加载了表单，但是如何将数据从SQL提取到组合框？

Question

Still learning C#

A comboBox is created and Tables called mainCat and subCat is created.

I have a code , but i am stuck to understand on how to get the data from mainCat to the comboBox , which is then used by another comboBox for the subCat to set a subcategory.

The Get Connection is underlined red. Why?

Here is my code -

System.Data.SqlServerCe.SqlCeConnection con;
System.Data.SqlServerCe.SqlCeDataAdapter da;
DataSet ds1;

private void Form2_Load(object sender, EventArgs e)
{
    SqlConnection conn = new SqlConnection();
    conn.ConnectionString = "Data Source=.\\SQLEXPRESS;Initial Catalog=master;Integrated Security=True";
    conn.Open();
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    using (SqlConnection Con = GetConnection())
    {
        SqlDataAdapter da = new SqlDataAdapter("Select Category.Category ,Category.Id from Category", Con);
        SqlCommand cmd = new SqlCommand("SELECT * from MAINCAT");
        DataTable dt = new DataTable();
        da.Fill(dt);
        mainCatU.DataSource = dt;
        mainCatU.DisplayMember = "Category";
        mainCatU.ValueMember = "Id";
        mainCatU.Text = "<-Please select Category->";
        myComboBox.DropDownStyle = ComboBoxStyle.DropDownList;
    }

}

So i then i tried another code.. but still doesnt work..

public partial class User : Form
{
        System.Data.SqlServerCe.SqlCeConnection con;
        System.Data.SqlServerCe.SqlCeDataAdapter da;
        DataSet ds1;

        private void User_Load(object sender, EventArgs e)
        {
            con = new System.Data.SqlServerCe.SqlCeConnection();
            con.ConnectionString = "Data Source=PEWPEWDIEPIE\\SQLEXPRESS;Integrated Security=True";
            con.Open();
            MessageBox.Show("Database connected");
            ds1 = new DataSet();
            string sql = "SELECT * from MAINCAT";
            da = new System.Data.SqlServerCe.SqlCeDataAdapter(sql, con);
            da.Fill(ds1, "SCSID");
            mainCatU.DataSource = ds1;
            con.Close();
            mainCatU.Text = "<-Please select Category->";
            mainCatU.DropDownStyle = ComboBoxStyle.DropDownList;
            mainCatU.Enabled = true;
        }
}

then i just used the data bound item function through the combobox GUI..

this.mAINCATTableAdapter.Fill(this.masterDataSet.MAINCAT);

but , the box didn't show any value , except "System.Data.DataRowView" in the comboBox

==================================================================================

System.Data.SqlServerCe.SqlCeConnection con; //not used at the moment
System.Data.SqlServerCe.SqlCeDataAdapter da; //not used at the moment
DataSet ds1;

private void User_Load(object sender, EventArgs e)
{

    SqlConnection conn = new SqlConnection();
    conn.ConnectionString = "Data Source=PEWPEWDIEPIE\\SQLEXPRESS;Initial Catalog=master;Integrated Security=True";
    conn.Open();
    MessageBox.Show("Database connected");

    SqlDataAdapter da = new SqlDataAdapter("SELECT * from MAINCAT", conn);
    ds1 = new DataSet();
    da.Fill(ds1, "MainCat");
    mainCatU.DisplayMember = "maincat";
    mainCatU.ValueMember = "maincat";
    mainCatU.DataSource = ds1.Tables["MAINCAT"];

}

===============

and the combo box is still not showing anything from the database table

Answer 1

You need to create the function GetConnection() :

public string ConnectionString { get; set;}

public SqlConnection GetConnection()
{
    SqlConnection cn = new SqlConnection(ConnectionString);
    return cn;
}

TBH, unless you want to do something in GetConnection , you might be better just creating it inline:

using (SqlConnection Con = new SqlConnection(ConnectionString))
{

EDIT:

Based on your revised question, I think the problem now may be here:

mainCatU.DisplayMember = "maincat";
mainCatU.ValueMember = "maincat";
mainCatU.DataSource = ds1.Tables["MAINCAT"];

My guess is that your table structure is not maincat.maincat. The display and value members should be set to the field name that you wish to display.

Answer 2

I am really sorry I am a vb developer but the concept is same to populate data into combo using sql is

Declare SQLConnection Declare SQLDataReader Declare SQLCommand

Try
    If Con.State = ConnectionState.Closed Then
        Con.Open()

    cmd.Connection = Con
    cmd.CommandText = "Select field1, field2 from table"


    dr = cmd.ExecuteReader()

    ' Fill a combo box with the datareader
    Do While dr.Read = True
        ComboBoxName.Items.Add(dr.GetString(0))
        ComboBoxName.Items.Add(dr.GetString(1))
    Loop

    Con.Close()
    End If

Catch ex As Exception
    MsgBox(ex.Message)

End Try

Hope it works for you.