从Linq扩展方法中的表达式获取价值

Question

I searched the whole internet and tried many things, but I am not able to get the value from my expression. It would be very cool if anybody could help me...

bye Markus

public static class LinqExtension
{
    public static IQueryable<T> GetFilteredByStatusList<T>(this IQueryable<T> source, Expression<Func<T, int>> expression)
    {
        // So I can compile the expression.
        // In some posts I have found, that I have to call the compiled method, but the method needs the T object.
        // I have no idea how to acces the value of T.
        Func<T, int> method = expression.Compile();
            //EDIT
            // Here I need the int value to pass it in a service method like:
            // Service.GetStatusById("int value from expression");
            //EDIT END
        return source;
    }
}

-- EDIT I have a query and in this query I have to call a method which needs a dynamic value from the current query item. This method already exists and I get this to work after a the query with a for loop through the query and call this method on every item in this loop. But I think this is not a very fast solution.

So I to call this method inside the query and that is why I try to implement this with an extension method.

Following the extension method call:

return query = query
            .Join(entities.tblTaskgroupGlobal, x => x.lngAssignMain_id, y => y.id, (x, y) => new { x = x, y = y })
            .WhereIf(taskFilterModel.StatusFilterList.Count() > 0, xy => taskFilterModel.StatusFilterList.Contains(xy.y.lngConstantStatus_id))
            .GetFilteredByStatusList(xy => xy.x.lngAssignMain_id)
            .Select(xy => xy.x);

-- EDIT END

Answer 1

Wow, the whole internet! That must have taken a while! :)

The expression you have compiled is now expecting to take an object of type T and return a value of type int.

I would imagine that you want to enumerate source and apply your method to it.

For example:

foreach (T item in source)
{ 
    yield return method(item);
}

But I think the better question is - how are you planning to use this method? Are you sure that an Expression is what you need?

Answer 2

The simplest solution would be:

return source.Select(expression);

But then again, if that's what you REALLY wanted to do you wouldn't need to write your own GetFilteredByStatusList at all.