C# objects and Postgresql

Hi, this is my first post, I'm pretty new to all this stuff, infact I haven't really tried programming in 4 years so please excuse if this is easy because for me it's stupid hard .... anyway

I'm having a problem, I'm using PostgreSQL V8.3.3-1, Npgsql and Visual studio 2008 c#.

I'm passed a dictionary of key, value (key been a string, value been an object). The code works fine with key and value both been strings but postgresql doesn't seem to like it when the value is an object. How can I correct my code to work when passed an object? I realise that rowsaffected is currently only storing the first value, that's fine. I just want to be able to run a query using an object datatype for now.
I feel DbType.Object might be the problem.

current error is: "InvalidCastException is unhandled by user code"

sql is a query

"SELECT username FROM users WHERE age = :Value1"

Code:

public static string GetSql(string sql, Dictionary<string, object> d)
        {
            string connection = System.Configuration.ConfigurationManager.Connecti  onStrings["POSTGRESQL_LOCAL"].ConnectionString;
            
            using (NpgsqlConnection conn = new NpgsqlConnection(connection))
            {
                conn.Open();
                using (NpgsqlCommand command = new NpgsqlCommand(sql, conn))
                {
                    int i = 0;
 
                    foreach (KeyValuePair<string, object> kvp in d)
                    {
                        command.Parameters.Add(new NpgsqlParameter(kvp.Key, DbType.Object));
                        command.Parameters[i].Value = kvp.Value;
                        i++;
                    }
                    string rowsaffected = "error: No data";
                    using (NpgsqlDataReader dr = command.ExecuteReader())
                    {
                        while (dr.Read())
                        {
                            rowsaffected = dr[0].ToString();
                        }
                    }
 
                    conn.Close();
                    return rowsaffected;
 
                }
 
            }
            
        }

I hope I placed this on the correct forum.

Thanks

 

For clarity, when I say both key and value are strings I mean I change the datatype to string for the value as well.