Database SqlParameterCollection only accepts nonnull SqlParameter type objects, not String objects

我一直有例外

The SqlParameterCollection only accepts non-null SqlParameter type objects, not String objects

同时执行以下代码:

string StrQuery;
using (SqlConnection conn = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=SanFransiscoData;Integrated Security=True;Pooling=False"))
{
    using (SqlCommand comm = new SqlCommand())
    {
        comm.Connection = conn;
        conn.Open();
       // SqlParameter author = new SqlParameter("@author", dataGridView1.Rows[0].Cells[0].Value.ToString());
        comm.Parameters.Add("@author", SqlDbType.VarChar);
        comm.Parameters.Add("@title", SqlDbType.NVarChar);
        comm.Parameters.Add("@genre", SqlDbType.VarChar);
        comm.Parameters.Add("@price", SqlDbType.Float);
        comm.Parameters.Add("@publish_date", SqlDbType.Date);
        comm.Parameters.Add("@description", SqlDbType.NVarChar);
        comm.Parameters.Add("@bookid", SqlDbType.VarChar);
        for (int i = 0; i < dataGridView1.Rows.Count; i++)
        {
            StrQuery = "INSERT INTO BooksData VALUES(@author,@title,@genre,@price,@publish_date,@description,@bookid)";
            comm.Parameters.Add(dataGridView1.Rows[i].Cells[0].Value.ToString());
            comm.Parameters.Add(dataGridView1.Rows[i].Cells[1].Value.ToString());
            comm.Parameters.Add(dataGridView1.Rows[i].Cells[2].Value.ToString());
            comm.Parameters.Add(Convert.ToDecimal(dataGridView1.Rows[i].Cells[3].Value));
            comm.Parameters.Add(Convert.ToDateTime(dataGridView1.Rows[i].Cells[4].Value));
            comm.Parameters.Add(dataGridView1.Rows[i].Cells[5].Value.ToString());
            comm.Parameters.Add(dataGridView1.Rows[i].Cells[6].Value.ToString());
            comm.CommandText = StrQuery;
            comm.ExecuteNonQuery();
        }
    }
}

请告诉我我哪里出错了.