[转载]C#中控件的拖拽操作

原文地址：C#中控件的拖拽操作作者：richaolas

下面描述与拖放操作相关的事件的引发方式以及引发时间。

DoDragDrop 方法确定当前光标位置下的控件。然后它将检查该控件是否是有效的放置目标。

如果该控件是有效的放置目标，则 GiveFeedback 事件以指定的拖放效果引发。有关拖放效果的列表，请参见 DragDropEffects 枚举。

跟踪鼠标光标位置、键盘状态和鼠标按钮状态的更改。

如果鼠标移出一个控件，则引发该控件 DragLeave 事件。

如果鼠标进入另一个控件，则引发该控件的 DragEnter 事件。

如果鼠标移动但停留在同一个控件中，则引发 DragOver 事件。

如果更改了键盘或鼠标按钮状态(经过调试发现其实每次鼠标的移动也会引发QueryContinueDrag这个事件)，则引发 QueryContinueDrag 事件并根据事件的QueryContinueDragEventArgs 的 Action 属性值确定是否继续拖动、放置数据或取消操作。

如果 DragAction 的值为 Continue，将引发 DragOver 事件以继续该操作，并引发 GiveFeedback 事件，同时产生新效果，以便能够设置适当的可视反馈。有关有效放置效果的列表，请参见 DragDropEffects 枚举。

注意：
DragOver 和 GiveFeedback 事件成对出现，从而当鼠标从放置目标上移动时，就能够为用户提供有关鼠标位置的最新反馈。

如果 DragAction 的值为 Drop，则放置效果值将返回到源，以便源应用程序可以对源数据执行适当的操作；例如，如果是移动操作，则剪切数据。

如果 DragAction 的值为 Cancel，则引发 DragLeave 事件。

综上所述，我们发现一个简单的拖拽过程需要处理的事件也是非常多的，做软件要细心，这除了敏锐的头脑外还需要一种态度。
开始拖拽 --> (loop (QueryContinueDrag, GiveFeedback)) --> 结束
当然这期间可能需要处理DragLeave、DragEnter、DragOver，一般情况 DragEnter 是必须的，而如果需要随时捕获鼠标在目的控件上的位置以改变目的控件的显示状态时，也需要处理DragOver事件。

DoDragDrop 会直到拖拽结束后才向下执行。这个非常关键!!

示例代码:

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

namespace WindowsFormsApplication10
{
public class Form1 : System.Windows.Forms.Form
{
private System.Windows.Forms.ListBox ListDragSource;
private System.Windows.Forms.ListBox ListDragTarget;
private System.Windows.Forms.CheckBox UseCustomCursorsCheck;
private System.Windows.Forms.Label DropLocationLabel;

private int indexOfItemUnderMouseToDrag;
private int indexOfItemUnderMouseToDrop;

private Rectangle dragBoxFromMouseDown;
private Point screenOffset;

private Cursor MyNoDropCursor;
private Cursor MyNormalCursor;

/// The main entry point for the application.
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

public Form1()
{
this.ListDragSource = new System.Windows.Forms.ListBox();
this.ListDragTarget = new System.Windows.Forms.ListBox();
this.UseCustomCursorsCheck = new System.Windows.Forms.CheckBox();
this.DropLocationLabel = new System.Windows.Forms.Label();

this.SuspendLayout();

// ListDragSource
this.ListDragSource.Items.AddRange(new object[] {"one", "two", "three", "four",
"five", "six", "seven", "eight",
"nine", "ten"});
this.ListDragSource.Location = new System.Drawing.Point(10, 17);
this.ListDragSource.Size = new System.Drawing.Size(120, 225);
this.ListDragSource.MouseDown += new System.Windows.Forms.MouseEventHandler(this.ListDragSource_MouseDown);
this.ListDragSource.QueryContinueDrag += new System.Windows.Forms.QueryContinueDragEventHandler(this.ListDragSource_QueryContinueDrag);
this.ListDragSource.MouseUp += new System.Windows.Forms.MouseEventHandler(this.ListDragSource_MouseUp);
this.ListDragSource.MouseMove += new System.Windows.Forms.MouseEventHandler(this.ListDragSource_MouseMove);
this.ListDragSource.GiveFeedback += new System.Windows.Forms.GiveFeedbackEventHandler(this.ListDragSource_GiveFeedback);

