Flat Resultset to Treeview Hierarchy

The above data can be converted into a heirarchial data source with XML data using LINQ with GroupBy extension method, and then it can be bound to the TreeView as shown below:

protected void Page_Load(object sender, EventArgs e)
{
    DataTable dt1 = new DataTable("DataTable1");
    dt1.Columns.Add("C1", typeof(string), null);
    dt1.Columns.Add("C2", typeof(string), null);
    dt1.Columns.Add("C3", typeof(string), null);
    dt1.Columns.Add("C4", typeof(string), null);
    dt1.Columns.Add("C5", typeof(string), null);
    dt1.Columns.Add("C6", typeof(string), null);
    dt1.Columns.Add("C7", typeof(string), null);
    dt1.Rows.Add("1", "Parent1", "10", "Child1", "501", "GrandChild1", "Text");
    dt1.Rows.Add("1", "Parent1", "20", "Child2", "502", "GrandChild2", "Text");
    dt1.Rows.Add("1", "Parent1", "30", "Child3", "503", "GrandChild3", "Text");
    dt1.Rows.Add("1", "Parent1", "40", "Child4", "0", "", "Text");
    dt1.Rows.Add("2", "Parent2", "10", "Child1", "505", "GrandChild4", "Text");
    dt1.Rows.Add("2", "Parent2", "10", "Child1", "506", "GrandChild5", "Text");
    dt1.Rows.Add("2", "Parent2", "20", "Child2", "507", "GrandChild6", "Text");
    dt1.Rows.Add("2", "Parent2", "30", "Child3", "508", "GrandChild7", "Text");
    dt1.Rows.Add("2", "Parent2", "40", "Child4", "0", "", "Text");

    XElement root = new XElement("Root");
    var query = dt1.AsEnumerable().GroupBy(dr => dr.Field<string>("C1")).Select(par => {
        XElement parent = new XElement("Parent");
        parent.Add(new XAttribute("Id", par.Key));
        parent.Add(new XAttribute("Name", par.First().Field<string>("C2")));
        par.GroupBy(dr1 => dr1.Field<string>("C3")).Select(ch => {
            XElement child = new XElement("Child");
            child.Add(new XAttribute("Id", ch.Key));
            child.Add(new XAttribute("Name", ch.First().Field<string>("C4")));
            ch.Select(gch => {
                if (gch.Field<string>("C5") != "0") {
                    XElement gChild = new XElement("GrandChild");
                    gChild.Add(new XAttribute("Id", gch.Field<string>("C5")));
                    gChild.Add(new XAttribute("Name", gch.Field<string>("C6")));
                    gChild.Add(new XAttribute("Value", gch.Field<string>("C7")));
                    child.Add(gChild);
                }
                return gch;
            }).Count();
            parent.Add(child);
            return ch;
        }
    ).Count();
        root.Add(parent);
        return par;
    }).Count();
    XmlDataSource1.Data = root.ToString();
}

The LINQ query is of deferred execution model, so that it will not be executed until it is iterated or a scalar method like Count is called on it.

Hence, in the above code Count extension method is used to force immediate execution of the query.

The ASP.NET markup for the TreeView is as follows:

<form id="form1" runat="server">
<div>
    <asp:TreeView ID="TreeView1" runat="server" DataSourceID="XmlDataSource1">
        <DataBindings>
            <asp:TreeNodeBinding DataMember="Parent" TextField="Name" ValueField="Id" />
            <asp:TreeNodeBinding DataMember="Child" TextField="Name" ValueField="Id" />
            <asp:TreeNodeBinding DataMember="GrandChild" TextField="Name" ValueField="Id" />
        </DataBindings>
    </asp:TreeView>

    <asp:XmlDataSource ID="XmlDataSource1" runat="server"
         XPath="Root/Parent"></asp:XmlDataSource>
</div>
</form>

The XML data generated by the LINQ query is as follows

<Root>
  <Parent Id="1" Name="Parent1">
    <Child Id="10" Name="Child1">
      <GrandChild Id="501" Name="GrandChild1" Value="Text" />
    </Child>
    <Child Id="20" Name="Child2">
      <GrandChild Id="502" Name="GrandChild2" Value="Text" />
    </Child>
    <Child Id="30" Name="Child3">
      <GrandChild Id="503" Name="GrandChild3" Value="Text" />
    </Child>
    <Child Id="40" Name="Child4" />
  </Parent>
  <Parent Id="2" Name="Parent2">
    <Child Id="10" Name="Child1">
      <GrandChild Id="505" Name="GrandChild4" Value="Text" />
      <GrandChild Id="506" Name="GrandChild5" Value="Text" />
    </Child>
    <Child Id="20" Name="Child2">
      <GrandChild Id="507" Name="GrandChild6" Value="Text" />
    </Child>
    <Child Id="30" Name="Child3">
      <GrandChild Id="508" Name="GrandChild7" Value="Text" />
    </Child>
    <Child Id="40" Name="Child4" />
  </Parent>
</Root>

And the TreeView is as follows:

Parent1
    Child1
        GrandChild1
    Child2
        GrandChild2
    Child3
        GrandChild3
    Child4
Parent2
    Child1
        GrandChild4
        GrandChild5
    Child2
        GrandChild6
    Child3
        GrandChild7
    Child4