如何使用 Catch 區塊中的特定例外狀況

一般而言，比起使用基本 catch 陳述式，攔截特定例外狀況類型會是更佳的程式設計做法。

發生例外狀況時，該例外狀況會在堆疊中向上傳遞，讓每個 catch 區塊都有機會處理。 Catch 陳述式的順序很重要。 請將針對特定例外狀況的 catch 區塊放在一般例外狀況的 catch 區塊之前，否則編譯器可能會發出錯誤。 藉由比對 catch 區塊中指定的例外狀況類型與例外狀況名稱，即可決定正確的 catch 區塊。 如果沒有特定 catch 區塊，則會由一般 catch 區塊 (如果有的話) 攔截例外狀況。

下列程式碼範例使用 try/catch 區塊來攔截 InvalidCastException。 此範例會建立具有員工層級 (Emlevel) 之單一屬性的類別，稱為 Employee。 PromoteEmployee 方法會採用一個物件並遞增員工層級。 InvalidCastException 會在 DateTime 執行個體傳遞給 PromoteEmployee 方法時發生。

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();
}

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.
        var e = (Employee) emp;
        // Increment employee level.
        e.Emlevel = e.Emlevel + 1;
    }

    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);
        }
    }
}

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

另請參閱

• 例外狀況