ASP.NET：DropDownList中的DataValueField

Question

I'm trying to create two dropdownlist, the first one with a list of users, and the second one visible only when i click the OK button, showing a list of accounts for the user. The aspx page has the following code:

User:
<asp:SqlDataSource id="sourceUsers" runat="server" ConnectionString="<%$ ConnectionStrings:ct2012 %>"
    SelectCommand="SELECT name, id FROM users" />
<asp:DropDownList id="codUsers" runat="server" DataSourceID="sourceUsers"
    DataTextField="name" DataValueField="id" AutoPostBack="true" >
</asp:DropDownList>
<br />
Conto:
<asp:SqlDataSource id="sourceAccounts" runat="server" ConnectionString="<%$ ConnectionStrings:ct2012 %>" />
<asp:DropDownList id="accUsers" runat="server" DataSourceID="sourceAccounts"
    DataTextField="code" DataValueField="id" Visible="false" >
</asp:DropDownList>
<br />
<asp:Button ID="btOK" text="OK" OnClick="cmdNew_Click" runat="server" />

The c# code to handle the button event is:

protected void cmdNew_Click(object sender, EventArgs e)
    {
        sourceAccounts.SelectCommand = "SELECT code, id FROM accounts WHERE ID_user=" + codUsers.DataValueField;
        accUsers.Visible = true;
    }

The problem is that codUsers.DataValueField returns me the string "id" instead of the value of the index....

Answer 1

Use:

`codUsers.SelectedValue` instead of  `codUsers.DataValueField`

Or

codUsers.SelectedItem.Value

Or

codUsers.Items[codUsers.SelectedIndex].Value

Answer 2

Try

codUsers.SelectedValue;

This one should work for you.

Answer 3

try using codUsers.SelectedValue;

sourceAccounts.SelectCommand = "SELECT code, id FROM accounts WHERE ID_user=" + 
codUsers.SelectedValue;

You are setting yourself up for possible SQL Injection tho. (Maybe not in this particular case because you are getting a value that is pre-defined, but lets say this value would come from a textbox, you would be better of with parametrized queries:

 string myVariable = someTextBox.Text;
 string sqlString = "Select something from someTable where someField=@someVariable";
 oCmd.Parameters.AddWithValue("@someVariable", myVariable);

Answer 4

It should be .SelectedValue Property.

So the SelectCommand will be as:

sourceAccounts.SelectCommand = "SELECT code, id FROM accounts WHERE ID_user=" + 
                                codUsers.SelectedValue;

DataValueField will be used at the time of Assignment of Value field to the DropDownList.

and

DataTextField for Text Field.

Answer 5

codUsers.DataValueField is the string id , which you set here: DataValueField="id"

What you're looking for is codUsers.SelectedValue .

Please note also that using string concatenation for your SQL statements is a very bad idea. A quick Google search for "ADO .NET Parameterized Queries" turned up an old tutorial on the subject . It's been a while since I've used plain ADO.NET for data access, but I'm sure there's still a lot of information out there.

Basically you want to use the query parameter tools to build your query, not just string concatenation. (Both for SQL injection reasons and for performance reasons, as well as just being cleaner code in general.) Though I recommend taking it a step further and using something like LINQ to SQL or Entity Framework for your data access, which parameterizes for you behind the scenes and simply gives you an object model to work with instead of direct database access.

Answer 6

Use Only

codUsers.SelectedItem.Text

it will return as it is dropdownlist field data