MainForm.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Picscut
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
            PicsList.Init();
        }

        private void MainForm_Load(object sender, EventArgs e)
        {                            
        }

        Size FitRatio(Size container, Size target)
        {
            Size sw = new Size(container.Width, target.Height * container.Width / target.Width);
            Size sh = new Size(target.Width * container.Height / target.Height, container.Height);
            if (sw.Height < container.Height) return sw;
            else return sh;
        }

        private void PicsList_SelectedIndexChanged(object sender, EventArgs e)
        {
            Cropper.Image = PicsList.SelectedPicture.Bitmap;            
            AdjustCropperSize();
        }

        private void AdjustCropperSize()
        {
            if (Cropper.Image == null) 
                return;

            Cropper.Size = FitRatio(PicViewer.Size, Cropper.Image.Size);
            Cropper.Left = (PicViewer.Width - Cropper.Width) / 2;
            Cropper.Top = (PicViewer.Height - Cropper.Height) / 2;            
            Cropper.EffectizeSize = Cropper.Image.Size;
            Cropper.Invalidate();
        }

        private void PicViewer_Resize(object sender, EventArgs e)
        {
            AdjustCropperSize();
        }      

        private void PicsList_PicsSizeChanged(object sender, EventArgs e)
        {

        }

        bool boundsUpdating = false;
        private void Cropper_SelectionBoundsChanged(object arg1, EventArgs arg2)
        {
            if (boundsUpdating)
                return;
            boundsUpdating = true;
            SelTopBox.Value = Cropper.SelectionBounds.Top;
            SelLeftBox.Value = Cropper.SelectionBounds.Left;
            SelWidthBox.Value = Cropper.SelectionBounds.Width;
            SelHeightBox.Value = Cropper.SelectionBounds.Height;
            boundsUpdating = false;
        }

        int clamp(int x, int a, int b)
        {
            if (x < a) return a;
            if (x > b) return b;
            return x;
        }

        private void SelBox_ValueChanged(object sender, EventArgs e)
        {
            if (boundsUpdating)
                return;
            boundsUpdating = true;

            int left = clamp((int)SelLeftBox.Value, 0, Cropper.Image.Width);
            int top = clamp((int)SelTopBox.Value, 0, Cropper.Image.Height);
            int width = (int)SelWidthBox.Value;
            int height = (int)SelHeightBox.Value;

            Cropper.SelectionBounds = new Rectangle(left, top, width, height);
            boundsUpdating = false;
        }

        private void SelBox_KeyUp(object sender, KeyEventArgs e)
        {
            // Trigger ValueChanged while typing
            var v = (sender as NumericUpDown).Value;
        }

        private void SaveOverBtn_Click(object sender, EventArgs e)
        {
            foreach(var pic in PicsList.Pictures)
            {
                var path = pic.Path;
                var dir = Path.Combine(Path.GetDirectoryName(path), "cropped");
                path = Path.Combine(dir, Path.GetFileName(path));
                Directory.CreateDirectory(dir);
                pic.Crop(Cropper.SelectionBounds).Save(path);
            }
        }

        private void PicsList_DragEnter(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop)) e.Effect = DragDropEffects.Copy;
        }

        private void PicsList_DragDrop(object sender, DragEventArgs e)
        {
            bool wasEmpty = PicsList.Pictures.Count == 0;
            string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
            foreach(var file in files)
            {
                try
                {
                    PicsList.AddPicture(file);
                }
                catch(Exception ex)
                {
                    MessageBox.Show($"Could not add file {file}\n\nReason: {ex.Message}", "Error");
                }
            }    
            if(wasEmpty) // so no selection was active
            {
                PicsList.SetSelected(0, true);
                if (!Cropper.Visible)
                    Cropper.Show();
            }
        }      
    }
}