How to make components disappear when I select something from the list?

What I’m trying to do: When I select “PRODUSE” from the dropdownlist the following happens. “nume1” and “salariu” disappear / become inaccessible. The same goes for the other components in the list. What’s wrong with my if? Why doesn’t it work.

    if (conectare.State == ConnectionState.Open) {
        conectare.Close();
    }
    conectare.Open();
}

protected void Button1_Click(object sender, EventArgs e)//butonul de insert
{
    if (DropDownList2.SelectedValue == "1")
    {
        denumire1.Enabled = false;
        salariu.Enabled = false;

        SqlCommand cmd = conectare.CreateCommand();
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = "insert into PRODUSE values(' " + denumire.Text + " ',' " + anfabricatie.Text + " ')";
        cmd.ExecuteNonQuery();

        denumire.Text = "";
        anfabricatie.Text = "";

        display();
    }
    else if (DropDownList2.SelectedValue=="2") {
        denumire.Enabled = false;
        anfabricatie.Enabled = false;
            
        SqlCommand cmd = conectare.CreateCommand();
        cmd.CommandType = CommandType.Text;
        cmd.CommandText = "insert into FUNCTII values(' " + denumire1.Text + " ',' " + salariu.Text + " ')";
        cmd.ExecuteNonQuery();

        denumire1.Text = "";
        salariu.Text="";
       
        display2();
    }
}

public void display() {// AFISAM TABELA PRODUSE
    SqlCommand cmd = conectare.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "select * from PRODUSE";
    cmd.ExecuteNonQuery();

    DataTable dt = new DataTable();
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    da.Fill(dt);
    GridView1.DataSource = dt;
    GridView1.DataBind();
}

public void display2() // afisam tabela FUNCTII
{
    SqlCommand cmd = conectare.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "select * from FUNCTII";
    cmd.ExecuteNonQuery();

    DataTable dt = new DataTable();
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    da.Fill(dt);
    GridView1.DataSource = dt;
    GridView1.DataBind();
}

protected void Button2_Click(object sender, EventArgs e)
{
    SqlCommand cmd = conectare.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "delete from PRODUSE where Denumire=' " + denumire.Text + " '";
    cmd.ExecuteNonQuery();

    denumire.Text = "";
    display();
}

protected void Button3_Click(object sender, EventArgs e)
{
    SqlCommand cmd = conectare.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "update PRODUSE set Denumire='"+denumire.Text+"',AnFabricatie='"+anfabricatie.Text+"' where IdProdus=" +Convert.ToInt32(idvechi.Text) + " ";
    cmd.ExecuteNonQuery();

    denumire.Text = "";
    anfabricatie.Text = "";

    display();
}

protected void Button4_Click(object sender, EventArgs e)
{
    display();
}

-HTML-

<table>
            <tr>
                <td>Denumire</td>
                <td><asp:TextBox ID="denumire" runat="server"></asp:TextBox></td>
            </tr>
            <tr>
                <td>AnFabricatie</td>
                <td><asp:TextBox ID="anfabricatie" runat="server"></asp:TextBox></td>
            </tr>
             <tr>
                <td>Denumire</td>
                <td><asp:TextBox ID="denumire1" runat="server"></asp:TextBox></td>
            </tr>
            <tr>
                <td>Salariu</td>
                <td><asp:TextBox ID="salariu" runat="server"></asp:TextBox></td>
            </tr>
            <tr>
                <td colspan="2" align="center">
                    <asp:Button ID="Button1" runat="server" Text="Insert" OnClick="Button1_Click" />
                    <asp:Button ID="Button2" runat="server" Text="Delete" OnClick="Button2_Click" />
                    <asp:Button ID="Button3" runat="server" Text="Update" OnClick="Button3_Click" />
                    <asp:Button ID="Button4" runat="server" Text="View" OnClick="Button4_Click" />
                                            
                </td>
            </tr>
            <tr>
                <td>Id-ul pentru update</td>
                <td>
                    <asp:TextBox ID="idvechi" runat="server"></asp:TextBox>
                </td>
            </tr>
        </table>

        <asp:DropDownList ID="DropDownList2" runat="server" Height="16px" Width="253px" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged">
            <asp:ListItem ID="produse" Value="1">PRODUSE</asp:ListItem>
            <asp:ListItem ID="functii" Value="2">FUNCTII</asp:ListItem>
            <asp:ListItem ID="angajati" Value="3">ANGAJATI</asp:ListItem>
            <asp:ListItem ID="categorii" Value="4">CATEGORII_PROD</asp:ListItem>
            <asp:ListItem ID="comenzi" Value="5">COMEZNI</asp:ListItem>
            <asp:ListItem ID="clienti" Value="6">CLIENTI</asp:ListItem>
            <asp:ListItem ID="vanzari" Value="7">VANZARI</asp:ListItem>
        </asp:DropDownList>

        <br />
        <asp:GridView ID="GridView1" runat="server"></asp:GridView>

    </div>
</form>

Answers:

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Method 1

You need to add the AutoPostback="true" to your DropDownList2 and also add the appropriate code into DropDownList2_SelectedIndexChanged

By default the asp:DropDownList does not postback when changed see https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.listcontrol.autopostback?view=netframework-4.8

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