CS 112D Lecture Notes - Exception Handling

Exception Handling

• To catch (handle) an exception, use a try block together with a catch block.
• Try is a keyword that begins a block of code in which any exceptions that occur will be handled. If an exception occurs in a try block, the code jumps out of that try block to the appropriate catch block.
• Catch is a keyword that begins a block of code in which a class of exception is handled. The code in a catch block is executed only if an exception is thrown from within the immediately preceding try block. Each catch block specifies a specific exception class. That catch block is only executed if an Exception of that class (or a subclass of it) is thrown.
• A catch block is much like a function, with a single parameter of the exception object. This "function" is called automatically when a matching exception is thrown from the try block immediately preceding it.
• Finally is a keyword that begins a block of code after a set of catch blocks, containing code that is always executed whether or not an exception occurred. The "finally" block is optional when handling exceptions.
• If an exception is not caught by the function in which it is thrown, then the exception propagates to the calling function. In other words, exceptions travel down the call stack until they are caught. If no function catches the exception, the program terminates and displays an error message (a run-time error).
• Here are some common Exception classes you might want to handle:
  • InvalidCastException - occurs when an error occurs trying to convert data types, such as using CInt with a non-numeric string argument.
  • NullReferenceException - occurs when a reference variable storing the value Nothing has one of its members accessed, such as a method being called on it.
  • Data.ConstraintException - occurs when a UniqueConstraint is violated in a DataSet or a DataTable.
  • Data.InvalidConstraintException - occurs when a ForeignKeyConstraint is violated in a DataSet or DataTable.
  • DBConcurrencyException - occurs when a DataAdapter finds no match for the "where" clause in an update command it executed. Generally, this is because of a concurrency violation: someone else modified the data you were modifying, and their change was saved in the database before yours was.
  • System.Data.SqlClient.SqlException - occurs when a database error occurs while executing a SqlCommand object. (Such as foreign key violation in the database, not caught by DataSet)
  • System.IO.FileNotFoundException - occurs when a file cannot be opened (e.g. a text file)

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