Entity Framework 4.1: Unable to cast from DbQuery to ObjectQuery

Question

I have the following code:

public void DeleteAccountsForMonth(int year, int month)
{
    var result = from acm in this._database.AccountsOnMonth
                     


        

Answer 1

That is because your _database is derived from DbContext and your AccountsOfMonth is DbSet<>. In such case you cannot use ObjectQuery directly because DbSet<> produces DbQuery<> which is not convertible to ObjectQuery<>.

You must either use DbQuery<> directly:

var result = from acm in this._database.AccountsOnMonth
             where ((acm.Year == year) && (acm.Month == month))
             select acm.Id;
var query = (DbQuery<int>)result;

string sql = string.Format(
    "DELETE FROM [AccountsOnMonth] WHERE [AccountsOnMonth].[Id] IN ({0})",
    query.ToString()
);

Or you must first convert your context to ObjectContext and create ObjectSet<>:

var objectContext = ((IObjectContextAdapter)_database).ObjectContext;
var set = objectContext.CreateObjectSet<AccountsOnMonth>();
var resut = from acm in set
            where ((acm.Year == year) && (acm.Month == month))
            select acm.Id;

The problem with first approach is that DbQuery doesn't offer Parameters collection - just another example of simplification in DbContext API which only makes it harder to use.

Answer 2

In my case I really needed the parameters and I found this workaround:

var query = (DbQuery<int>)result;

FieldInfo internalQueryField = query.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Instance).Where(f => f.Name.Equals("_internalQuery")).FirstOrDefault();
var internalQuery = internalQueryField.GetValue(query);
FieldInfo objectQueryField = internalQuery.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Instance).Where(f => f.Name.Equals("_objectQuery")).FirstOrDefault();
ObjectQuery<int> objectQuery = objectQueryField.GetValue(internalQuery) as ObjectQuery<int>;

foreach (ObjectParameter objectParam in objectQuery.Parameters)
{
    SqlParameter sqlParam = new SqlParameter(objectParam.Name, objectParam.Value);
    // Etc...
}

I'm implementing SqlCacheDependency with Entity Framework. :-)