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How to: Use Specific Exceptions in a Catch Block

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When an exception occurs, it is passed up the stack and each catch block is given the opportunity to handle it. The order of catch statements is important. Put catch blocks targeted to specific exceptions before a general exception catch block or the compiler might issue an error. The proper catch block is determined by matching the type of the exception to the name of the exception specified in the catch block. If there is no specific catch block, the exception is caught by a general catch block, if one exists.

The following code example uses a try/catch block to catch an InvalidCastException. The sample creates a class called Employee with a single property, employee level (Emlevel). A method, PromoteEmployee, takes an object and increments the employee level. An InvalidCastException occurs when a DateTime instance is passed to the PromoteEmployee method.

Example

Imports System
Public Class Employee
   'Create employee level property.
   
   Public Property Emlevel() As Integer
      Get
         Return emlevel
      End Get
      Set
         emlevel = value
      End Set
   End Property
   Private emlevel As Integer
End Class 
 
Public Class Ex13
   
   Public Shared Sub PromoteEmployee(emp As [Object])
      'Cast object to Employee.
      Dim e As Employee = CType(emp, Employee)
      ' Increment employee level.
      e.Emlevel = e.Emlevel + 1
   End Sub 'PromoteEmployee
   
   Public Shared Sub Main()
      Try
         Dim o = New Employee()
         Dim newyears As New DateTime(2001, 1, 1)
         'Promote the new employee.
         PromoteEmployee(o)
         'Promote DateTime; results in InvalidCastException as newyears is not an employee instance.
         PromoteEmployee(newyears)
      Catch e As InvalidCastException
         Console.WriteLine(("Error passing data to PromoteEmployee method. " + e))
      End Try
   End Sub 'Main
End Class 'Ex13

using System;
public class Employee
{
   //Create employee level property.
   public int Emlevel
   {
      get
         {
         return(emlevel);
         }
      set
         {
         emlevel = value;
         }
   }
   int emlevel;
}

public class Ex13 
{
   public static void PromoteEmployee(Object emp)
   {
   //Cast object to Employee.
   Employee e = (Employee) emp;
   // Increment employee level.
   e.Emlevel = e.Emlevel + 1;
   }

   public static void Main()
   {
   try
      {
   Object o = new Employee();
   DateTime newyears = new DateTime(2001, 1, 1);
   //Promote the new employee.
   PromoteEmployee(o);
   //Promote DateTime; results in InvalidCastException as newyears is not an employee instance.
   PromoteEmployee(newyears);
      }
   catch (InvalidCastException e)
      {
      Console.WriteLine("Error passing data to PromoteEmployee method. " + e);
      }
   }
}

The common language runtime catches exceptions that are not caught by a catch block. Depending on how the runtime is configured, either a debug dialog box appears, or the program stops executing and a dialog box with exception information appears.