//переменные класса 
private bool isDragging = false;
private Point oldPos;
 //в конструкторе 
this.MouseDown += new MouseEventHandler(MyForm_MouseDown);
this.MouseMove += new MouseEventHandler(MyForm_MouseMove);
this.MouseUp += new MouseEventHandler(MyForm_MouseUp);
 //методы 
private void MyForm_MouseDown(object sender, MouseEventArgs e)
{
 this.isDragging = true;
 this.oldPos = new Point();
 this.oldPos.X = e.X;
 this.oldPos.Y = e.Y;
}
private void MyForm_MouseMove(object sender, MouseEventArgs e)
{
 if(this.isDragging)
 {
  Point tmp = new Point(this.Location.X, this.Location.Y);
  tmp.X += e.X - this.oldPos.X;
  tmp.Y += e.Y - this.oldPos.Y;
  this.Location = tmp;
 }
}
private void MyForm_MouseUp(object sender, MouseEventArgs e)
{
 this.isDragging = false;
}

if (e.Data.GetDataPresent(DataFormats.Text))
	e.Effect = DragDropEffects.Copy;

string str = e.Data.GetData(DataFormats.Text).ToString();
TextBox.Text = str;//присвоить значение строки