mysql = System.Collections.Generic.List中的C＃结果

Question

in table "skupina" we have values "pc1,pc2,pc3,..." . But in this code I have result in mysql System.Collections.Generic.List , but should be "pc1,pc2,pc3,...." . Where is my mistake?

private void button1_Click(object sender, EventArgs e)
{
    using (MySqlConnection cnn = new MySqlConnection("Server=10.7.18.35;Database=OitDB;Uid=martin;Pwd=;"))
    {
        MySqlDataAdapter da = new MySqlDataAdapter("SELECT namepc FROM skupina where nazovskup= 'mojask' ", cnn); 
        DataSet ds = new DataSet();
        da.Fill(ds, "skupina");

        List<string> skName = new List<string>();
        foreach (DataRow row in ds.Tables["skupina"].Rows)
        {
            skName.Add(row["namepc"].ToString());

            string constring = "Server=10.7.18.35;Database=OitDB;Uid=martin;Pwd=;";
            var Query = "INSERT INTO OitDB.skup(uzivatel)VALUES('" + skName + "')";
            MySqlConnection conDatabase = new MySqlConnection(constring);
            MySqlCommand cmdDatabase = new MySqlCommand(Query, conDatabase);
            MySqlDataReader myReader;

            try
            {
                conDatabase.Open();
                myReader = cmdDatabase.ExecuteReader();
                MessageBox.Show("Správa odoslaná!");
                while (myReader.Read())
                {
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }           
    }
}

here is error

Answer 1

You're implicitly calling ToString() on a List<T> . That doesn't do what you expect it to, because List<T> doesn't override ToString() . You can use

// Note: the ToArray() call is only required if you're targeting .NET 3.5, which
// is massively out of date, and really shouldn't be used IMO.  
string.Join(", ", skName.ToArray())

to get a comma-separated result... but you really shouldn't do it where you're currently doing it. Instead, you should use parameterized SQL, eg

var query = "INSERT INTO OitDB.skup (uzivatel) VALUES (@name)";
...
cmdDatabase.Parameters.Add("@name", MySqlDbType.VarChar).Value
    = string.Join(", ", skName.ToArray());

Always, always use parameterized SQL for values. You should only ever build SQL dynamically for non-value parts - and in that case you need to be really, really careful.

I would also suggest using using directives for your command and data reader... and currently you've got two different connections open, which seems unnecessary...

Answer 2

I guess you are left with flattening your List to a string like:

 // Convert the ArrayList to a string
    string separator = "|"; // Any string that doesn't collide with content
    string arrayListAsString = String.Join(separator, skName.ToArray());

Then save the string in the database. When reading the string from the database you'll probably want to convert it back to an ArrayList:

// Convert the string back to an List
    ArrayList list = new ArrayList();
    list.AddRange(arrayListAsString.Split(new string[]{separator}, StringSplitOptions.RemoveEmptyEntries));