Bitmap function giving parameter not valid error

Question

First some background. I a relative new to C# programming and have been lear ning it by creating games following several tutorials. I am running VS2015 Community on Win8.1. I am following a game development tutorial that was originally written to be run under XNA. I am at the point where I am creating a level editor using Windows Forms. So far the code is working as the level editor form displays without problems. The next step, with which I am having the issue, is to load a spritesheet into the editor so levels can be created/modified. Here is the method to load the spritesheet:

private void LoadImageList()
    {
        string filepath = Application.StartupPath +
           @"\Content\PlatformTiles";
       Bitmap tileSheet = new Bitmap(filepath);
        int tilecount = 0;
        for (int y = 0; y < tileSheet.Height / TileMap.TileHeight; y++)
        {
            for (int x = 0; x < tileSheet.Width / TileMap.TileWidth; x++)
            {
                Bitmap newBitmap = tileSheet.Clone(new
                    System.Drawing.Rectangle(
                        x * TileMap.TileWidth,
                        y * TileMap.TileHeight,
                        TileMap.TileWidth,
                        TileMap.TileHeight),
                        System.Drawing.Imaging.PixelFormat.DontCare);

                imgListTiles.Images.Add(newBitmap);
                string itemName = "";
                if (tilecount == 0)
                {
                    itemName = "Empty";
                }
                if (tilecount == 1)
                {
                    itemName = "White";
                }
                listTiles.Items.Add(new
                    ListViewItem(itemName, tilecount++));
            }
        }
    }

    private void MapEditor_Load(object sender, EventArgs e)
    {
        LoadImageList();
    }

    private void exitToolStripMenuItem_Click(object sender, EventArgs e)
    {
        game.Exit();
        Application.Exit();
    }
  }
}

The spritesheet is a .png file named PlatformTiles and located in the Content folder for the project. The file was loaded using Monogame's Content manager. When I build the project, Debug stops at the line where the "filepath" variable is being assigned to Bitmap. I get the error message shown in the following screen shot:

I have researched as much as I could using Google, MSDN, Monogame, and similar sites. Though there are other similar instances of programmers getting the message, they are not working on things like I am. So I am posting to get advice on what I can do to fix the problem. If there are any questions please let me know. Thank you.

Answer 1

You need to give the extension as well when you're passing in the filename

string filepath = Application.StartupPath +
           @"\Content\PlatformTiles.png";