Question

private void button2_Click(object sender, EventArgs e)
{
    OleDbConnection cn = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;data source=c://library//lib.mdb");
    OleDbDataAdapter cmd = new OleDbDataAdapter("select * from entry where LTRIM(subjet)=?", cn);
    cmd.SelectCommand.Parameters.AddWithValue("1", comboBox1.Text);
    DataSet fg = new DataSet();
    cmd.Fill(fg);

    label1.Text = "No. of Rows:-    " + fg.Tables[0].Rows.Count.ToString();
    dataGridView1.DataSource = fg.Tables[0];
}

Answer 1

If this is a simple, small application then something like this would suffice…

   private void button2_Click(object sender, EventArgs e) 
   {
        DataRetriever dr = new DataRetriever();
        DataSet fg = dr.GetData(comboBox.Text);
        label1.Text = "No. of Rows:-    " + fg.Tables[0].Rows.Count.ToString();
        dataGridView1.DataSource = fg.Tables[0];
    }

public class DataRetriever
{
    public void GetData(string text)
    {
        OleDbConnection cn = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;data source=c://library//lib.mdb");

        OleDbDataAdapter cmd = new OleDbDataAdapter("select * from entry where LTRIM(subjet)=?", cn);
        cmd.SelectCommand.Parameters.AddWithValue("1", text);
        DataSet fg = new DataSet();
        cmd.Fill(fg);
   }
}

If this is a complex application that you will want to look into multi-tier application design.