private void button2_Click(object sender, EventArgs e)
        {
            if (txtDeptName.Text == "")
            {
                MessageBox.Show("请输入要添加的部门名称。");
                return;
            }

            if (checkBox1.Checked)
            {
                foreach (TreeNode childnode in treeView1.SelectedNode.Nodes)
                {
                    if (childnode.Text == txtDeptName.Text)
                    {
                        MessageBox.Show("此部门已经存在。");
                        return;
                    }
                }
                treeView1.SelectedNode.Nodes.Add(txtDeptName.Text);
                treeView1.SelectedNode.Expand();
            }
            else
            {
                foreach (TreeNode node in treeView1.Nodes)
                {
                    if (node.Text == txtDeptName.Text)
                    {
                        MessageBox.Show("此部门已经存在。");
                        return;
                    }
                }
                treeView1.Nodes.Add(txtDeptName.Text);
            }

        }

        private void AddNode(TreeNodeCollection treeNodes, String value)
        {
            foreach (TreeNode childnode in treeNodes)
            {
                if (childnode.Text == value)
                {
                    MessageBox.Show("此部门已经存在。");
                    return;
                }
            }
            treeNodes.Add(txtDeptName.Text);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (txtDeptName.Text == "")
            {
                MessageBox.Show("请输入要添加的部门名称。");
                return;
            }

            if (checkBox1.Checked)
            {
                if (treeView1.SelectedNode != null)
                {
                    AddNode(treeView1.SelectedNode.Nodes, txtDeptName.Text);
                }
            }
            else
            {
                AddNode(treeView1.Nodes, txtDeptName.Text);
            }
        }

------解决方案--------------------
C# code
private void button2_Click(object sender, EventArgs e)
{
    if (txtDeptName.Text == "")
    {
        MessageBox.Show("请输入要添加的部门名称。");
        return;
    }

    var nodes = checkBox1.Checked ? treeView1.SelectedNode.Nodes : treeView1.Nodes;
    bool exit = nodes.Cast<TreeNode>().Any(v1 => v1.Text != txtDeptName.Text);
    if (!exit)
    {
        MessageBox.Show("此部门已经存在。");
        return;
    }

    nodes.Add(txtDeptName.Text);
    if (checkBox1.Checked)
    {
        treeView1.SelectedNode.Expand();
    }
}