Populate TreeView with arbitrary depth from DataTable

Question

I have a DataTable with id, parent, label columns and want to populate a TreeView with it.

I'm new to c# and as far as I understand, there is no ready solution for it, unlike some other languages.

So I sorted the data hierarchically at DataTable level and wrote not a very pretty recursive method.

Is this a good solution or can it be greatly improved?

    using System.Data;

    public void Populate()
    {
        //data example
        var table = new DataTable();
        DataColumn column;
        DataRow row;
        column = new DataColumn(); column.DataType = Type.GetType("System.Int32"); column.ColumnName = "id"; table.Columns.Add(column);
        column = new DataColumn(); column.DataType = Type.GetType("System.Int32"); column.ColumnName = "parent"; table.Columns.Add(column);
        column = new DataColumn(); column.DataType = Type.GetType("System.String"); column.ColumnName = "label"; table.Columns.Add(column);
        row = table.NewRow(); row["id"] = 1; row["parent"] = DBNull.Value; row["label"] = "root1"; table.Rows.Add(row);
        row = table.NewRow(); row["id"] = 2; row["parent"] = 1; row["label"] = "child11"; table.Rows.Add(row);
        row = table.NewRow(); row["id"] = 3; row["parent"] = 1; row["label"] = "child12"; table.Rows.Add(row);
        row = table.NewRow(); row["id"] = 4; row["parent"] = DBNull.Value; row["label"] = "root2"; table.Rows.Add(row);
        row = table.NewRow(); row["id"] = 5; row["parent"] = 4; row["label"] = "child21"; table.Rows.Add(row);
        row = table.NewRow(); row["id"] = 6; row["parent"] = 5; row["label"] = "child211"; table.Rows.Add(row);
        // call method
        int i = 0;
        var result = new List<TreeNode>();
        var data = table.Rows;
        PopTree(ref data, DBNull.Value, ref i, ref result);
        treeView1.Nodes.AddRange(result.ToArray());
    }

    private void PopTree(ref DataRowCollection data, object parent, ref int i, ref List<TreeNode> result)
    {
        if (i >= data.Count || data[i]["parent"].ToString() != parent.ToString())
        {
            return;
        }
        var row = data[i];
        var children = new List<TreeNode>();
        i++;
        if (i < data.Count && row["id"].ToString() == data[i]["parent"].ToString())
        {
            PopTree(ref data, row["id"], ref i, ref children);
        }
        TreeNode node = new TreeNode(row["label"].ToString(), children.ToArray());
        result.Add(node);
        PopTree(ref data, parent, ref i, ref result);
    }

... or can it be greatly improved? That is a much more open question than you may realize. Perhaps read this SO thread for starters: Tree Data Structure in C# — radarbob
– radarbob, Commented Mar 14, 2022 at 7:58

Peter Csala · Accepted Answer · 2022-03-16 12:30:29Z

Let me focus only on the Populate function in this post.

Defining Columns

You can define columns in a really concise way thanks to the AddRange method of the DataColumnCollection class

table.Columns.AddRange(new[] {
    new DataColumn("id", typeof(int)),
    new DataColumn("parent", typeof(int)), 
    new DataColumn("label", typeof(string)),
});

I would also advice to use typeof operator over Type.GetType

since the former can give you compile time safety
whereas the latter can fail at runtime in case of a typo for example.

Populating Rows with data

Unfortunately DataRowCollection does not define an AddRange method so, you have to add the rows one-by one. Here the preferred approach is to define the data separately from the DataRow population.

Here I have created an array of ValueTuples to represent only the data.

var relationships = new (int Id, int? Parent, string Label)[] {
 (1, null, "root1"),
 (2, 1, "child11"),
 (3, 1, "child12"),
 (4, null, "root2"),
 (5, 4, "child21"),
 (6, 5, "child211"),
};

Finally here is a foreach to convert the data into a DataRow then add it to the DataTable

foreach (var relationship in relationships)
{
    var row = table.NewRow();
    row[0] = relationship.Id;
    if (relationship.Parent.HasValue)
        row[1] = relationship.Parent;
    else
        row[1] = DBNull.Value;
    row[2] = relationship.Label;
    table.Rows.Add(row);
}

It seems from the question text that the asker is interested in the PopTree algorithm and the Populate method is provided for the sake of having a working example. The advice in this answer is sound, but it seems not to answer the question. — phoog
– phoog, Commented Mar 17, 2022 at 8:11
@phoog Yes that's right the question is more about the PopTree. But that Populate code was shouting for improvement :) — Peter Csala
– Peter Csala, Commented Mar 17, 2022 at 8:13

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