Asp.net 从数据库中填充 DropDownList 的正确方法是什么

您可以将DropDownList绑定到数据源(DataTable、List、DataSet、SqlDataSource等).

例如，如果要使用DataTable:

ddlSubject.DataSource = subjectsTable;
ddlSubject.DataTextField = "SubjectNamne";
ddlSubject.DataValueField = "SubjectID";
ddlSubject.DataBind();

EDIT - More complete example

private void LoadSubjects()
{

    DataTable subjects = new DataTable();

    using (SqlConnection con = new SqlConnection(connectionString))
    {

        try
        {
            SqlDataAdapter adapter = new SqlDataAdapter("SELECT SubjectID, SubjectName FROM Students.dbo.Subjects", con);
            adapter.Fill(subjects);

            ddlSubject.DataSource = subjects;
            ddlSubject.DataTextField = "SubjectNamne";
            ddlSubject.DataValueField = "SubjectID";
            ddlSubject.DataBind();
        }
        catch (Exception ex)
        {
            // Handle the error
        }

    }

    // Add the initial item - you can add this even if the options from the
    // db were not successfully loaded
    ddlSubject.Items.Insert(0, new ListItem("<Select Subject>", "0"));

}

要通过标记(而不是代码隐藏)设置初始值，请指定选项并将Append数据边界Items属性设置为true:

<asp:DropDownList ID="ddlSubject" runat="server" AppendDataBoundItems="true">
    <asp:ListItem Text="<Select Subject>" Value="0" />
</asp:DropDownList>

然后，您可以将DropDownList绑定到代码隐藏中的数据源(只需记住删除:

ddlSubject.Items.Insert(0, new ListItem("<Select Subject>", "0"));

从代码隐藏，否则您将有两个""项.