想实现，选择不同项时，datagridview显示不同内容

请问一下，我的表单，我想实现在下拉菜单选择不同项时，，一点查询按钮，dgv显示对应的内容，现在问题是，我选择下拉项时，内容似乎不变化，只显示第二项的内容，且和第一项部分内容，我的表单和代码如下，请问哪里的问题呢，请给我指导一下吧；

```c#
 private void btnsearch_Click(object sender, EventArgs e)
        {
            //连接数据库
            conn = new SqlConnection("server=192.168.100.247;database=Whmesinfo;user=sa;password=whyy@2021");

            conn.Open();

            //查询条件  
    
            if(this.comboBox1.SelectedIndex == 0)

            SqlString = "select * from w_Qctable1 where productcode like '%" + textBox2.Text + "%' and productname like '%" + textBox3.Text + "%' and batchNo like '%" + textBox1.Text + "%'  and [date] >= '" + dateTimePicker1.Value.ToString("yyyy-MM-dd HH:mm:ss") + "'and [date] <= '" + dateTimePicker2.Value.ToString("yyyy-MM-dd HH:mm:ss") + "' order by date asc ";
       
            //加载数据并显示

            try
            {
                //查询条件和SqlConnection连接
                SqlCommand cmd = new SqlCommand(SqlString, conn);
                //数据适配器
                SqlDataAdapter sda = new SqlDataAdapter();
                sda.SelectCommand = cmd;
               
                DataSet ds = new DataSet();

                sda.Fill(ds, "w_Qctable1");

                dt = ds.Tables["w_Qctable1"];

                dataGridView1.DataSource = dt.DefaultView;

                this.dataGridView1.DefaultCellStyle.WrapMode = DataGridViewTriState.True;//让查询的数据换行显示；

               //this.dataGridView1.AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.AllCells;//自动调整行的高度；

                dataGridView1.Columns[0].Visible = false;
                dataGridView1.Columns[1].HeaderText = "日期";
                dataGridView1.Columns[2].HeaderText = "产品编号";
                dataGridView1.Columns[3].HeaderText = "产品名称";
                dataGridView1.Columns[4].HeaderText = "批号";
                dataGridView1.Columns[5].HeaderText = "报告编号";
                dataGridView1.Columns[6].HeaderText = "工序";
                dataGridView1.Columns[7].HeaderText = "水分（％）";
                dataGridView1.Columns[8].HeaderText = "溶散时限（分钟)";
                dataGridView1.Columns[9].HeaderText = "鉴别";
                dataGridView1.Columns[10].HeaderText = "含量";
                dataGridView1.Columns[11].HeaderText = "粒度%";
                dataGridView1.Columns[12].HeaderText = "PH值";
                dataGridView1.Columns[13].HeaderText = "相对密度";
                dataGridView1.Columns[14].HeaderText = "乙醇量％";
                dataGridView1.Columns[15].HeaderText = "正丁醇提取物％";
                dataGridView1.Columns[16].HeaderText = "检验结论";
                dataGridView1.Columns[17].HeaderText = "备注";
               

            }
            catch
            { }
            finally
            {
                conn.Close();
            }

            if (this.comboBox1.SelectedIndex == 1)

                SqlString = "select * from w_Qctablew where productname like '%" + textBox3.Text + "%' and batchNo like '%" + textBox1.Text + "%'  and [sampledate] >= '" + dateTimePicker1.Value.ToString("yyyy-MM-dd HH:mm:ss") + "'and [sampledate] <= '" + dateTimePicker2.Value.ToString("yyyy-MM-dd HH:mm:ss") + "' order by sampledate asc ";

            //加载数据并显示

            try
            {
                //查询条件和SqlConnection连接
                SqlCommand cmd = new SqlCommand(SqlString, conn);
                //数据适配器
                SqlDataAdapter sda = new SqlDataAdapter();
                sda.SelectCommand = cmd;
                
                DataSet ds = new DataSet();

                sda.Fill(ds, "w_Qctablew");

                dt = ds.Tables["w_Qctablew"];

                dataGridView1.DataSource = dt.DefaultView;

                this.dataGridView1.DefaultCellStyle.WrapMode = DataGridViewTriState.True;//让查询的数据换行显示；

                dataGridView1.Columns[0].Visible = false;

                dataGridView1.Columns[1].HeaderText = "取样日期";
                dataGridView1.Columns[2].HeaderText = "报告日期";
                dataGridView1.Columns[3].HeaderText = "产品名称";
                dataGridView1.Columns[4].HeaderText = "批号";
                dataGridView1.Columns[5].HeaderText = "报告编号";
                dataGridView1.Columns[6].HeaderText = "工序";
                dataGridView1.Columns[7].HeaderText = "需氧菌cfu/g(ml)";
                dataGridView1.Columns[8].HeaderText = "霉菌及酵母菌cfu/g(ml)";
                dataGridView1.Columns[9].HeaderText = "大肠埃希菌/g(ml)";
                dataGridView1.Columns[10].HeaderText = "耐胆盐革兰阴性菌cfu/g(ml)";
                dataGridView1.Columns[11].HeaderText = "沙门菌/10g(ml)";
                dataGridView1.Columns[12].HeaderText = "检验结论";
                dataGridView1.Columns[13].HeaderText = "备注";
            
            }
            catch
            { }
            finally
            {
                conn.Close();
            }

        }

你if没有大括号，程序会顺序执行