simplify OleDbDataReader code

Question

I made a program, that reads data in the database I use the OleDbDataReader but the problem is I have different tables, this codes works perfectly but I found it a little bit "hardcoded" or recursive here is my sample code

        private void loadMilk()
        {
            cn.Open();
            OleDbDataReader reader = null;
            OleDbCommand cmd = new OleDbCommand("select* from Milk", cn);
            reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                Milk.Add(reader["Product"].ToString());
            }
            cn.Close();
        }

I need to repeat this again and again just to read what's on the other table (e.g., "select* from Fruit then "select* from Classics....) Is there any way so that I will not repeat this code again and again? thanks.:)

Answer 1

You can refactor that method into something like this:

private IList<string> Load(string tableName, string columnName)
{
    var result = new List<string>();
    cn.Open();
    OleDbDataReader reader = null;
    OleDbCommand cmd = new OleDbCommand(string.Format("select* from {0}", tableName), cn);
    reader = cmd.ExecuteReader();
    while (reader.Read())
    {
        result.Add(reader[columnName].ToString());
    }
    cn.Close();

    return result;
}

Your code sample will be:

var milkItems = Load("Milk", "Product");
var classicItems = Load("Classics", "..."); //Enter the column here.

Edit:

You might want something a little more specific (eg. storing a List<SomeObject> instead of just List<string>). Let's suppose you sometimes you want to return a list of Person, and also you want to read a list of Building. Then you can write something like this (not compiled & tested):

private IList<T> Load<T>(string tableName, Func<OleDbDataReader, T> selector)
{
    IList<T> result = new List<T>();
    cn.Open();
    OleDbDataReader reader = null;
    OleDbCommand cmd = new OleDbCommand(string.Format("select* from {0}", tableName), cn);
    reader = cmd.ExecuteReader();
    while (reader.Read())
    {
        result.Add(selector(reader));
    }
    cn.Close();

    return result;
}

and you can call it like:

Func<OleDbDataReader, Person> selector = x => new Person { Name = x["Person"].ToString() };
Load("People", selector);

Answer 2

private void loadMilk(string TableName, string itemValue)
        {
            string SQLString = String.Format("select * from {0}",TableName);

            cn.Open();
            OleDbDataReader reader = null;
            OleDbCommand cmd = new OleDbCommand(SQLString, cn);
            reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                Milk.Add(reader[ItemValue].ToString());
            }
            cn.Close();
        }

Answer 3

Not sure what type "Milk" is. Try:

private void loadObjectsFrom(string tableName, object obj, string column)
        {
            cn.Open();
            OleDbDataReader reader = null;
            OleDbCommand cmd = new OleDbCommand("select* from " + tableName, cn);
            reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                obj.Add(reader[column].ToString());
            }
            cn.Close();
        }

Answer 4

Just pass a table name as a parameter:

    private void loadMilk(string tableName)
    {
        cn.Open();
        OleDbDataReader reader = null;
        OleDbCommand cmd = new OleDbCommand(string.Format("select* from {0}",tableName), cn);
        reader = cmd.ExecuteReader();
        while (reader.Read())
        {
            Milk.Add(reader["Product"].ToString());
        }
        cn.Close();
    }