如何：在 Catch 块中使用特定异常
更新：2007 年 11 月
发生异常时，异常沿堆栈向上传递，每个 Catch 块都有机会处理它。Catch 语句的顺序很重要。将针对特定异常的 Catch 块放在常规异常 Catch 块的前面，否则编译器可能会发出错误。确定正确 Catch 块的方法是将异常的类型与 Catch 块中指定的异常名称进行匹配。如果没有特定的 Catch 块，则由可能存在的常规 Catch 块捕捉异常。
下面的代码示例使用 try/catch 块捕获 InvalidCastException。该示例创建一个名为 Employee 的类，它带有一个属性：职员级别 (Emlevel)。PromoteEmployee 方法取得对象并增加职员级别。将 DateTime 实例传递给 PromoteEmployee 方法时，发生 InvalidCastException。
示例
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 'Employee
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);
}
}
}
公共语言运行库捕捉 Catch 块没有捕捉的异常。根据运行库的配置，或者出现一个调试对话框，或者程序停止执行并出现一个包含异常信息的对话框。有关调试的信息，请参见调试和分析应用程序。