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
                 where ((acm.Year == year) && (acm.Month == month))
                 select acm.Id;
    var query = (ObjectQuery<int>)result;

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

    var parameters = new List<System.Data.SqlClient.SqlParameter>();
    foreach (ObjectParameter parameter in query.Parameters)
    {
        parameters.Add(new System.Data.SqlClient.SqlParameter {
            ParameterName = parameter.Name,
            Value = parameter.Value
        });
    }

    this._database.Database.ExecuteSqlCommand(sql, parameters.ToArray());
}

Basically, what I'm trying to do is to delete a bulk of data from a context (get a query result, get SQL and execute it). But I'm having a problem when casting result to ObjectQuery. The exception that gives is

Unable to cast object of type 'System.Data.Entity.Infrastructure.DbQuery1[System.Int32]' to type 'System.Data.Objects.ObjectQuery1[System.Int32]'.

Can anybody give any hint to solve this? Thanks!

EDIT: Ladislav first solution helped me solve the problem, but it happenned a little problem with the SQL parameters of the generated SQL query, i.e. the SQL query generated by query.ToString() was this:

DELETE FROM [SncAccountOnMonths] WHERE [SncAccountOnMonths].[Id] IN (
    SELECT [Extent1].[Id] AS [Id]
    FROM [dbo].[SncAccountOnMonths] AS [Extent1]
    WHERE ([Extent1].[Year] = @p__linq__0) AND ([Extent1].[Month] = @p__linq__1))

The problem was that the variables @p__linq__0 and @p__linq__1 where not declared and so the query gave the error "Must declare the scalar variable @p_linq_0" (I sure it would give the same error for variable @p__linq__1). To "declare" them I need to pass them as arguments of the ExecuteSqlCommand(). And so, the final solution for the initial answer is the code below:

public void DeleteAccountsForMonth(int year, int month)
{
    var result = (this._database.AccountsOnMonth
        .Where(acm => (acm.Year == year) && (acm.Month == month)))
        .Select(acm => acm.Id);
    var query = (DbQuery<int>)result;

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

    this._database.Database.ExecuteSqlCommand(sql,
        new SqlParameter("p__linq__0", year),
        new SqlParameter("p__linq__1", month)
    );
}

By the way, I assume the variables generated always have the format @p__linq__, unless Microsoft's Entity Framework Team changes it in any future EF update...

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. :-)