Cascading DropDownList in Windows Forms Application using C# and VB.Net

Hi rickabbam,

Check this example. Now please take its reference and correct your code.

Database

I have made use of the following tables with the schema as follows.

Country

State

City

You can download the database table SQL by clicking the download link below.

Download

Form Design

Namespaces

C#

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

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Code

C#

private void Form1_Load(object sender, EventArgs e)
{
    cmbState.Enabled = false;
    cmbCity.Enabled = false;
    string strQuery = "SELECT CountryId, CountryName FROM Countries";
    cmbCountry.ValueMember = "CountryId";
    cmbCountry.DisplayMember = "CountryName";
    cmbCountry.DataSource = GetData(strQuery, null).Tables[0];
}

private void cmbCountry_SelectedIndexChanged(object sender, EventArgs e)
{
    string strQuery = "SELECT StateId, StateName FROM States WHERE CountryID = @CountryID";
    SqlParameter[] sqlParameter = new SqlParameter[1];
    sqlParameter[0] = new SqlParameter("@CountryID", cmbCountry.SelectedValue);
    cmbState.Enabled = true;
    cmbState.DisplayMember = "StateName";
    cmbState.ValueMember = "StateId";
    cmbState.DataSource = this.GetData(strQuery, sqlParameter).Tables[0];
}

private void cmbState_SelectedIndexChanged(object sender, EventArgs e)
{
    string strQuery = "SELECT CityId, CityName FROM Cities WHERE StateId = @StateId";
    SqlParameter[] sqlParameter = new SqlParameter[1];
    sqlParameter[0] = new SqlParameter("@StateId", cmbState.SelectedValue);
    cmbCity.Enabled = true;
    cmbCity.DisplayMember = "CityName";
    cmbCity.ValueMember = "CityId";
    cmbCity.DataSource = this.GetData(strQuery, sqlParameter).Tables[0];
}

private DataSet GetData(string query, SqlParameter[] sqlParameter)
{
    string strConnString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    DataSet ds = new DataSet();
    SqlCommand cmd = new SqlCommand(query, con);
    if (sqlParameter != null)
    {
        cmd.Parameters.AddRange(sqlParameter);
    }
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    da.Fill(ds);
    return ds;
}

VB.Net

Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs)
    cmbState.Enabled = False
    cmbCity.Enabled = False
    Dim strQuery As String = "SELECT CountryId, CountryName FROM Countries"
    cmbCountry.ValueMember = "CountryId"
    cmbCountry.DisplayMember = "CountryName"
    cmbCountry.DataSource = GetData(strQuery, Nothing).Tables(0)
End Sub

Private Sub cmbCountry_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim strQuery As String = "SELECT StateId, StateName FROM States WHERE CountryID = @CountryID"
    Dim sqlParameter As SqlParameter() = New SqlParameter(0) {}
    sqlParameter(0) = New SqlParameter("@CountryID", cmbCountry.SelectedValue)
    cmbState.Enabled = True
    cmbState.DisplayMember = "StateName"
    cmbState.ValueMember = "StateId"
    cmbState.DataSource = Me.GetData(strQuery, sqlParameter).Tables(0)
End Sub

Private Sub cmbState_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim strQuery As String = "SELECT CityId, CityName FROM Cities WHERE StateId = @StateId"
    Dim sqlParameter As SqlParameter() = New SqlParameter(0) {}
    sqlParameter(0) = New SqlParameter("@StateId", cmbState.SelectedValue)
    cmbCity.Enabled = True
    cmbCity.DisplayMember = "CityName"
    cmbCity.ValueMember = "CityId"
    cmbCity.DataSource = Me.GetData(strQuery, sqlParameter).Tables(0)
End Sub

Private Function GetData(ByVal query As String, ByVal sqlParameter As SqlParameter()) As DataSet
    Dim strConnString As String = ConfigurationManager.ConnectionStrings("conString").ConnectionString
    Dim con As SqlConnection = New SqlConnection(strConnString)
    Dim ds As DataSet = New DataSet()
    Dim cmd As SqlCommand = New SqlCommand(query, con)
    If sqlParameter IsNot Nothing Then
        cmd.Parameters.AddRange(sqlParameter)
    End If

    Dim da As SqlDataAdapter = New SqlDataAdapter(cmd)
    da.Fill(ds)
    Return ds
End Function

Screenshot