Problem with Generic Linq OrderBy function

问题:

I saw the following function in a posting which allows one to order data using a generic expression:

public static IOrderedQueryable<T> OrderBy<T, TKey>(
  this IQueryable<T> source, Expression<Func<T, TKey>> func, bool isDescending) {
  return isDescending ? source.OrderByDescending(func) : source.OrderBy(func);
}

When I try to use this function I get an error saying "The type or namespace name "TKey' could not be found (are you missing a using directive or an assembly reference?)". I'm doing something dumb here but I can't figure it out.

Edit:

After doing a bit more research, I think my problem is in building the Expression that I pass into it. Is it possible to build an expression that can contain different types? Let's say my dataset has a string, an int, and a bool and I want to use the generic function above to sort by any of the items. How do I do this?

I have this working now:

if (IsString)
{
   Expression<Func<T, string>> expString = ...;
   // call orderBy with expString
}
else if (IsInt)
{
   Expression<Func<T, int>> expInt;
   // call orderBy w/ expInt
}
:

I want something like:

Expression<Func<T, {something generic!}>> exp;
if (IsString)
    exp = ...;
else if (IsInt)
    exp = ...;
:
// call orderBy with exp

回答1:

One quick observation: You don't really need to use a lambda expression (Expression<Func<T,TKey>>). A simple delegate (Func<T,TKey>) is fine.

That said, I think the answer you might be looking for is this:

Func<T,IComparable> func = null;
if (IsString)
    func = (T a) => a.SomeStringValue;
else if (IsInt)
    func = (T a) => a.SomeIntValue;
// call orderBy with exp

回答2:

My goal in this was to eliminate a lot of repetitious code. In addition to handling the ascending/descending my "OrderBy" function handles some other common logic has well. Assuming the function definition in the original posting, one can simply do this:

if ( {need to sort by integer})
    query = OrderBy(objectT, a => a.myIntegerField, asc);
else if ( {need to sort by string})
    query = OrderBy(objectT, a=> a.myStringField, asc);
:

回答3:

The expression can only have one type; my preferred answer here would be something like:

IQueryable<T> query = ...
if({case 1}) {
    query = query.OrderBy(x=>x.SomeValue);
} else if({case 2}) {
    query = query.OrderBy(x=>x.SomeOtherValue);
} ...

However, if you want to do something more flexible, you'd probably need to get into custom Expression writing; something more like this.

回答4:

I think what you may be looking for is the ability to have a dynamic orderby clause within linq. For some good articles on the topic see

http://blogs.msdn.com/swiss_dpe_team/archive/2008/06/05/composable-linq-to-sql-query-with-dynamic-orderby.aspx

or

http://www.equivalence.co.uk/archives/819

or

http://www.rocksthoughts.com/blog/archive/2008/01/24/linq-to-sql-dynamic-queries.aspx

回答5:

Take a look at this answer

My generic handler for sorting is:

• "dgvProcessList" is my dataGridView
• "Process" is my object binded to it

• "e" is my DataGridViewCellMouseEventArgs

              PropertyInfo column = (new Process()).GetType().GetProperties().Where(x => x.Name == dgvProcessList.Columns[e.ColumnIndex].Name).First();
          if (isSortedASC == true)
              dgvProcessList.DataSource = ((List<Process>)dgvProcessList.DataSource).OrderByDescending(x => column.GetValue(x, null)).ToList();
          else
              dgvProcessList.DataSource = ((List<Process>)dgvProcessList.DataSource).OrderBy(x => column.GetValue(x, null)).ToList();
  
          isSortedASC = !isSortedASC;
          dgvProcessList.ClearSelection();
  

Cheers