要將二叉樹轉換為雙向鏈表，可以使用中序遍歷來實現。具體步驟如下：

1. 創建一個類來表示雙向鏈表的節點，包括指向前一個節點和后一個節點的指針。

public class Node
{
    public int val;
    public Node prev;
    public Node next;

    public Node(int val)
    {
        this.val = val;
        this.prev = null;
        this.next = null;
    }
}

2. 創建一個類來表示二叉樹的節點，包括左子節點和右子節點。

public class TreeNode
{
    public int val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode(int val)
    {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}

3. 編寫一個遞歸函數來實現中序遍歷，并在遍歷過程中將二叉樹轉換為雙向鏈表。

public class Solution
{
    private Node prev;

    public Node Convert(TreeNode root)
    {
        if (root == null)
            return null;

        Node dummy = new Node(-1);
        prev = dummy;

        InOrder(root);

        Node head = dummy.next;
        head.prev = null;

        return head;
    }

    private void InOrder(TreeNode node)
    {
        if (node == null)
            return;

        InOrder(node.left);

        Node current = new Node(node.val);
        prev.next = current;
        current.prev = prev;
        prev = current;

        InOrder(node.right);
    }
}

4. 在主函數中調用Convert方法，將二叉樹轉換為雙向鏈表。

class Program
{
    static void Main(string[] args)
    {
        TreeNode root = new TreeNode(4);
        root.left = new TreeNode(2);
        root.right = new TreeNode(5);
        root.left.left = new TreeNode(1);
        root.left.right = new TreeNode(3);

        Solution solution = new Solution();
        Node head = solution.Convert(root);

        // 遍歷雙向鏈表
        Node currentNode = head;
        while (currentNode != null)
        {
            Console.Write(currentNode.val + " ");
            currentNode = currentNode.next;
        }
    }
}

運行上面的代碼，即可將二叉樹轉換為雙向鏈表，并輸出雙向鏈表的值。