// ListDragTarget
this.ListDragTarget.AllowDrop = true;
this.ListDragTarget.Location = new System.Drawing.Point(154, 17);
this.ListDragTarget.Size = new System.Drawing.Size(120, 225);
this.ListDragTarget.DragOver += new System.Windows.Forms.DragEventHandler(this.ListDragTarget_DragOver);
this.ListDragTarget.DragDrop += new System.Windows.Forms.DragEventHandler(this.ListDragTarget_DragDrop);
this.ListDragTarget.DragEnter += new System.Windows.Forms.DragEventHandler(this.ListDragTarget_DragEnter);
this.ListDragTarget.DragLeave += new System.EventHandler(this.ListDragTarget_DragLeave);

// UseCustomCursorsCheck
this.UseCustomCursorsCheck.Location = new System.Drawing.Point(10, 243);
this.UseCustomCursorsCheck.Size = new System.Drawing.Size(137, 24);
this.UseCustomCursorsCheck.Text = "Use Custom Cursors";

// DropLocationLabel
this.DropLocationLabel.Location = new System.Drawing.Point(154, 245);
this.DropLocationLabel.Size = new System.Drawing.Size(137, 24);
this.DropLocationLabel.Text = "None";

// Form1
this.ClientSize = new System.Drawing.Size(292, 270);
this.Controls.AddRange(new System.Windows.Forms.Control[] {this.ListDragSource,
this.ListDragTarget, this.UseCustomCursorsCheck,
this.DropLocationLabel});
this.Text = "drag-and-drop Example";

this.ResumeLayout(false);

}

private void ListDragSource_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
// Get the index of the item the mouse is below.
indexOfItemUnderMouseToDrag = ListDragSource.IndexFromPoint(e.X, e.Y);

if (indexOfItemUnderMouseToDrag != ListBox.NoMatches)
{

// Remember the point where the mouse down occurred. The DragSize indicates
// the size that the mouse can move before a drag event should be started.
Size dragSize = SystemInformation.DragSize;

// Create a rectangle using the DragSize, with the mouse position being
// at the center of the rectangle.
dragBoxFromMouseDown = new Rectangle(new Point(e.X - (dragSize.Width / 2),
e.Y - (dragSize.Height / 2)), dragSize);
}
else
// Reset the rectangle if the mouse is not over an item in the ListBox.
dragBoxFromMouseDown = Rectangle.Empty;

}

private void ListDragSource_MouseUp(object sender, System.Windows.Forms.MouseEventArgs e)
{
// Reset the drag rectangle when the mouse button is raised.
dragBoxFromMouseDown = Rectangle.Empty;
}

private void ListDragSource_MouseMove(object sender, System.Windows.Forms.MouseEventArgs e)
{
//注意这里的严密性, 最好不使用e.Button == MouseButtons.Left, 因为e.Button还可能具有其他的属性
if ((e.Button & MouseButtons.Left) == MouseButtons.Left)
{
// If the mouse moves outside the rectangle, start the drag.
//这里做得太好了，防止意外的拖拽，不是说鼠标动了一下就要拖拽!
if (dragBoxFromMouseDown != Rectangle.Empty &&
!dragBoxFromMouseDown.Contains(e.X, e.Y))
{
// Create custom cursors for the drag-and-drop operation.
try
{
MyNormalCursor = new Cursor("3dwarro.cur");
MyNoDropCursor = new Cursor("3dwno.cur");
}
catch
{
// An error occurred while attempting to load the cursors, so use
// standard cursors.
UseCustomCursorsCheck.Checked = false;
}
finally
{
// The screenOffset is used to account for any desktop bands
// that may be at the top or left side of the screen when
// determining when to cancel the drag drop operation.
screenOffset = SystemInformation.WorkingArea.Location;

// Proceed with the drag-and-drop, passing in the list item.
DragDropEffects dropEffect = ListDragSource.DoDragDrop(ListDragSource.Items[indexOfItemUnderMouseToDrag], DragDropEffects.All | DragDropEffects.Link);
//拖拽结束后继续执行!
// If the drag operation was a move then remove the item.
if (dropEffect == DragDropEffects.Move)
{
ListDragSource.Items.RemoveAt(indexOfItemUnderMouseToDrag);

// Selects the previous item in the list as long as the list has an item.
if (indexOfItemUnderMouseToDrag > 0)
ListDragSource.SelectedIndex = indexOfItemUnderMouseToDrag - 1;

else if (ListDragSource.Items.Count > 0)
// Selects the first item.
ListDragSource.SelectedIndex = 0;
}

