C#上移，下移TreeView中的树节点顺序

public Form1()
{
InitializeComponent();
}
TreeNode preNode, nextNode, currentNode;
int g_tag;
string g_text;
private void Form1_Load(object sender, EventArgs e)
{
TreeNode tn = new TreeNode();
tn.Nodes.Add("北京");
tn.Tag = 1;
tn.Nodes.Add("湖北");
tn.Tag = 2;
tn.Nodes.Add("上海");
tn.Tag = 3;
tn.Nodes.Add("天津");
tn.Tag = 4;
treeView1.Nodes.Add(tn);

}
//上移
private void button2_Click(object sender, EventArgs e)
{
currentNode = treeView1.SelectedNode;
if (currentNode == null)
{
return;
}
else
{

preNode = currentNode.PrevNode;
if (preNode == null)
{
return;
}
else
{
g_text= preNode.Text;
g_tag=Convert.ToInt32( preNode.Tag);
preNode.Tag= currentNode.Tag;
preNode.Text = currentNode.Text;
currentNode.Tag = g_tag;
currentNode.Text = g_text;
}
}

}
//下移
private void button3_Click(object sender, EventArgs e)
{

currentNode = treeView1.SelectedNode;
if (currentNode == null)
{
return;
}
else
{

preNode = currentNode.NextNode;
if (preNode == null)
{
return;
}
else
{
g_text = preNode.Text;
g_tag = Convert.ToInt32(preNode.Tag);
preNode.Tag = currentNode.Tag;
preNode.Text = currentNode.Text;
currentNode.Tag = g_tag;
currentNode.Text = g_text;
}
}
}