string strsql = "Select Department from UserInfo group by Department";
DataSet DsforTree = new DataSet();
DsforTree = ObjectMaster.GetDatasetBySql(strsql, "UserInfo");//这是获取DataSet
for (int i = 0; i < DsforTree.Tables[0].Rows.Count; i++)
{
  treeView1.Nodes.Add(DsforTree.Tables["UserInfo"].Rows[i]["Department"].ToString());
  treeView1.Nodes[i].ImageIndex = 0;
  string SQLsentence = "Select * from UserInfo where Department='" + treeView1.Nodes[i].Text + "'";
   DataSet DsforTreeofaDepartment = new DataSet();
   DsforTreeofaDepartment = ObjectMaster.GetDatasetBySql(SQLsentence, "UserInfo");
   for (int j = 0; j < DsforTreeofaDepartment.Tables[0].Rows.Count; j++)
   {
     treeView1.Nodes[i].Nodes.Add(DsforTreeofaDepartment.Tables["UserInfo"].Rows[j]["nickname"].ToString());
      treeView1.Nodes[i].Nodes[j].ImageIndex = 2;
   }
}

------解决方案--------------------
将数据先加载到一个 DataTable 中，这个不需要多说了吧

假设 DataTable 中字段名为 id, dept, name
窗体上有一 TreeView 控件名为 treeView1

下面的代码仅供参考

C# code


private void Form1_Load(object sender, EventArgs e)
{
    // 模拟数据加载
    DataTable table = new DataTable();

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

    table.Rows.Add(new object[] { 1, "财务部", "小红" });
    table.Rows.Add(new object[] { 2, "业务部", "小明" });
    table.Rows.Add(new obj