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VB.NET Conditional Block Statements
VB.NET Conditional Block StatementsThe supporting VB.NET Conditional Block Statements are Else, If…Then…Else, On Error, Select Case, Then, Try...Catch...Finally Else StatementIntroduces a group of statements to be run or compiled if no other conditional group of statements has been run or compiled. RemarksThe Else keyword can be used in these contexts: See alsoIf...Then...Else StatementConditionally executes a group of statements, depending on the value of an expression. Syntax' Multiline syntax:
If condition [ Then ]
[ statements ]
[ ElseIf elseifcondition [ Then ]
[ elseifstatements ] ]
[ Else
[ elsestatements ] ]
End If
' Single-line syntax:
If condition Then [ statements ] [ Else [ elsestatements ] ]
Quick links to example codeThis article includes several examples that illustrate uses of the If...Then...Else statement: Parts
condition If the expression is a Nullable Boolean variable that evaluates to Nothing, the condition is treated as if the expression is False and the Else block is executed.
Then
statements
elseifcondition
elseifstatements
elsestatements
End If RemarksMultiline syntaxWhen an If...Then...Else statement is encountered, condition is tested. If condition is True, the statements following Then are executed. If condition is False, each ElseIf statement (if there are any) is evaluated in order. When a True elseifcondition is found, the statements immediately following the associated ElseIf are executed. If no elseifcondition evaluates to True, or if there are no ElseIf statements, the statements following Else are executed. After executing the statements following Then, ElseIf, or Else, execution continues with the statement following End If. The ElseIf and Else clauses are both optional. You can have as many ElseIf clauses as you want in an If...Then...Else statement, but no ElseIf clause can appear after an Else clause. If...Then...Else statements can be nested within each other. In the multiline syntax, the If statement must be the only statement on the first line. The ElseIf, Else, and End If statements can be preceded only by a line label. The If...Then...Else block must end with an End If statement. Tip The Select...Case Statement might be more useful when you evaluate a single expression that has several possible values. Single-Line syntaxYou can use the single-line syntax for a single condition with code to execute if it's true. However, the multiple-line syntax provides more structure and flexibility and is easier to read, maintain, and debug. What follows the Then keyword is examined to determine whether a statement is a single-line If. If anything other than a comment appears after Then on the same line, the statement is treated as a single-line If statement. If Then is absent, it must be the start of a multiple-line If...Then...Else. In the single-line syntax, you can have multiple statements executed as the result of an If...Then decision. All statements must be on the same line and be separated by colons. See also
On Error StatementEnables an error-handling routine and specifies the location of the routine within a procedure; can also be used to disable an error-handling routine. The On Error statement is used in unstructured error handling and can be used instead of structured exception handling. Structured exception handling is built into .NET, is generally more efficient, and so is recommended when handling runtime errors in your application. Without error handling or exception handling, any run-time error that occurs is fatal: an error message is displayed, and execution stops. Note The Error keyword is also used in the Error Statement, which is supported for backward compatibility. Syntax
VB
On Error { GoTo [ line | 0 | -1 ] | Resume Next }
Parts
RemarksNote We recommend that you use structured exception handling in your code whenever possible, rather than using unstructured exception handling and the On Error statement. For more information, see Try...Catch...Finally Statement. An "enabled" error handler is one that is turned on by an On Error statement. An "active" error handler is an enabled handler that is in the process of handling an error. If an error occurs while an error handler is active (between the occurrence of the error and a Resume, Exit Sub, Exit Function, or Exit Property statement), the current procedure's error handler cannot handle the error. Control returns to the calling procedure. If the calling procedure has an enabled error handler, it is activated to handle the error. If the calling procedure's error handler is also active, control passes back through previous calling procedures until an enabled, but inactive, error handler is found. If no such error handler is found, the error is fatal at the point at which it actually occurred. Each time the error handler passes control back to a calling procedure, that procedure becomes the current procedure. Once an error is handled by an error handler in any procedure, execution resumes in the current procedure at the point designated by the Resume statement. Note An error-handling routine is not a Sub procedure or a Function procedure. It is a section of code marked by a line label or a line number. Number PropertyError-handling routines rely on the value in the Number property of the Err object to determine the cause of the error. The routine should test or save relevant property values in the Err object before any other error can occur or before a procedure that might cause an error is called. The property values in the Err object reflect only the most recent error. The error message associated with Err.Number is contained in Err.Description. Throw StatementAn error that is raised with the Err.Raise method sets the Exception property to a newly created instance of the Exception class. In order to support the raising of exceptions of derived exception types, a Throw statement is supported in the language. This takes a single parameter that is the exception instance to be thrown. The following example shows how these features can be used with the existing exception handling support:
VB
On Error GoTo Handler
Throw New DivideByZeroException()
Handler:
If (TypeOf Err.GetException() Is DivideByZeroException) Then
' Code for handling the error is entered here.
