq to objects
Have a look at the CS Code Samples. There are a dynamic Linq examples.
From the samples:
Northwind db = new Northwind(connString);
db.Log = Console.Out;
var query =
db.Customers.Where("City == @0 and Orders.Count >= @1", "London", 10).
OrderBy("CompanyName").
Select("New(CompanyName as Name, Phone)");
Order by Code:
public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string ordering, params object[] values) {
return (IQueryable<T>)OrderBy((IQueryable)source, ordering, values);
}
public static IQueryable OrderBy(this IQueryable source, string ordering, params object[] values) {
if (source == null) throw new ArgumentNullException("source");
if (ordering == null) throw new ArgumentNullException("ordering");
ParameterExpression[] parameters = new ParameterExpression[] {
Expression.Parameter(source.ElementType, "") };
ExpressionParser parser = new ExpressionParser(parameters, ordering, values);
IEnumerable<DynamicOrdering> orderings = parser.ParseOrdering();
Expression queryExpr = source.Expression;
string methodAsc = "OrderBy";
string methodDesc = "OrderByDescending";
foreach (DynamicOrdering o in orderings) {
queryExpr = Expression.Call(
typeof(Queryable), o.Ascending ? methodAsc : methodDesc,
new Type[] { source.ElementType, o.Selector.Type },
queryExpr, Expression.Quote(Expression.Lambda(o.Selector, parameters)));
methodAsc = "ThenBy";
methodDesc = "ThenByDescending";
}
return source.Provider.CreateQuery(queryExpr);
}
But be sure that you check User Input!