When Using statement should be used?

Question

A lot of people on SO pointed out that I should use using statement more often. So I am practicing.

The problem is that I can't decide when to use it and when not to use it. When I think I should use it, then I get errors such as in this example (PS. HashPhrase is a class created by me):

        using (HashPhrase hash = new HashPhrase())
        {
            connection.ConnectionString =
                "Provider=Microsoft.ACE.OLEDB.12.0;" +
                "Data Source=" + filePath + ";" +
                "Persist Security Info=False;" +
                "Jet OLEDB:Database Password=" + hash.ShortHash(pass) + ";";
        }

But it gives me an error: 'Password_Manager.HashPhrase': type used in a using statement must be implicitly convertible to 'System.IDisposable'

But in this example it works fine:

    using (OleDbConnection connection = new OleDbConnection())
    {
        connection.ConnectionString =
            "Provider=Microsoft.ACE.OLEDB.12.0;" +
            "Data Source=" + filePath + ";" +
            "Persist Security Info=False;" +
            "Jet OLEDB:Database Password=" + hash.ShortHash(pass) + ";";

        using (OleDbCommand command = new OleDbCommand(sql, connection))
        {
            try
            {
                connection.Open();
                command.ExecuteNonQuery();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error: " + ex.Message);
            }
        }
    }

Are there any quick guidelines how to determine when using statement should be used?

Answer 1

Your question already touches on the answer.

If the type implements the IDisposable interface then you should aim to use using whenever possible (that is, always, unless there's a compelling reason not to).

And if the type doesn't implement IDisposable then you can't use using, and the compiler will tell you so, as you've already discovered.