The following Catch statement conditions are the least specific, and will catch all exceptions that derive from the Exception class. Catch the more specific exceptions before the less specific ones. If you do this, the order of the Catch clauses is significant because they are examined in order. You can use more than one Catch statement in a Try…Catch statement. If no Catch block is found, Visual Basic displays an unhandled exception message to the user and stops execution of the program. If a matching Catch statement is not found, Visual Basic examines the method that called the current method, and so on up the call stack. When an exception is thrown in a Try block, Visual Basic looks for the Catch statement that handles the exception. If you expect that a particular exception might occur during a particular section of code, put the code in a Try block and use a Catch block to retain control and handle the exception if it occurs.Ī Try…Catch statement consists of a Try block followed by one or more Catch clauses, which specify handlers for various exceptions. Statement(s) that are executed after all other error processing has occurred. A Finally block is always executed when execution leaves any part of the Try.Catch statement. The Finally statement will still be executed. Execution resumes with the code immediately following the End Try statement. Keyword that breaks out of the structure. Statement(s) to handle errors that occur in the associated Try block. Used with When keyword to specify circumstances under which the error is caught. Typically used to filter by error number. Any expression that describes a generic filter. Must be implicitly convertible to Boolean. A When clause is applied only after checking the type of the exception, and expression may refer to the identifier representing the exception. A Catch statement with a When clause catches exceptions only when expression evaluates to True. If the value of exception is of the type specified by type or of a derived type, the identifier becomes bound to the exception object. If omitted, the Catch statement catches any exception. Used with Catch to specify the error caught. The initial value of exception is the value of the thrown error. If an exception occurs when processing the Try block, each Catch statement is examined in textual order to determine whether it handles the exception, with exception representing the exception that has been thrown. Programmers familiar with languages where a "while" loop uses "end," or languages limited to closing a "while" loop using parenthetical-type containers, will want to take note of the "wend" command.Provides a way to handle some or all possible errors that may occur in a given block of code, while still running code. "Wend" is used as an "end" command for a "while" loop in Visual Basic.
The familiar "if/then/else," "while" and "for each/next" arguments are all available for use in Visual Basic programming. The basic use of loops and conditional arguments in Visual Basic programming is largely identical to other modern programming languages. The command "Output = MyString like MyPattern" will return a "true" value to Output if MyString adheres to the pattern defined by MyPattern. "Like" compares a string against a pattern. The command "Output = YesNoA eqv YesNoB" will set the variable Output "true" if both YesNoA and YesNoB are true. "Eqv" performs a comparison against two logical variables. Other Visual Basic commands categorized as operators: Comparative operators such as "and" and "or" are also used the same as you'd see them in other languages. Mathematical symbols such as "+", "-", "/" and "*" serve their usual purpose. "Public" defines a variable that can be used externally (i.e., by procedures other than the one in which it is created).īasic math functionality falls under the "operators" category. (I.e., it does not lose its value every time the particular procedure is ended.) "Static" is similar to what might be called a "global" in other languages a "static" variable does not lose its assigned value until the entire program has been terminated. Examples: "dim MyString as string" or "n = dim n as integer." "Dim" (and "As") is used to declare a variable. Declarations can be a stumbling point for Visual Basic beginners because much of the syntax used is unique to Visual Basic. Programming languages use commands called "declarations" to define and populate variables.
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