Insert item to DropDownList from Database based on another two DropDownList using C# and VB.Net in ASP.Net

Hi micah,

I have created sample that full-fill your requirement.

HTML

<table border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td>
            Inches
        </td>
        <td>
            <asp:DropDownList ID="ddlInches" runat="server" OnSelectedIndexChanged="SelectedIndexChanged"
                AutoPostBack="true" Width="150px">
                <asp:ListItem Text="Select" Value="Select" />
                <asp:ListItem Text="5inches" Value="5inches" />
                <asp:ListItem Text="6inches" Value="6inches" />
                <asp:ListItem Text="9inches" Value="9inches" />
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            Category
        </td>
        <td>
            <asp:DropDownList ID="ddlCategory" runat="server" OnSelectedIndexChanged="SelectedIndexChanged"
                AutoPostBack="true" Width="150px">
                <asp:ListItem Text="Select" Value="Select" />
                <asp:ListItem Text="Machin mold" Value="Machin mold" />
                <asp:ListItem Text="Hand mold" Value="Hand mold" />
                <asp:ListItem Text="Hollow" Value="Hollow" />
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            Item
        </td>
        <td>
            <asp:DropDownList ID="ddlItem" runat="server" OnSelectedIndexChanged="SelectedIndexChanged"
                AutoPostBack="true" Width="150px">
                <asp:ListItem Text="Select" Value="Select" />
                <asp:ListItem Text="block" Value="block" />
                <asp:ListItem Text="Interlock" Value="Interlock" />
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>
            Quantity
        </td>
        <td>
            <asp:DropDownList ID="ddlQuantity" runat="server" Width="150px">
            </asp:DropDownList>
        </td>
    </tr>
</table>

C#

protected void SelectedIndexChanged(object sender, EventArgs e)
{
    ddlQuantity.Items.Clear();
    if (ddlInches.SelectedIndex > 0 && ddlCategory.SelectedIndex > 0 && ddlItem.SelectedIndex > 0)
    {
        string inches = ddlInches.SelectedItem.Text;
        string category = ddlCategory.SelectedItem.Text;
        string item = ddlItem.SelectedItem.Text;
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[6] { 
            new DataColumn("ID"), new DataColumn("UserName"), new DataColumn("Inches"), 
            new DataColumn("Category"), new DataColumn("Item"), new DataColumn("qty") });
        dt.Rows.Add(1, "mic33", "5inches", "Machin mold", "block", "30");
        dt.Rows.Add(2, "mic33", "6inches", "Hand mold", "Interlock", "70");
        dt.Rows.Add(3, "mic33", "9inches", "Hollow", "block", "120");
        DataRow[] dr = dt.Select("Inches = '" + inches + "' AND Category = '" + category + "' AND Item = '" + item + "'");
        if (dr.Length > 0)
        {
            string quantity = dr[0].ItemArray[5].ToString();
            ddlQuantity.Items.Insert(0, new ListItem("Select", "Select"));
            ddlQuantity.Items.Insert(1, new ListItem(quantity, quantity));
            ddlQuantity.Items.FindByText(quantity).Selected = true;
        }
    }
}

VB.Net

Protected Sub SelectedIndexChanged(sender As Object, e As EventArgs)
    ddlQuantity.Items.Clear()
    If ddlInches.SelectedIndex > 0 AndAlso ddlCategory.SelectedIndex > 0 AndAlso ddlItem.SelectedIndex > 0 Then
        Dim inches As String = ddlInches.SelectedItem.Text
        Dim category As String = ddlCategory.SelectedItem.Text
        Dim item As String = ddlItem.SelectedItem.Text
        Dim dt As New DataTable()
        dt.Columns.AddRange(New DataColumn(5) {New DataColumn("ID"), New DataColumn("UserName"), New DataColumn("Inches"), New DataColumn("Category"), New DataColumn("Item"), New DataColumn("qty")})
        dt.Rows.Add(1, "mic33", "5inches", "Machin mold", "block", "30")
        dt.Rows.Add(2, "mic33", "6inches", "Hand mold", "Interlock", "70")
        dt.Rows.Add(3, "mic33", "9inches", "Hollow", "block", "120")
        Dim dr As DataRow() = dt.[Select]((Convert.ToString((Convert.ToString((Convert.ToString("Inches = '") & inches) + "' AND Category = '") & category) + "' AND Item = '") & item) + "'")
        If dr.Length > 0 Then
            Dim quantity As String = dr(0).ItemArray(5).ToString()
            ddlQuantity.Items.Insert(0, New ListItem("Select", "Select"))
            ddlQuantity.Items.Insert(1, New ListItem(quantity, quantity))
            ddlQuantity.Items.FindByText(quantity).Selected = True
        End If
    End If
End Sub

Screenshot