How to: Use Specific Exceptions in a Catch Block
Microsoft Silverlight will reach end of support after October 2021. Learn more.
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.
See Also