End If
Notice that the On Error GoTo statement traps all errors, regardless of the exception class. On Error Resume NextOn Error Resume Next causes execution to continue with the statement immediately following the statement that caused the run-time error, or with the statement immediately following the most recent call out of the procedure containing the On Error Resume Next statement. This statement allows execution to continue despite a run-time error. You can place the error-handling routine where the error would occur rather than transferring control to another location within the procedure. An On Error Resume Next statement becomes inactive when another procedure is called, so you should execute an On Error Resume Next statement in each called routine if you want inline error handling within that routine. Note The On Error Resume Next construct may be preferable to On Error GoTo when handling errors generated during access to other objects. Checking Err after each interaction with an object removes ambiguity about which object was accessed by the code. You can be sure which object placed the error code in Err.Number, as well as which object originally generated the error (the object specified in Err.Source). On Error GoTo 0On Error GoTo 0 disables error handling in the current procedure. It doesn't specify line 0 as the start of the error-handling code, even if the procedure contains a line numbered 0. Without an On Error GoTo 0 statement, an error handler is automatically disabled when a procedure is exited. On Error GoTo -1On Error GoTo -1 disables the exception in the current procedure. It does not specify line -1 as the start of the error-handling code, even if the procedure contains a line numbered -1. Without an On Error GoTo -1 statement, an exception is automatically disabled when a procedure is exited. To prevent error-handling code from running when no error has occurred, place an Exit Sub, Exit Function, or Exit Property statement immediately before the error-handling routine, as in the following fragment:
VB
Public Sub InitializeMatrix(ByVal Var1 As Object, ByVal Var2 As Object) On Error GoTo ErrorHandler ' Insert code that might generate an error here Exit Sub ErrorHandler: ' Insert code to handle the error here Resume Next End Sub Here, the error-handling code follows the Exit Sub statement and precedes the End Sub statement to separate it from the procedure flow. You can place error-handling code anywhere in a procedure. Untrapped ErrorsUntrapped errors in objects are returned to the controlling application when the object is running as an executable file. Within the development environment, untrapped errors are returned to the controlling application only if the proper options are set. See your host application's documentation for a description of which options should be set during debugging, how to set them, and whether the host can create classes. If you create an object that accesses other objects, you should try to handle any unhandled errors they pass back. If you cannot, map the error codes in Err.Number to one of your own errors and then pass them back to the caller of your object. You should specify your error by adding your error code to the VbObjectError constant. For example, if your error code is 1052, assign it as follows:
VB
Err.Number = vbObjectError + 1052 Caution System errors during calls to Windows dynamic-link libraries (DLLs) do not raise exceptions and cannot be trapped with Visual Basic error trapping. When calling DLL functions, you should check each return value for success or failure (according to the API specifications), and in the event of a failure, check the value in the Err object's LastDLLError property. RequirementsNamespace: Microsoft.VisualBasic Assembly: Visual Basic Runtime Library (in Microsoft.VisualBasic.dll) See also
Select...Case StatementRuns one of several groups of statements, depending on the value of an expression. SyntaxSelect [ Case ] testexpression
[ Case expressionlist
[ statements ] ]
[ Case Else
[ elsestatements ] ]
End Select
Parts
RemarksIf testexpression matches any Case expressionlist clause, the statements following that Case statement run up to the next Case, Case Else, or End Select statement. Control then passes to the statement following End Select. If testexpression matches an expressionlist clause in more than one Case clause, only the statements following the first match run. The Case Else statement is used to introduce the elsestatements to run if no match is found between the testexpression and an expressionlist clause in any of the other Case statements. Although not required, it is a good idea to have a Case Else statement in your Select Case construction to handle unforeseen testexpression values. If no Case expressionlist clause matches testexpression and there is no Case Else statement, control passes to the statement following End Select. You can use multiple expressions or ranges in each Case clause. For example, the following line is valid. Case 1 To 4, 7 To 9, 11, 13, Is > maxNumber Note The Is keyword used in the Case and Case Else statements is not the same as the Is Operator, which is used for object reference comparison. You can specify ranges and multiple expressions for character strings. In the following example, Case matches any string that is exactly equal to "apples", has a value between "nuts" and "soup" in alphabetical order, or contains the exact same value as the current value of testItem. Case "apples", "nuts" To "soup", testItem The setting of Option Compare can affect string comparisons. Under Option Compare Text, the strings "Apples" and "apples" compare as equal, but under Option Compare Binary, they do not. Note A Case statement with multiple clauses can exhibit behavior known as short-circuiting. Visual Basic