The Try Statement

The Try statement provides a mechanism for catching exceptions that occur during execution of a nested statements block. The Try statement also enables the user to specify a block of statements that are always executed when control leaves the Try statement.

try-statement:

Try block catch-clauses End Try

Try block finally-clause End Try

Try block catch-clauses finally-clause End Try

catch-clauses:

Except specific-catch-clauses else-catch-clauseopt

Except block

specific-catch-clauses:

specific-catch-clause

specific-catch-clauses specific-catch-clause

specific-catch-clause:

On exception-variable-declarationopt class-type Do

Beginopt block Endopt;

exception-variable-declaration:

identifier :

else-catch-clause:

Else

Beginopt block Endopt;

finally-clause:

Finally block

There are three possible forms of the Try statement:

A Try statements block followed by one or more Except statements blocks.
A Try statements block followed by a Finally statements block.
A Try statements block followed by one or several Except statements blocks followed by a Finally statements block.

If a handler type is specified for the Except block, this type must derive from System.Exception.

If both the handler type and identifier are specified in the Except block, an exception variable of the given name and type is declared. The exception variable is similar to a local variable with a scope that includes the statement block in the Except block. During the handler execution, the exception variable represents the exception currently being handled.

The Except handler without the specified handler type is named a general purpose handler. The Try statement may contain only one general purpose handler, and this handler is to be the last in the exception handler chain.

Example

Imports System;
Public Class TestException
    Public Sub Test(a, b: integer);
    Begin
        //Procedure code
        //Test ratio of parameters A and B
        If <Test condition> Then
            Raise New ArgumentException("Invalid ratio of parameters A and B");
        //Testing valid range of parameter values
        Elseif <Test condition> Then
            Raise New ArgumentOutOfRangeException("Parameter values are outside the valid range");
        Else
            //Main procedure code executed if
            //all parameters tests are passed
        End If;
    End Sub Test;
End Class TestException;

Public Class Program
    Public Sub Main();
    Var
        Obj: TestException = New TestException();
    Begin
        Try
            //Execute the Test procedure with some parameter values
            Obj.Test(<Parameter values>);
        Except On Ex: ArgumentException Do
            Begin
                //Handle exception
                //when ratio of parameters A and B is invalid
            End;
        On Ex: ArgumentOutOfRangeException Do
            Begin
                //Handle exception
                //when values of parameters A and B lie outside the valid range
            End;
        Else
            Begin
                //Handle all other exceptions
            End;
        Finally
            //Always executable block
        End Try;
    End Sub Main;
End Class Program;