你调用的对象是空的。 C＃和访问

Question

hello am working on back up access database using C#, i have four groups which is access configuration , database selection, database backup and database restore. so on data configuration i have data source textbox user id textbox and password textbox on database selection i have database combbox so that i can select one so i wrote this code

 public partial class Form11 : Form
{
    private OleDbConnection conn;
    private OleDbCommand command;
    private OleDbDataReader reader;
    string ole = "";
    string connectionString = "";


    public Form11()
    {
        InitializeComponent();
    }

    private void BtnConnect_Click(object sender, EventArgs e)
    {
        try
        {
            conn.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;" +
                                   @"Data Source = "+txtDataSource.Text+"; User Id="+txtUserId.Text+"; Password="+txtPassword.Text+"";
            conn = new OleDbConnection(connectionString);
            conn.Open();
            ole = "EXEC sp_databases";
            command = new OleDbCommand(ole, conn);
            reader = command.ExecuteReader();
            cmbDatabases.Items.Clear();
            while (reader.Read())
            {
                cmbDatabases.Items.Add(reader[0].ToString());
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }
}

the problem is i keep seeing object reference not set to an instance of an object, here i use access database but on SQL i didn't have such problem, please help me out with this thing. thank you.

Answer 1

Conn is an object and is not instantiated yet when you are using conn.ConnectionString property
just flip the order this 2 lines

  try
        {
            conn = new OleDbConnection();
            conn.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;" +
                                   @"Data Source = "+txtDataSource.Text+"; User Id="+txtUserId.Text+"; Password="+txtPassword.Text+"";

Answer 2

In your code, you are using your connection before instantiating your connection object. You need this first:

conn = new OleDbConnection();

The error should have referenced a line in your code that you were getting this problem - often relatively easy to track back from there to see what is null.