evaluates the clauses from left to right, and if one produces a match with testexpression, the remaining clauses are not evaluated. Short-circuiting can improve performance, but it can produce unexpected results if you are expecting every expression in expressionlist to be evaluated. For more information on short-circuiting, see Boolean Expressions. If the code within a Case or Case Else statement block does not need to run any more of the statements in the block, it can exit the block by using the Exit Select statement. This transfers control immediately to the statement following End Select. Select Case constructions can be nested. Each nested Select Case construction must have a matching End Select statement and must be completely contained within a single Case or Case Else statement block of the outer Select Case construction within which it is nested. See alsoThen StatementIntroduces a statement block to be compiled or executed if a tested condition is true. RemarksThe Then keyword can be used in these contexts: See alsoTry...Catch...Finally StatementProvides a way to handle some or all possible errors that may occur in a given block of code, while still running code. Syntax
VB
Try
[ tryStatements ]
[ Exit Try ]
[ Catch [ exception [ As type ] ] [ When expression ]
[ catchStatements ]
[ Exit Try ] ]
[ Catch ... ]
[ Finally
[ finallyStatements ] ]
End Try
Parts
RemarksIf 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. A Try…Catch statement consists of a Try block followed by one or more Catch clauses, which specify handlers for various exceptions. When an exception is thrown in a Try block, Visual Basic looks for the Catch statement that handles the exception. 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. If no Catch block is found, Visual Basic displays an unhandled exception message to the user and stops execution of the program. You can use more than one Catch statement in a Try…Catch statement. If you do this, the order of the Catch clauses is significant because they are examined in order. Catch the more specific exceptions before the less specific ones. The following Catch statement conditions are the least specific, and will catch all exceptions that derive from the Exception class. You should ordinarily use one of these variations as the last Catch block in the Try...Catch...Finally structure, after catching all the specific exceptions you expect. Control flow can never reach a Catch block that follows either of these variations.
When a Try…Catch…Finally statement is nested in another Try block, Visual Basic first examines each Catch statement in the innermost Try block. If no matching Catch statement is found, the search proceeds to the Catch statements of the outer Try…Catch…Finally block. Local variables from a Try block are not available in a Catch block because they are separate blocks. If you want to use a variable in more than one block, declare the variable outside the Try...Catch...Finally structure. Tip The Try…Catch…Finally statement is available as an IntelliSense code snippet. In the Code Snippets Manager, expand Code Patterns - If, For Each, Try Catch, Property, etc, and then Error Handling (Exceptions). For more information, see Code Snippets. Finally blockIf you have one or more statements that must run before you exit the Try structure, use a Finally block. Control passes to the Finally block just before it passes out of the Try…Catch structure. This is true even if an exception occurs anywhere inside the Try structure. A Finally block is useful for running any code that must execute even if there is an exception. Control is passed to the Finally block regardless of how the Try...Catch block exits. The code in a Finally block runs even if your code encounters a Return statement in a Try or Catch block. Control does not pass from a Try or Catch block to the corresponding Finally block in the following cases:
It is not valid to explicitly transfer execution into a Finally block. Transferring execution out of a Finally block is not valid, except through an exception. If a Try statement does not contain at least one Catch block, it must contain a Finally block. Tip If you do not have to catch specific exceptions, the Using statement behaves like a Try…Finally block, and guarantees disposal of the resources, regardless of how you exit the block. This is true even with an unhandled exception. For more information, see Using Statement. Exception argumentThe Catch block exception argument is an instance of the Exception class or a class that derives from the Exception class. The Exception class instance corresponds to the error that occurred in the Try block. The properties of the Exception object help to identify the cause and location of an exception. For example, the StackTrace property lists the called methods that led to the exception, helping you find where the error occurred in the code. Message returns a message that describes the exception. HelpLink returns a link to an associated Help file. InnerException returns the Exception object that caused the current exception, or it returns Nothing if there is no original Exception. Considerations when using a Try…Catch statementUse a Try…Catch statement only to signal the occurrence of unusual or unanticipated program events. Reasons for this include the following:
You do not always need a Try…Catch statement to check for a condition that is likely to occur. The following example checks whether a file exists before trying to open it. This reduces the need for catching an exception thrown by the OpenText method.
VB
Private Sub TextFileExample(ByVal filePath As String)
' Verify that the file exists.
If System.IO.File.Exists(filePath) = False Then
Console.Write("File Not Found: " & filePath)
Else
' Open the text file and display its contents.
