Hi irshad1231,

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

HTML

<asp:TreeView ID="TreeView1" runat="server" ImageSet="XPFileExplorer" NodeIndent="15"
    OnSelectedNodeChanged="TreeView1_SelectedNodeChanged">
    <HoverNodeStyle Font-Underline="True" ForeColor="#6666AA" />
    <NodeStyle Font-Names="Tahoma" Font-Size="8pt" ForeColor="Black" HorizontalPadding="2px"
        NodeSpacing="0px" VerticalPadding="2px"></NodeStyle>
    <ParentNodeStyle Font-Bold="False" />
    <SelectedNodeStyle BackColor="#B5B5B5" Font-Underline="False" HorizontalPadding="0px" VerticalPadding="0px" />
</asp:TreeView>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.PopulateTreeView(this.GetData("SELECT Id, Name FROM VehicleTypes"), 0, null);
    }
}

private void PopulateTreeView(DataTable dtParent, int parentId, TreeNode treeNode)
{
    foreach (DataRow row in dtParent.Rows)
    {
        TreeNode child = new TreeNode { Text = row["Name"].ToString(), Value = row["Id"].ToString() };
        if (parentId == 0)
        {
            TreeView1.Nodes.Add(child);
            PopulateTreeView(this.GetData("SELECT Id, Name FROM VehicleSubTypes WHERE VehicleTypeId = " + child.Value), int.Parse(child.Value), child);
        }
        else
        {
            treeNode.ChildNodes.Add(child);
        }
    }
}

private DataTable GetData(string query)
{
    DataTable dt = new DataTable();
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                sda.Fill(dt);
            }
        }
        return dt;
    }
}

protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)
{
    string parent = "";
    int parentId = 0;
    string child = "";
    int childId = 0;
    if (this.TreeView1.SelectedNode.Parent == null)
    {
        parent = this.TreeView1.SelectedNode.Text;
        parentId = Convert.ToInt32(this.TreeView1.SelectedNode.Value);
        Response.Redirect("SectionContents.aspx?PID=0&courseid=" + GetID());
    }
    else
    {
        parent = this.TreeView1.SelectedNode.Parent.Text;
        parentId = Convert.ToInt32(this.TreeView1.SelectedNode.Parent.Value);
        child = this.TreeView1.SelectedNode.Text;
        childId = Convert.ToInt32(this.TreeView1.SelectedNode.Value);
        Response.Redirect("SectionContents.aspx?PID=" + parentId + "&courseid=" + GetID());
    }
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.PopulateTreeView(Me.GetData("SELECT Id, Name FROM VehicleTypes"), 0, Nothing)
    End If
End Sub

Private Sub PopulateTreeView(dtParent As DataTable, parentId As Integer, treeNode As TreeNode)
    For Each row As DataRow In dtParent.Rows
        Dim child As New TreeNode() With { _
         .Text = row("Name").ToString(), _
         .Value = row("Id").ToString() _
        }
        If parentId = 0 Then
            TreeView1.Nodes.Add(child)
            PopulateTreeView(Me.GetData("SELECT Id, Name FROM VehicleSubTypes WHERE VehicleTypeId = " + child.Value), Integer.Parse(child.Value), child)
        Else
            treeNode.ChildNodes.Add(child)
        End If
    Next
End Sub

Private Function GetData(query As String) As DataTable
    Dim dt As New DataTable()
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As New SqlConnection(constr)
        Using cmd As New SqlCommand(query)
            Using sda As New SqlDataAdapter()
                cmd.CommandType = CommandType.Text
                cmd.Connection = con
                sda.SelectCommand = cmd
                sda.Fill(dt)
            End Using
        End Using
        Return dt
    End Using
End Function

Protected Sub TreeView1_SelectedNodeChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim parent As String = ""
    Dim parentId As Integer = 0
    Dim child As String = ""
    Dim childId As Integer = 0

    If Me.TreeView1.SelectedNode.Parent Is Nothing Then
        parent = Me.TreeView1.SelectedNode.Text
        parentId = Convert.ToInt32(Me.TreeView1.SelectedNode.Value)
        Response.Redirect("SectionContents.aspx?PID=0&courseid=" & GetID())
    Else
        parent = Me.TreeView1.SelectedNode.Parent.Text
        parentId = Convert.ToInt32(Me.TreeView1.SelectedNode.Parent.Value)
        child = Me.TreeView1.SelectedNode.Text
        childId = Convert.ToInt32(Me.TreeView1.SelectedNode.Value)
        Response.Redirect("SectionContents.aspx?PID=" & parentId & "&courseid=" & GetID())
    End If
End Sub