Hey there, let's work through this issue clearly. That error message is straightforward—your DataList1 has both a DataSource property set in your code-behind and a DataSourceID linked to a data source control (like SqlDataSource) in your markup. ASP.NET strictly prohibits using both at the same time, since they're two different ways to bind data to the control. Let's break down two solid solutions, plus fix those "new errors" you mentioned when removing one of the definitions.

Option 1: Keep Using DataSourceID (Frontend Data Source Control) #

This is the simpler route if you're using a visual data source control like SqlDataSource.

Step 1: Remove Manual Data Binding Code #

First, delete any lines in your .cs file that set DataList1.DataSource or call DataList1.DataBind(). When using DataSourceID, ASP.NET handles data binding automatically—you don't need to do it manually.

Step 2: Configure DropDownList to Trigger Data Refresh #

Make sure your DropDownList is set to post back when its selection changes, and link it to your data source control's filter parameter:

• Add AutoPostBack="true" to your DropDownList markup (this fixes the "data doesn't update when selecting a new category" common error).
• Update your data source control to use the DropDownList's selected value as a filter parameter.

Example markup:

<!-- DropDownList for categories -->
<asp:DropDownList ID="DropDownList1" runat="server" 
    AutoPostBack="true"
    DataSourceID="SqlDataSource_Categories"
    DataTextField="CategoryName"
    DataValueField="CategoryID">
</asp:DropDownList>

<!-- Data source for category list -->
<asp:SqlDataSource ID="SqlDataSource_Categories" runat="server"
    ConnectionString="<%$ ConnectionStrings:YourDbConn %>"
    SelectCommand="SELECT CategoryID, CategoryName FROM Categories">
</asp:SqlDataSource>

<!-- Data source for filtered items (linked to DropDownList) -->
<asp:SqlDataSource ID="SqlDataSource_FilteredItems" runat="server"
    ConnectionString="<%$ ConnectionStrings:YourDbConn %>"
    SelectCommand="SELECT * FROM Items WHERE CategoryID = @SelectedCategory">
    <SelectParameters>
        <asp:ControlParameter 
            Name="SelectedCategory"
            ControlID="DropDownList1"
            PropertyName="SelectedValue" />
    </SelectParameters>
</asp:SqlDataSource>

<!-- DataList bound to filtered data source -->
<asp:DataList ID="DataList1" runat="server" DataSourceID="SqlDataSource_FilteredItems">
    <!-- Your ItemTemplate goes here -->
    <ItemTemplate>
        <h3><%# Eval("ItemName") %></h3>
        <p><%# Eval("ItemDescription") %></p>
    </ItemTemplate>
</asp:DataList>

Common Fix for New Errors Here #

If your DataList still doesn't update, double-check:

• AutoPostBack="true" is on the DropDownList (no postback = no data refresh).
• The ControlParameter in your data source matches the DropDownList's ControlID and PropertyName (case-sensitive!).

Option 2: Keep Using DataSource (Code-Behind Binding) #

Choose this if you need custom logic for filtering (e.g., complex queries, business rules).

Step 1: Remove DataSourceID from DataList Markup #

Delete the DataSourceID="..." attribute from your DataList in the frontend. Your DataList markup should look like this:

<asp:DataList ID="DataList1" runat="server">
    <!-- Your ItemTemplate -->
</asp:DataList>

Step 2: Set Up DropDownList Postback and Binding Logic #

1. Add AutoPostBack="true" to your DropDownList.
2. In your code-behind, handle the SelectedIndexChanged event to filter and bind data manually. Also, bind initial data on page load (only on the first load, not postbacks).

Example code-behind:

using System.Data;
using System.Data.SqlClient;
using System.Configuration;

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        // Bind initial category list to DropDownList
        BindCategoryDropDown();
        // Bind initial filtered data to DataList (default to first category)
        RefreshFilteredData(DropDownList1.SelectedValue);
    }
}

private void BindCategoryDropDown()
{
    string connString = ConfigurationManager.ConnectionStrings["YourDbConn"].ConnectionString;
    using (SqlConnection conn = new SqlConnection(connString))
    {
        string query = "SELECT CategoryID, CategoryName FROM Categories";
        SqlCommand cmd = new SqlCommand(query, conn);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        
        DropDownList1.DataSource = dt;
        DropDownList1.DataTextField = "CategoryName";
        DropDownList1.DataValueField = "CategoryID";
        DropDownList1.DataBind();
    }
}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    // Refresh DataList with selected category's data
    RefreshFilteredData(DropDownList1.SelectedValue);
}

private void RefreshFilteredData(string categoryId)
{
    string connString = ConfigurationManager.ConnectionStrings["YourDbConn"].ConnectionString;
    using (SqlConnection conn = new SqlConnection(connString))
    {
        string query = "SELECT * FROM Items WHERE CategoryID = @CategoryID";
        SqlCommand cmd = new SqlCommand(query, conn);
        cmd.Parameters.AddWithValue("@CategoryID", categoryId);
        
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        da.Fill(dt);
        
        DataList1.DataSource = dt;
        DataList1.DataBind();
    }
}

Common Fix for New Errors Here #

• If you get a "no data source" error: Ensure you're calling RefreshFilteredData in Page_Load (inside !IsPostBack) to bind initial data.
• If filtering doesn't work: Verify that DropDownList1.SelectedValue is passing the correct ID to your query (use debugging to check the value).

Which Option Should You Choose? #

• Go with Option 1 if you prefer visual, drag-and-drop data binding with minimal code.
• Go with Option 2 if you need flexibility for custom filtering, data transformation, or integration with business logic.

内容的提问来源于stack exchange，提问作者Adarsh