This is what my tile map looks like (red highlighting added for clarity):

My code:

private List<List<int>> _layoutLayer = new List<List<int>>(50); // Layout Layer
private List<List<int>> _trLayer = new List<List<int>>(50); // Transitional Layer
private List<List<int>> _entityLayer = new List<List<int>>(50); // Object Layer
private List<List<int>> _logicLayer = new List<List<int>>(50); // Logic Layer

// To add a tile
if (inputManager.IsMouseHeld(true))
{
    try
    {
        switch (_currentLayer)
        {
            case LayerRepresentation.LayoutLayer:
                _layoutLayer[_gridMouseY][_gridMouseX] = _currentTile;
                break;
            case LayerRepresentation.TransitionalLayer:
                _trLayer[_gridMouseY][_gridMouseX] = _currentTile;
                break;
            case LayerRepresentation.EntityLayer:
                _entityLayer[_gridMouseY][_gridMouseX] = _currentTile;
                break;
            case LayerRepresentation.LogicLayer:
                _logicLayer[_gridMouseY][_gridMouseX] = _currentTile;
                break;
        }
    }
    catch
    {
         // Player clicked outside of map bounds, add specified amount of tiles to map
    }
}

What I need:

When the player clicks outside of map bounds for example on the point A, add the amount of tiles between point A and the map to the map's height. Do the same with point B, but add to the map width instead.

What I've tried:

    // Inside the above catch block
    if (_mapHeight - _gridMouseY == 0 && _gridMouseX < _mapWidth && _gridMouseX > -1)
    {
        List<int> tempDefaultLayout = new List<int>();
        List<int> tempDefaultOther = new List<int>();

        for (int i = 0; i < _layoutLayer[0].Count; i++)
        {
            tempDefaultLayout.Add(0);
            tempDefaultOther.Add(-1);
        }
        _layoutLayer.Insert(_gridMouseY, tempDefaultLayout);
        _trLayer.Insert(_gridMouseY, tempDefaultOther);
        _entityLayer.Insert(_gridMouseY, tempDefaultOther);
        _logicLayer.Insert(_gridMouseY, tempDefaultOther);
        _mapHeight++;
    }
    else if (_mapWidth - _gridMouseX == 0 && _gridMouseY < _mapHeight && _gridMouseY > -1)
    {
        for (int y = 0; y < _layoutLayer.Count; y++)
        {
            for (int x = 0; x < _layoutLayer[y].Count; x++)
            {
                if (_layoutLayer[y].Count <= _mapWidth)
                {
                    _layoutLayer[y].Add(0);
                    _trLayer[y].Add(-1);
                    _entityLayer[y].Add(-1);
                    _logicLayer[y].Add(-1);
                }
            }
        }
        _mapWidth++;
    }
    return;

The problems:

The first part is about adding tiles to the height (only one row at a time) Adding the temp Lists to the layer Lists with List.Add() doesn't work, I don't know why. I used List.Insert instead.

The second part is used to add a single column to the map one at a time.

Loading a map doesn't work because when the loaded map's size is larger than the default programmed size (_mapWidth and _mapHeight) in the Map class. My Load() method:

if (File.Exists(path))
{
    float version;

    using (BinaryReader reader = new BinaryReader(File.Open(path, FileMode.Open)))
    {
        version = reader.ReadSingle();

        if (version < _version)
        {
            return -1; // The map you are trying to open has been made with an outdated version of the Editor.
        }

        if (version > _version)
        {
            return -2; // The map you are trying to open has been made with a newer version of the Editor.
        }

        _name = reader.ReadString();
        _mapWidth = reader.ReadInt32();
        _mapHeight = reader.ReadInt32();

        for (int y = 0; y < _mapHeight; y++)
        {
            for (int x = 0; x < _mapWidth; x++)
            {
                _layoutLayer[y][x] = reader.ReadInt32();
                _trLayer[y][x] = reader.ReadInt32();
                _entityLayer[y][x] = reader.ReadInt32();
                _logicLayer[y][x] = reader.ReadInt32();
            }
        }
    };

    return 0; // Success
}