如何：在 Catch 块中使用特定异常
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 emlevelValue
End Get
Set
emlevelValue = Value
End Set
End Property
Private emlevelValue As Integer = 0
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
Public Shared Sub Main()
Try
Dim o As Object = 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.Message)
End Try
End Sub
End Class
using System;
public class Employee
{
//Create employee level property.
public int Emlevel
{
get
{
return(emlevel);
}
set
{
emlevel = value;
}
}
private int emlevel = 0;
}
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.Message);
}
}
}
using namespace System;
public ref class Employee
{
public:
Employee()
{
emlevel = 0;
}
//Create employee level property.
property int Emlevel
{
int get()
{
return emlevel;
}
void set(int value)
{
emlevel = value;
}
}
private:
int emlevel;
};
public ref class Ex13
{
public:
static void PromoteEmployee(Object^ emp)
{
//Cast object to Employee.
Employee^ e = (Employee^) emp;
// Increment employee level.
e->Emlevel++;
}
static void Main()
{
try
{
Object^ o = gcnew Employee();
DateTime^ newyears = gcnew 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->Message);
}
}
};
int main()
{
Ex13::Main();
}
公共语言运行时捕捉 Catch 块没有捕捉的异常。 根据运行时的配置，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。