// Dispose of the cursors since they are no longer needed.
if (MyNormalCursor != null)
MyNormalCursor.Dispose();

if (MyNoDropCursor != null)
MyNoDropCursor.Dispose();
}
}
}
}
private void ListDragSource_GiveFeedback(object sender, System.Windows.Forms.GiveFeedbackEventArgs e)
{
// Use custom cursors if the check box is checked.
if (UseCustomCursorsCheck.Checked)
{
// Sets the custom cursor based upon the effect.
e.UseDefaultCursors = false;
if ((e.Effect & DragDropEffects.Move) == DragDropEffects.Move)
Cursor.Current = MyNormalCursor;
else
Cursor.Current = MyNoDropCursor;
}
}
private void ListDragTarget_DragOver(object sender, System.Windows.Forms.DragEventArgs e)
{
// Determine whether string data exists in the drop data. If not, then
// the drop effect reflects that the drop cannot occur.
if (!e.Data.GetDataPresent(typeof(System.String)))
{
e.Effect = DragDropEffects.None;
DropLocationLabel.Text = "None - no string data.";
return;
}
// Set the effect based upon the KeyState.
if ((e.KeyState & (8 + 32)) == (8 + 32) &&
(e.AllowedEffect & DragDropEffects.Link) == DragDropEffects.Link)
{
// KeyState 8 + 32 = CTL + ALT
// Link drag-and-drop effect.
e.Effect = DragDropEffects.Link;
}
else if ((e.KeyState & 32) == 32 &&
(e.AllowedEffect & DragDropEffects.Link) == DragDropEffects.Link)
{
// ALT KeyState for link.
e.Effect = DragDropEffects.Link;
}
else if ((e.KeyState & 4) == 4 &&
(e.AllowedEffect & DragDropEffects.Move) == DragDropEffects.Move)
{
// SHIFT KeyState for move.
e.Effect = DragDropEffects.Move;
}
else if ((e.KeyState & 8) == 8 &&
(e.AllowedEffect & DragDropEffects.Copy) == DragDropEffects.Copy)
{
// CTL KeyState for copy.
e.Effect = DragDropEffects.Copy;
}
else if ((e.AllowedEffect & DragDropEffects.Move) == DragDropEffects.Move)
{
// By default, the drop action should be move, if allowed.
e.Effect = DragDropEffects.Move;
}
else
e.Effect = DragDropEffects.None;

// Get the index of the item the mouse is below.

// The mouse locations are relative to the screen, so they must be
// converted to client coordinates.

indexOfItemUnderMouseToDrop =
ListDragTarget.IndexFromPoint(ListDragTarget.PointToClient(new Point(e.X, e.Y)));

// Updates the label text.
if (indexOfItemUnderMouseToDrop != ListBox.NoMatches)
{
DropLocationLabel.Text = "Drops before item #" + (indexOfItemUnderMouseToDrop + 1);
}
else
DropLocationLabel.Text = "Drops at the end.";

}
private void ListDragTarget_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
// Ensure that the list item index is contained in the data.
if (e.Data.GetDataPresent(typeof(System.String)))
{
Object item = (object)e.Data.GetData(typeof(System.String));
// Perform drag-and-drop, depending upon the effect.
if (e.Effect == DragDropEffects.Copy ||
e.Effect == DragDropEffects.Move)
{
// Insert the item.
if (indexOfItemUnderMouseToDrop != ListBox.NoMatches)
ListDragTarget.Items.Insert(indexOfItemUnderMouseToDrop, item);
else
ListDragTarget.Items.Add(item);
}
}
// Reset the label text.
DropLocationLabel.Text = "None";
}
private void ListDragSource_QueryContinueDrag(object sender, System.Windows.Forms.QueryContinueDragEventArgs e)
{

// Cancel the drag if the mouse moves off the form.
ListBox lb = sender as ListBox;
if (lb != null)
{
Form f = lb.FindForm();
// Cancel the drag if the mouse moves off the form. The screenOffset
// takes into account any desktop bands that may be at the top or left
// side of the screen.
if (((Control.MousePosition.X - screenOffset.X) < f.DesktopBounds.Left) ||
((Control.MousePosition.X - screenOffset.X) > f.DesktopBounds.Right) ||
((Control.MousePosition.Y - screenOffset.Y) < f.DesktopBounds.Top) ||
((Control.MousePosition.Y - screenOffset.Y) > f.DesktopBounds.Bottom))
{

e.Action = DragAction.Cancel;
}
}
}
private void ListDragTarget_DragEnter(object sender, System.Windows.Forms.DragEventArgs e)
{
// Reset the label text.
DropLocationLabel.Text = "None";
}
private void ListDragTarget_DragLeave(object sender, System.EventArgs e)
{
// Reset the label text.
DropLocationLabel.Text = "None";
}
}

}