Help about most recently used file(MRU)

There are several possibilities. I will explain one using Application Settings.

First, in the solution explorer, right-click on your project, then choose properties, and select the Settings tab.
In the Name column, enter Mru
In the Type column, select System.Collections.Specialized.StringCollection
In the Scope column, select User
In the Value column, click on the small button to edit the collection, enter any value, then click on OK, then click again on the button, remove the value and click OK again to make the collection empty. Without this trick, the collection wouldn't be instanciated and you would have an exception will running your code.

OK you have created your MRU list and it will be saved in your application settings file.

Now let's add some code in your form. I suppose you already created a menu with two entries: Open and Recent files.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        //fill the menu items
        UpdateMRUMenu();
        //this event will be fired if the user selects a file in the MRU
        recentFilesToolStripMenuItem.DropDownItemClicked += new ToolStripItemClickedEventHandler(recentFilesToolStripMenuItem_DropDownItemClicked);
    }

    //user clicked on File, Recent files
    void recentFilesToolStripMenuItem_DropDownItemClicked(object sender,
        ToolStripItemClickedEventArgs e)
    {
        //the user clicked on a file in the MRU. Let's open it.
        OpenFile(e.ClickedItem.Text);
    }

    // updates the menu items
    private void UpdateMRUMenu()
    {
        recentFilesToolStripMenuItem.DropDownItems.Clear();
        foreach (string path in Properties.Settings.Default.Mru)
            recentFilesToolStripMenuItem.DropDownItems.Add(path);
    }

    // add this file to the MRU
    void AddToMRU(string path)
    {
        //add the file to the list
        Properties.Settings.Default.Mru.Insert(0, path);
        //just keep the 5 recentest elements
        while (Properties.Settings.Default.Mru.Count > 5)
            Properties.Settings.Default.Mru.RemoveAt(Properties.Settings.Default.Mru.Count - 1);
        //save the settings file
        Properties.Settings.Default.Save();
        //update the menu items
        UpdateMRUMenu();
    }

    //user clicked on File, Open
    private void openToolStripMenuItem_Click(object sender, EventArgs e)
    {
        OpenFileDialog dlg = new OpenFileDialog();
        if (dlg.ShowDialog() == DialogResult.OK)
        {
            string path = dlg.FileName;
            //add this file to the MRU
            AddToMRU(path);
            //open the file
            OpenFile(path);
        }
    }

    //open your file here
    void OpenFile(string path)
    {
        ...
    }
}