Реализация интерфейса - C# (179705)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
 
namespace File_Copier
{
    public partial class FrmFileCopier : Form
    {
        private const int MaxLevel = 2;
        public FrmFileCopier()
        {
            InitializeComponent();
            FillDirectoryTree(tvwSource, true);
            FillDirectoryTree(tvwTargetDir, false);
        }
 
        private void FillDirectoryTree(TreeView tvw, bool isSource)
        {
            tvw.Nodes.Clear();
            string[] strDrives = Environment.GetLogicalDrives();
            foreach(string rootDirectoryName in strDrives)
            {
                try
                {
                    DirectoryInfo dir = new DirectoryInfo(rootDirectoryName);
                    dir.GetDirectories();
                    TreeNode ndRoot = new TreeNode(rootDirectoryName);//create ndRoot to hold the directories
                    tvw.Nodes.Add(ndRoot);
                    if (isSource) { GetSubDirectoryNodes(ndRoot, ndRoot.Text, true, 1); }
                    else { GetSubDirectoryNodes(ndRoot, ndRoot.Text, false, 1); }
                }
                catch { }
                Application.DoEvents();
            }
        }
 
        private void GetSubDirectoryNodes(TreeNode parentNode, string fullName, bool getFileNames, int level)
        {
            DirectoryInfo dir = new DirectoryInfo(fullName);
            DirectoryInfo[] dirSubs = dir.GetDirectories();
            foreach(DirectoryInfo dirSub in dirSubs)
            {
                if ((dirSub.Attributes & FileAttributes.Hidden) != 0) { continue; }
                TreeNode subNode = new TreeNode(dirSub.Name);
                parentNode.Nodes.Add(subNode);
                if (level < MaxLevel)
                {
                    GetSubDirectoryNodes(subNode, dirSub.FullName, getFileNames, level + 1);
                }
                if (getFileNames)
                {
                    FileInfo[] files = dir.GetFiles();
                    foreach(FileInfo file in files)
                    {
                        TreeNode fileNode = new TreeNode(file.Name);
                        parentNode.Nodes.Add(fileNode);
                    }
                }
            }
        }
 
        private void label1_Click(object sender, EventArgs e)
        {
 
        }
 
        private void tvwSource_AfterCheck(object sender, TreeViewEventArgs e)
        {
            SetCheck(e.Node, e.Node.Checked);
        }
 
        private void SetCheck(TreeNode node, bool check)
        {
            foreach(TreeNode n in node.Nodes)
            {
                n.Checked = check; //check the node
                if (n.Nodes.Count != 0) { SetCheck(n, check); }
            }
        }
 
        private void tvwSource_BeforeExpand(object sender, TreeViewCancelEventArgs e)
        {
            tvwExpand(sender, e.Node);
        }
 
        private void tvwExpand(object sender, TreeNode currentNode)
        {
            TreeView tvw = (TreeView)sender;
            bool getFiles = (tvw == tvwSource);
            string fullName = currentNode.FullPath;
            currentNode.Nodes.Clear();
            GetSubDirectoryNodes(currentNode, fullName, getFiles, 1);
        }
 
        private void tvwTargetDir_BeforeExpand(object sender, TreeViewCancelEventArgs e)
        {
            tvwExpand(sender, e.Node);
        }
 
        private void tvwTargetDir_AfterSelect(object sender, TreeViewEventArgs e)
        {
            string theFullPath = e.Node.FullPath;
            if (theFullPath.EndsWith(""))
            {
                theFullPath = theFullPath.Substring(0, theFullPath.Length - 1);
            }
            txtTargetDir.Text = theFullPath;
        }
 
        private void btnClear_Click(object sender, EventArgs e)
        {
            foreach(TreeNode node in tvwSource.Nodes) { SetCheck(node, false); }
        }
 
        private void btnCopy_Click(object sender, EventArgs e)
        {
            List<FileInfo> fileList = GetFileList();
            foreach(FileInfo file in fileList)
            {
                try
                {
                    lblStatus.Text = "Copying " + txtTargetDir.Text + "" + file.Name + "...";
                    Application.DoEvents();
                    file.CopyTo(txtTargetDir.Text + "" + file.Name, chkOverwrite.Checked);
                }
                catch(Exception ex) { MessageBox.Show(ex.Message); }
            }
            lblStatus.Text = "Done.";
            //Application.DoEvents();
        }
 
        private List<FileInfo> GetFileList()
        {
            List<string> fileNames = new List<string>();
            foreach(TreeNode theNode in tvwSource.Nodes)
            {
                GetCheckedFiles(theNode, fileNames);
            }
            List<FileInfo> fileList = new List<FileInfo>();
            foreach(string fileName in fileNames)
            {
                FileInfo file = new FileInfo(fileName);
                if (file.Exists) { fileList.Add(file); }
            }
            IComparer<FileInfo> comparer = (IComparer<FileInfo>) new FileComparer();// создаем интерфейсную ссылку на объект класса
            fileList.Sort(comparer);//Почему мы здесь передаем ссылку на объект, в котором реализована только одна ф-ия 
//Compaprer, а не на саму функцию? Она же не вызывается сама по себе...
            return fileList;
        }
        private void GetCheckedFiles(TreeNode node, List<string> fileNames)
        {
            if(node.Nodes.Count==0)//if this is a leaf...
            {
                if(node.Checked)//if the node was checked...
                {
                    fileNames.Add(node.FullPath);//if the node was checked...
                }
            }
            else
            {
                foreach(TreeNode n in node.Nodes) { GetCheckedFiles(n, fileNames); }
            }
        }
        public class FileComparer: IComparer<FileInfo>
        {
            public int Comparer(FileInfo file1, FileInfo file2)// почему компилятор не видит здесь реализацию интерфейса?
//пробовал изменить название на IComparer<FileInfo>.Comparer, но безуспешно... Почему?
            {
                if (file1.Length > file2.Length) { return -1; }
                if (file1.Length < file2.Length) { return 1; }
                return 0;
            }
 
            int IComparer<FileInfo>.Compare(FileInfo x, FileInfo y)// этой функции в примере нет, но без нее не работает - 
// не реализован интерфейс
            {
                throw new NotImplementedException();
            }
        }
 
        private void btnDelete_Click(object sender, EventArgs e)
        {
            System.Windows.Forms.DialogResult result =
                MessageBox.Show(
                    "Are you quite sure?",
                    "Delete Files",
                    MessageBoxButtons.OKCancel,
                    MessageBoxIcon.Exclamation,
                    MessageBoxDefaultButton.Button2);
            if(result== System.Windows.Forms.DialogResult.OK)
            {
                List<FileInfo> fileNames = GetFileList();
                foreach (FileInfo file in fileNames)
                {
                    try
                    {
                        lblStatus.Text = "Deleting " + txtTargetDir.Text + "" + file.Name + "...";
                        Application.DoEvents();
                        file.Delete();
                    }
                    catch (Exception ex) { MessageBox.Show(ex.Message); }
                }
                lblStatus.Text = "Done.";
                Application.DoEvents();
            }
        }
 
        private void btnCancel_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }
    }
}