In this article I will explain with an example, how to populate Cascading DropDownList from SQL Server in ASP.Net using C# and VB.Net.
Note: Cascading DropDownLists are series of DropDownList where the ListItem of each DropDownList is depend on the selection made in the previous one.
 
 

Database

I have made use of the following three tables Countries, States and Cities with the schema as follows.

Countries Table

Populate Cascading DropDownList from Database in ASP.Net Example
 

States Table

Populate Cascading DropDownList from Database in ASP.Net Example
 

Cities Table

Populate Cascading DropDownList from Database in ASP.Net Example
 
Note: You can download the database table SQL by clicking the download link below.
           Download SQL file
 
 

HTML Markup

The HTML Markup consists of following controls:
DropDownList – For selecting data from database in DropDownList.

Properties

The DropDownList have been assigned with the AutoPostBack property set to TRUE.

Event Handlers

The DropDownList have been assigned with an OnSelectedIndexChanged event handler.
<table>
    <tr>
        <td>Country:</td>
        <td>
            <asp:DropDownList ID="ddlCountries" runat="server" AutoPostBack="true" OnSelectedIndexChanged="Country_Changed"></asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>State:</td>
        <td>
            <asp:DropDownList ID="ddlStates" runat="server" AutoPostBack="true" OnSelectedIndexChanged="State_Changed"></asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>City:</td>
        <td>
            <asp:DropDownList ID="ddlCities" runat="server" AutoPostBack="true" OnSelectedIndexChanged="City_Changed"></asp:DropDownList>
        </td>
    </tr>
</table>
 
 

Populating the DropDownList from Database using C# and VB.Net

The BindDropDownList method will be called to populate DropDownList based on selection of the current DropDownList.
This method accepts the reference of the DropDownList, the SQL Query, and the name of the Columns to be set in DataTextField and DataValueField properties of DropDownList respectively and a DefaultText.
Inside this method, the accepted SQL query is passed as parameter to SqlCommand class and using its ExecuteReader method, the records are fetched from the SQL Server database.
Note: For more details on ExecuteReader method, please refer my article Using SQLCommand ExecuteReader Example in ASP.Net with C# and VB.Net in ASP.Net with C# and VB.Net.
 
Finally, the referenced DropDownList will be populated and the DefaultText is set to the 0th position of DropDownList.
C#
private void BindDropDownList(DropDownList ddl, string query, string text, string value, string defaultText)
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                con.Open();
                ddl.DataSource = cmd.ExecuteReader();
                ddl.DataTextField = text;
                ddl.DataValueField = value;
                ddl.DataBind();
                con.Close();
            }
        }
    }
    ddl.Items.Insert(0, new ListItem(defaultText, "0"));
}
 
VB.Net
Private Sub BindDropDownList(ByVal ddl As DropDownList, ByVal query As String, ByVal text As String, ByVal value As String, ByVal defaultText As String)
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(constr)
        Using cmd As SqlCommand = New SqlCommand(query)
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                con.Open()
                ddl.DataSource = cmd.ExecuteReader()
                ddl.DataTextField = text
                ddl.DataValueField = value
                ddl.DataBind()
                con.Close()
            End Using
        End Using
    End Using
    ddl.Items.Insert(0, New ListItem(defaultText, "0"))
End Sub
 
 

Populating the Country DropDownList from Database

Inside the Page Load event handler, the DropDownList for displaying Country names is populated by making call to the BindDropDownList generic method (explained earlier).
Then, the Enabled property of DropDownList for displaying State and City names is set to FALSE and DefaultText is also set.
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        string  query = "SELECT CountryId,CountryName FROM Countries";
        this.BindDropDownList(ddlCountries, query, "CountryName", "CountryId", "Select Country");
        ddlStates.Enabled = false;
        ddlCities.Enabled = false;
        ddlStates.Items.Insert(0, new ListItem("Select State", "0"));
        ddlCities.Items.Insert(0, new ListItem("Select City", "0"));
    }
}
 
