ConnectionString 属性尚未初始化
public partial class User : Form
  {

   
  public User()
  {
  InitializeComponent();
   
  }

  public DataSet ds = new DataSet();
  SqlDataAdapter sda1 = null;

  string username;
  string userpsw;
  string userlevel;



  //编辑现有用户
  private void BT_edit_Click(object sender, EventArgs e)
  {
   
  sda1.Update(ds, "a");
  Databind();
  }


  //删除所选用户
  private void BT_delete_Click(object sender, EventArgs e)
  {
   
  }

  //重载控件
  private void BT_refresh_Click(object sender, EventArgs e)
  {
  // 刷新datagridview 把当前的form重新加载
  User_Load(sender, e);

  }

   

  //数据绑定方法

   


  public void Databind()
  {
  SqlConnection scon1 = new SqlConnection("Data Source=acer-PC;Initial Catalog=cmis;Integrated Security=True;Pooling=False");
  string sqlconnet1 = "select [username] '名称',[userpsw] '密码',[userlevel] '等级' from [users]";
  SqlCommand scmd1 = new SqlCommand(sqlconnet1, scon1);
  sda1 = new SqlDataAdapter(scmd1);
  sda1.SelectCommand = scmd1;
  SqlCommandBuilder scb = new SqlCommandBuilder(sda1);
   
  sda1.InsertCommand = scb.GetInsertCommand();
  sda1.DeleteCommand = scb.GetDeleteCommand();
  sda1.UpdateCommand = scb.GetUpdateCommand();

  try
  {
  scon1.Open();
  ds.Clear();
  sda1.Fill(ds, "a");
  this.DATA_user.DataSource = ds.Tables[0];

  }
  catch (Exception ex)
  {
  MessageBox.Show("数据库异常");
  }
  finally
  {
  scon1.Close();
  scon1.Dispose();
  scmd1.Dispose();
  sda1.Dispose();

  }
  }

  private void User_Load(object sender, EventArgs e)
  {
  Databind();
  }

------解决方案--------------------

C# code

SqlCommand scmd1 = new SqlCommand(sqlconnet1);

//下面行添加 ,scon1
  sda1 = new SqlDataAdapter(scmd1,scon1);
  sda1.SelectCommand = scmd1;
  SqlCommandBuilder scb = new SqlCommandBuilder(sda1);

------解决方案--------------------

ConnectionString配置不正确。