Dim sr As System.IO.StreamReader =
System.IO.File.OpenText(filePath)
Console.Write(sr.ReadToEnd)
sr.Close()
End If
End Sub
Ensure that code in Catch blocks can properly report exceptions to users, whether through thread-safe logging or appropriate messages. Otherwise, exceptions might remain unknown. Async methodsIf you mark a method with the Async modifier, you can use the Await operator in the method. A statement with the Await operator suspends execution of the method until the awaited task completes. The task represents ongoing work. When the task that's associated with the Await operator finishes, execution resumes in the same method. For more information, see Control Flow in Async Programs. A task returned by an Async method may end in a faulted state, indicating that it completed due to an unhandled exception. A task may also end in a canceled state, which results in an OperationCanceledException being thrown out of the await expression. To catch either type of exception, place the Await expression that's associated with the task in a Try block, and catch the exception in the Catch block. An example is provided later in this topic. A task can be in a faulted state because multiple exceptions were responsible for its faulting. For example, the task might be the result of a call to Task.WhenAll. When you await such a task, the caught exception is only one of the exceptions, and you can't predict which exception will be caught. An example is provided later in this topic. An Await expression can't be inside a Catch block or Finally block. IteratorsAn iterator function or Get accessor performs a custom iteration over a collection. An iterator uses a Yield statement to return each element of the collection one at a time. You call an iterator function by using a For Each...Next Statement. A Yield statement can be inside a Try block. A Try block that contains a Yield statement can have Catch blocks, and can have a Finally block. See the "Try Blocks in Visual Basic" section of Iterators for an example. A Yield statement cannot be inside a Catch block or a Finally block. If the For Each body (outside of the iterator function) throws an exception, a Catch block in the iterator function is not executed, but a Finally block in the iterator function is executed. A Catch block inside an iterator function catches only exceptions that occur inside the iterator function. Partial-trust situationsIn partial-trust situations, such as an application hosted on a network share, Try...Catch...Finally does not catch security exceptions that occur before the method that contains the call is invoked. The following example, when you put it on a server share and run from there, produces the error "System.Security.SecurityException: Request Failed." For more information about security exceptions, see the SecurityException class.
VB
Try
Process.Start("http://www.microsoft.com")
Catch ex As Exception
MsgBox("Can't load Web page" & vbCrLf & ex.Message)
End Try
In such a partial-trust situation, you have to put the Process.Start statement in a separate Sub. The initial call to the Sub will fail. This enables Try...Catch to catch it before the Sub that contains Process.Start is started and the security exception produced. Exception in a method called from a Try blockIn the following example, the CreateException method throws a NullReferenceException. The code that generates the exception is not in a Try block. Therefore, the CreateException method does not handle the exception. The RunSample method does handle the exception because the call to the CreateException method is in a Try block. See also
While...End While StatementRuns a series of statements as long as a given condition is True. SyntaxWhile condition
[ statements ]
[ Continue While ]
[ statements ]
[ Exit While ]
[ statements ]
End While
Parts
RemarksUse a While...End While structure when you want to repeat a set of statements an indefinite number of times, as long as a condition remains True. If you want more flexibility with where you test the condition or what result you test it for, you might prefer the Do...Loop Statement. If you want to repeat the statements a set number of times, the For...Next Statement is usually a better choice. Note The While keyword is also used in the Do...Loop Statement, the Skip While Clause and the Take While Clause. If condition is True, all of the statements run until the End While statement is encountered. Control then returns to the While statement, and condition is again checked. If condition is still True, the process is repeated. If it’s False, control passes to the statement that follows the End While statement. The While statement always checks the condition before it starts the loop. Looping continues while the condition remains True. If condition is False when you first enter the loop, it doesn’t run even once. The condition usually results from a comparison of two values, but it can be any expression that evaluates to a Boolean Data Type value (True or False). This expression can include a value of another data type, such as a numeric type, that has been converted to Boolean. You can nest While loops by placing one loop within another. You can also nest different kinds of control structures within one another. For more information, see Nested Control Structures. Exit WhileThe Exit While statement can provide another way to exit a While loop. Exit While immediately transfers control to the statement that follows the End While statement. You typically use Exit While after some condition is evaluated (for example, in an If...Then...Else structure). You might want to exit a loop if you detect a condition that makes it unnecessary or impossible to continue iterating, such as an erroneous value or a termination request. You can use Exit While when you test for a condition that could cause an endless loop, which is a loop that could run an extremely large or even infinite number of times. You can then use Exit While to escape the loop. You can place any number of Exit While statements anywhere in the While loop. When used within nested While loops, Exit While transfers control out of the innermost loop and into the next higher level of nesting. The Continue While statement immediately transfers control to the next iteration of the loop. For more information, see Continue Statement. See also
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