Entity Framework 4.1: Unable to cast from DbQuery to ObjectQuery

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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.

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