表單被加載，但如何從SQL中提取數據到組合框？

Question

仍在學習C＃

創建一個組合框，並創建一個名爲mainCat和SUBCAT表。

我有一個代碼，但我堅持瞭解如何將數據從mainCat獲取到組合框，然後由另一個組合框用於子控件設置子類別。

的獲取連接的紅色下劃線。爲什麼？

這裏是我的代碼 -

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; 
    } 

} 

所以我然後我嘗試另一個代碼..但還是不工作..

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; 
     } 
} 

然後我就通過下拉框使用數據綁定項功能GUI ..

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

但是，該框沒有顯示任何值，除了「System.Data.DataRowView」在comboBo中X

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

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"]; 

} 

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

和組合框仍然沒有顯示從數據庫表

Answer 1

您需要創建功能GetConnection()什麼：

public string ConnectionString { get; set;} 

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

TBH，除非你想要做的事在GetConnection，你可能會更好只是創造它內聯：

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

編輯：

根據修改後的問題，我認爲現在的問題可能是在這裏：

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

我的猜測是，你的表結構不maincat.maincat。顯示和值成員應設置爲您希望顯示的字段名稱。

Answer 2

，我真的很抱歉我是一個VB開發人員，但概念是相同的，以數據填充到使用SQL組合體被

申報的SQLConnection聲明SqlDataReader的聲明的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 

希望它爲你工作。