程序代码：

 错误提示在最下面一行，if语句执行没有问题。
private void button1_Click(object sender, EventArgs e)
        {
            OleDbConnection aConnection = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + "/" + "DB.mdb");
            aConnection.Open();
            string strName;
            strName = "insert into user(name,password,qx) values(" + textBox1.Text + ",'" + textBox2.Text + "'," + textBox3.Text + ")";
            if (textBox1.Text == "" | textBox2.Text == "" | textBox3.Text == "" )
            {
                MessageBox.Show("请填写要添加的数据");
                return;
            }
            OleDbDataAdapter myadapter = new OleDbDataAdapter(strName, aConnection); //执行查询
            DataSet ds = new DataSet(); //新建数据集
            myadapter.Fill(ds);//将SQL执行结果，填入数据集ds中可自加数据，进行验证
        }