Question

There is a table Customers with a nullable cstCredit column. The following works fine and can be iterated.

var query = _oContext.Customers.OrderBy(cst => cst.cstCredit);

However, the following will fail when iterating.

public IQueryable<Customer> AllSorted(Expression<Func<Customer, object>> orderby)
{
   return _oContext.Customers.OrderBy(orderby);
}

void test()
{
   var query = AllSorted(cst => cst.cstCredit);
   foreach (Customer oCustomer in query)
   {
   }
}

The message (translated from German) is

“The type System.Nullable can not be converted to System.Object. LINQ
to Entities only supports converting primitive or enumeration types”.

What have I done wrong?

Answer 1

Try writing the AllSorted method like this:

public IQueryable<Customer> AllSorted<TKey>(Expression<Func<Customer, TKey>> orderby)
{
   return _oContext.Customers.OrderBy(orderby);
}

It didn’t work because nullable types like int? are value types are do not derive from object ( and are instances of the System.Nullable struct) and hence Expression<Func<Customer, object>> orderby would not work with them.