WinForms drag-and-drop process selector.

ProcessSelector.cs

using System;
using System.ComponentModel;
using System.Drawing;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace Ace.UI.Forms {
    public partial class ProcessSelector : UserControl {
        #region Imports
        private delegate int HookProc(int nCode, IntPtr wParam, IntPtr lParam);

        private const int WH_MOUSE_LL = 14;
        private const int WM_LBUTTONUP = 0x202;
        
        [DllImport("user32.dll")]
        private static extern IntPtr SetWindowsHookEx(int hookType, HookProc lpfn, IntPtr hMod, int dwThreadId);

        [DllImport("user32.dll")]
        private static extern bool UnhookWindowsHookEx(IntPtr idHook);
		
        [DllImport("user32.dll")]
        private static extern int CallNextHookEx(IntPtr idHook, int nCode, IntPtr wParam, IntPtr lParam);

        [DllImport("user32.dll")]
        private static extern IntPtr WindowFromPoint(Point p);

        [DllImport("user32.dll")]
        private static extern IntPtr GetAncestor(IntPtr hwnd, uint flags);

        [DllImport("user32.dll")]
        private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out int processId);
        #endregion

        #region Properties
        public Process SelectedProcess { get; private set; }
        #endregion

        #region Fields
        private readonly HookProc _mouseHookProcedure;
        private IntPtr _hookId;
        private bool _dragging;
        #endregion

        public ProcessSelector() {
            InitializeComponent();
            Disposed += OnDisposed;
            _mouseHookProcedure = OnMouseEvent;
        }

        private void OnDisposed(object sender, EventArgs e) {
            UnhookMouseEvents();
        }
        
        private void TargetImage_MouseDown(object sender, MouseEventArgs e) {
            HookMouseEvents();
            _dragging = true;
            DoDragDrop(sender, DragDropEffects.None);
        }

        private int OnMouseEvent(int nCode, IntPtr wParam, IntPtr lParam) {
            if (_dragging && wParam.ToInt32() == WM_LBUTTONUP) {
                var window = GetAncestor(WindowFromPoint(MousePosition), 2);
                int processId;
                GetWindowThreadProcessId(window, out processId);
                var process = Process.GetProcessById(processId);
                
                _dragging = false;
                SetProcessInfo(process);
            }
            
            return CallNextHookEx(_hookId, nCode, wParam, lParam);
        }

        private void HookMouseEvents() {
            _hookId = SetWindowsHookEx(WH_MOUSE_LL, _mouseHookProcedure, IntPtr.Zero, 0);
            if (_hookId == IntPtr.Zero)
                throw new Win32Exception("Cannot perform SetWindowsHookEx.");
        }

        private void UnhookMouseEvents() {
            if (_hookId == IntPtr.Zero)
                return;

            UnhookWindowsHookEx(_hookId);
            _hookId = IntPtr.Zero;
        }

        private void SetProcessInfo(Process process) {
            if (InvokeRequired) {
                Invoke((Action<Process>) SetProcessInfo, process);
                return;
            }

            UnhookMouseEvents();

            if (process.Id == Process.GetCurrentProcess().Id)
                return;

            ProcessIdValue.Text = process.Id.ToString();
            ProcessNameValue.Text = process.ProcessName + ".exe";
            SelectedProcess = process;
        }
    }
}

ProcessSelector.Designer.cs

namespace Ace.UI.Forms {
    partial class ProcessSelector {
        /// <summary> 
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary> 
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing) {
            if (disposing && (components != null)) {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Component Designer generated code

