C#

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

namespace MouseMoveLabelDemo {
  public sealed class DemoForm : Form {
    [STAThread]
    private static void Main() {
      Application.Run(new DemoForm());
    }

    private readonly Point home;
    private Label itinerantLabel;
    private Size offset;

    public DemoForm() {
      home = new Point(10, 10);
      BackColor = Color.White;

      itinerantLabel = new Label();
      itinerantLabel.Text = "Move Me";
      itinerantLabel.BackColor = Color.Pink;
      itinerantLabel.BorderStyle = BorderStyle.FixedSingle;
      itinerantLabel.Location = home;
      itinerantLabel.Parent = this;
      itinerantLabel.MouseDown += Label_MouseDown;
      itinerantLabel.MouseMove += Label_MouseMove;
      itinerantLabel.MouseCaptureChanged += Label_CaptureLost;

      SizeChanged += DemoForm_SizeChanged;
    }

    private void DemoForm_SizeChanged(object sender, EventArgs e) {
      // In this simple demo we don't want to lose the label 
      // so just send it home whenever the form is resized
      itinerantLabel.Location = home;
    }

    private void Label_MouseDown(object sender, MouseEventArgs e) {
      Label lbl = (Label)sender;
      if (e.Button == MouseButtons.Left) {
        // Mouse is automatically captured on mouse down

        // All manipulations in Screen coordinates
        Point cursorPosition = Cursor.Position;
        Point labelPosition = PointToScreen(lbl.Location);

        // calculate offset of cursor from label origin
        offset = new Size(cursorPosition.X - labelPosition.X, cursorPosition.Y - labelPosition.Y);

        // Constrain cursor within a part of the ClientRectangle
        // (x1, y1) - (x2, y2)
        Rectangle cursorClippingRectangle = Rectangle.FromLTRB(
          // x1
          offset.Width,
          // y1
          offset.Height,
          // x2
          ClientRectangle.Width - (lbl.Width - offset.Width) + 1,
          // y2
          ClientRectangle.Height - (lbl.Height - offset.Height) + 1);
        Cursor.Clip = RectangleToScreen(cursorClippingRectangle);
      }
    }

    private void Label_MouseMove(object sender, MouseEventArgs e) {
      Label lbl = (Label)sender;
      if (lbl.Capture) {
        lbl.Location = PointToClient(Cursor.Position - offset);
      }
    }

    private void Label_CaptureLost(object sender, EventArgs e) {
      Cursor.Clip = Rectangle.Empty;
    }
  }
}

/// <summary>
/// Handle Left and right keys.
/// (This effectively gives us Shortcut keys for the Select / Deselect buttons)
/// </summary>
/// <param name="msg"></param>
/// <param name="keyData"></param>
/// <returns></returns>
protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
    {
    if (!tbFilter.ContainsFocus || (ModifierKeys & Keys.Control) != 0)
        {
        // Only Handle if not in filter textbox, or if Control is used as an override.
        if (keyData == Keys.Left || keyData == (Keys.Left | Keys.Control))
            {
            butDeselectAll.PerformClick();
            return true;
            }
        if (keyData == Keys.Right || keyData == (Keys.Right | Keys.Control))
            {
            butSelectAll.PerformClick();
            return true;
            }
        }
    return base.ProcessCmdKey(ref msg, keyData);
    }