ScreenCaptureApp/MainForm.cs

using System;
using System.Drawing;
using System.Windows.Forms;

namespace ScreenCaptureApp
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }

        private void btnCapture_Click(object sender, EventArgs e)
        {
            this.Hide();
            using (var captureForm = new ScreenCaptureForm())
            {
                if (captureForm.ShowDialog() == DialogResult.OK)
                {
                    SaveImage(captureForm.CapturedImage);
                }
            }
            this.Show();
        }

        private void SaveImage(Image image)
        {
            using (SaveFileDialog sfd = new SaveFileDialog())
            {
                sfd.Filter = "PNG Image|*.png|JPEG Image|*.jpg|Bitmap Image|*.bmp";
                sfd.Title = "Save Screenshot";
                sfd.FileName = "screenshot.png";

                if (sfd.ShowDialog() == DialogResult.OK)
                {
                    image.Save(sfd.FileName, GetImageFormat(sfd.FileName));
                }
            }
        }

        private System.Drawing.Imaging.ImageFormat GetImageFormat(string fileName)
        {
            string ext = System.IO.Path.GetExtension(fileName).ToLower();
            switch (ext)
            {
                case ".jpg": return System.Drawing.Imaging.ImageFormat.Jpeg;
                case ".jpeg": return System.Drawing.Imaging.ImageFormat.Jpeg;
                case ".bmp": return System.Drawing.Imaging.ImageFormat.Bmp;
                default: return System.Drawing.Imaging.ImageFormat.Png;
            }
        }
    }
}