        /// <summary> 
        /// Required method for Designer support - do not modify 
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent() {
            this.TargetImage = new System.Windows.Forms.PictureBox();
            this.ProcessIdLabel = new System.Windows.Forms.Label();
            this.StatusLabel = new System.Windows.Forms.Label();
            this.ProcessNameValue = new System.Windows.Forms.Label();
            this.ProcessNameLabel = new System.Windows.Forms.Label();
            this.ProcessIdValue = new System.Windows.Forms.Label();
            ((System.ComponentModel.ISupportInitialize)(this.TargetImage)).BeginInit();
            this.SuspendLayout();
            // 
            // TargetImage
            // 
            this.TargetImage.Image = global::Ace.UI.Properties.Resources.TargetIcon;
            this.TargetImage.Location = new System.Drawing.Point(0, 0);
            this.TargetImage.Name = "TargetImage";
            this.TargetImage.Size = new System.Drawing.Size(32, 32);
            this.TargetImage.SizeMode = System.Windows.Forms.PictureBoxSizeMode.AutoSize;
            this.TargetImage.TabIndex = 1;
            this.TargetImage.TabStop = false;
            this.TargetImage.MouseDown += new System.Windows.Forms.MouseEventHandler(this.TargetImage_MouseDown);
            // 
            // ProcessIdLabel
            // 
            this.ProcessIdLabel.Font = new System.Drawing.Font("Segoe UI", 9F, System.Drawing.FontStyle.Italic, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.ProcessIdLabel.Location = new System.Drawing.Point(38, 0);
            this.ProcessIdLabel.Name = "ProcessIdLabel";
            this.ProcessIdLabel.Size = new System.Drawing.Size(91, 15);
            this.ProcessIdLabel.TabIndex = 2;
            this.ProcessIdLabel.Text = "Process ID:";
            // 
            // StatusLabel
            // 
            this.StatusLabel.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right)));
            this.StatusLabel.Font = new System.Drawing.Font("Segoe UI", 9F, System.Drawing.FontStyle.Italic, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.StatusLabel.Location = new System.Drawing.Point(0, 33);
            this.StatusLabel.Name = "StatusLabel";
            this.StatusLabel.Size = new System.Drawing.Size(271, 23);
            this.StatusLabel.TabIndex = 8;
            this.StatusLabel.Text = "Drag the target icon over the game window...";
            this.StatusLabel.TextAlign = System.Drawing.ContentAlignment.MiddleCenter;
            // 
            // ProcessNameValue
            // 
            this.ProcessNameValue.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right)));
            this.ProcessNameValue.AutoEllipsis = true;
            this.ProcessNameValue.Location = new System.Drawing.Point(135, 17);
            this.ProcessNameValue.Name = "ProcessNameValue";
            this.ProcessNameValue.Size = new System.Drawing.Size(133, 15);
            this.ProcessNameValue.TabIndex = 5;
            // 
            // ProcessNameLabel
            // 
            this.ProcessNameLabel.Font = new System.Drawing.Font("Segoe UI", 9F, System.Drawing.FontStyle.Italic, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.ProcessNameLabel.Location = new System.Drawing.Point(38, 17);
            this.ProcessNameLabel.Name = "ProcessNameLabel";
            this.ProcessNameLabel.Size = new System.Drawing.Size(91, 15);
            this.ProcessNameLabel.TabIndex = 3;
            this.ProcessNameLabel.Text = "Process Name:";
            // 
            // ProcessIdValue
            // 
            this.ProcessIdValue.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right)));
            this.ProcessIdValue.AutoEllipsis = true;
            this.ProcessIdValue.Location = new System.Drawing.Point(135, 0);
            this.ProcessIdValue.Name = "ProcessIdValue";
            this.ProcessIdValue.Size = new System.Drawing.Size(133, 15);
            this.ProcessIdValue.TabIndex = 4;
            // 
            // ProcessSelector
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(7F, 15F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.BackColor = System.Drawing.SystemColors.Window;
            this.Controls.Add(this.TargetImage);
            this.Controls.Add(this.ProcessIdLabel);
            this.Controls.Add(this.StatusLabel);
            this.Controls.Add(this.ProcessIdValue);
            this.Controls.Add(this.ProcessNameValue);
            this.Controls.Add(this.ProcessNameLabel);
            this.Font = new System.Drawing.Font("Segoe UI", 9F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.Name = "ProcessSelector";
            this.Size = new System.Drawing.Size(271, 56);
            ((System.ComponentModel.ISupportInitialize)(this.TargetImage)).EndInit();
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion
        private System.Windows.Forms.PictureBox TargetImage;
        private System.Windows.Forms.Label ProcessIdLabel;
        private System.Windows.Forms.Label StatusLabel;
        private System.Windows.Forms.Label ProcessNameValue;
        private System.Windows.Forms.Label ProcessNameLabel;
        private System.Windows.Forms.Label ProcessIdValue;
    }
}