VB.Net
Protected Sub Page_Load(ByVal sender As Object, ByVal  e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim query As String = "SELECT CountryId,CountryName FROM Countries"
        Me.BindDropDownList(ddlCountries, query, "CountryName", "CountryId", "Select Country")
        ddlStates.Enabled = False
        ddlCities.Enabled = False
        ddlStates.Items.Insert(0, New ListItem("Select State", "0"))
        ddlCities.Items.Insert(0, New ListItem("Select City", "0"))
    End If
End Sub
 
 

Populating the State DropDownList from Database

When selection of Country DropDownList is changed, the value of the CountryId is fetched and SELECT query along with the ID of the DropDownList for displaying State and required details are passed as parameter to BindDropDownList method (explained earlier).
Finally, the Enabled property of the DropDownList for displaying State names is set to TRUE.
C#
protected void Country_Changed(object sender, EventArgs e)
{
    int countryId = int.Parse(ddlCountries.SelectedItem.Value);
    if (countryId > 0)
    {
        string query = string.Format("SELECT StateId,StateName FROM States WHERECountryId = {0}", countryId);
        this.BindDropDownList(ddlStates, query, "StateName", "StateId", "Select State");
        ddlStates.Enabled = true;
    }
}
 
VB.Net
Protected Sub Country_Changed(ByVal sender As Object, ByVal e As EventArgs)
    Dim countryId As Integer = Integer.Parse(ddlCountries.SelectedItem.Value)
    If countryId > 0 Then
        Dim query As String = String.Format("SELECT StateId, StateName FROM States WHERE CountryId = {0}", countryId)
        Me.BindDropDownList(ddlStates, query, "StateName", "StateId", "Select State")
        ddlStates.Enabled = True
    End If
End Sub
 
 

Populating the City DropDownList from Database

When selection of State, the value of the StateId is fetched and SELECT query along with the ID of the DropDownList for displaying City and required details are passed as parameter to BindDropDownList method (explained earlier).
Finally, the Enabled property of the DropDownList for displaying City names is set to TRUE.
C#
protected void State_Changed(object sender, EventArgs e)
{
    int stateId = int.Parse(ddlStates.SelectedItem.Value);
    if (stateId > 0)
    {
        string query = string.Format("SELECT CityId, CityName FROM Cities WHEREStateId = {0}", stateId);
        this.BindDropDownList(ddlCities, query, "CityName", "CityId", "Select City");
        ddlCities.Enabled = true;
    }
}
 
VB.Net
Protected Sub State_Changed(ByVal sender As Object, ByVal e As EventArgs)
    Dim stateId As Integer = Integer.Parse(ddlStates.SelectedItem.Value)
    If stateId > 0 Then
        Dim query As String = String.Format("SELECT CityId, CityName FROM Cities WHEREStateId = {0}", stateId)
        Me.BindDropDownList(ddlCities, query, "CityName", "CityId", "Select City")
        ddlCities.Enabled = True
    End If
End Sub
 
 

Displaying Selected Item of DropDownLists

When selection of City DropDownList is changed, the selected item of each DropDownList is displayed in JavaScript Alert Message Box using RegisterStartupScript method.
Finally, the Enabled property of DropDownList for displaying State and City names is set to FALSE.
C#
protected void City_Changed(object sender, EventArgs e)
{
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Country: " + ddlCountries.SelectedItem.Text +
                                                       "\\nState: " + ddlStates.SelectedItem.Text +
                                                       "\\nCity: " + ddlCities.SelectedItem.Text + "');", true);
    ddlStates.Enabled = false;
    ddlCities.Enabled = false;
}
 
VB.Net
Protected Sub City_Changed(ByVal sender As Object, ByVal e As EventArgs)
    ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('Country: " & ddlCountries.SelectedItem.Text &
                                        "\nState: " & ddlStates.SelectedItem.Text &
                                        "\nCity: " & ddlCities.SelectedItem.Text & "');", True)
    ddlStates.Enabled = False
    ddlCities.Enabled = False
End Sub
 
 

Screenshot

Populate Cascading DropDownList from Database in ASP.Net Example
 
 

Downloads

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