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UNIT4.2(VB).pptx

  1. 1. Handling Mouse Events Mouse events occur with mouse movements in forms and controls. Following are the various mouse events related with a Control class − MouseDown − it occurs when a mouse button is pressed MouseEnter − it occurs when the mouse pointer enters the control MouseHover − it occurs when the mouse pointer hovers over the control MouseLeave − it occurs when the mouse pointer leaves the control MouseMove − it occurs when the mouse pointer moves over the control
  2. 2. • MouseUp − it occurs when the mouse pointer is over the control and the mouse button is released • MouseWheel − it occurs when the mouse wheel moves and the control has focus
  3. 3. • The MouseEventArgs object is used for handling mouse events. It has the following properties − • Buttons − indicates the mouse button pressed • Clicks − indicates the number of clicks • Delta − indicates the number of detents the mouse wheel rotated • X − indicates the x-coordinate of mouse click • Y − indicates the y-coordinate of mouse click
  4. 4. Handling Keyboard Events • Following are the various keyboard events related with a Control class − • KeyDown − occurs when a key is pressed down and the control has focus • KeyPress − occurs when a key is pressed and the control has focus • KeyUp − occurs when a key is released while the control has focus
  5. 5. properties − • Alt − it indicates whether the ALT key is pressed • Control − it indicates whether the CTRL key is pressed • Handled − it indicates whether the event is handled • KeyCode − stores the keyboard code for the event • KeyData − stores the keyboard data for the event
  6. 6. • KeyValue − stores the keyboard value for the event • Modifiers − it indicates which modifier keys (Ctrl, Shift, and/or Alt) are pressed • Shift − it indicates if the Shift key is pressed
  7. 7. Exception Handling • An exception is a problem that arises during the execution of a program. An exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero. • Exceptions provide a way to transfer control from one part of a program to another. VB.Net exception handling is built upon four keywords - Try, Catch, Finally and Throw.
  8. 8. • Try − A Try block identifies a block of code for which particular exceptions will be activated. It's followed by one or more Catch blocks. • Catch − A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The Catch keyword indicates the catching of an exception. • Finally − The Finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For example, if you open a file, it must be closed whether an exception is raised or not. • Throw − A program throws an exception when a problem shows up. This is done using a Throw keyword.
  9. 9. Exception Class Description System.IO.IOException Handles I/O errors. System.IndexOutOfRangeException Handles errors generated when a method refers to an array index out of range. System.ArrayTypeMismatchException Handles errors generated when type is mismatched with the array type. System.NullReferenceException Handles errors generated from deferencing a null object. System.DivideByZeroException Handles errors generated from dividing a dividend with zero. System.InvalidCastException Handles errors generated during typecasting. System.OutOfMemoryException Handles errors generated from insufficient free memory. System.StackOverflowException Handles errors generated from stack overflow.

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