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Boxing und Unboxing (C#-Programmierhandbuch)
int i = 123; // The following line boxes i. object o = i;
o = 123; i = (int)o; // unboxing
// String.Concat example. // String.Concat has many versions. Rest the mouse pointer on // Concat in the following statement to verify that the version // that is used here takes three object arguments. Both 42 and // true must be boxed. Console.WriteLine(String.Concat("Answer", 42, true)); // List example. // Create a list of objects to hold a heterogeneous collection // of elements. List<object> mixedList = new List<object>(); // Add a string element to the list. mixedList.Add("First Group:"); // Add some integers to the list. for (int j = 1; j < 5; j++) { // Rest the mouse pointer over j to verify that you are adding // an int to a list of objects. Each element j is boxed when // you add j to mixedList. mixedList.Add(j); } // Add another string and more integers. mixedList.Add("Second Group:"); for (int j = 5; j < 10; j++) { mixedList.Add(j); } // Display the elements in the list. Declare the loop variable by // using var, so that the compiler assigns its type. foreach (var item in mixedList) { // Rest the mouse pointer over item to verify that the elements // of mixedList are objects. Console.WriteLine(item); } // The following loop sums the squares of the first group of boxed // integers in mixedList. The list elements are objects, and cannot // be multiplied or added to the sum until they are unboxed. The // unboxing must be done explicitly. var sum = 0; for (var j = 1; j < 5; j++) { // The following statement causes a compiler error: Operator // '*' cannot be applied to operands of type 'object' and // 'object'. //sum += mixedList[j] * mixedList[j]); // After the list elements are unboxed, the computation does // not cause a compiler error. sum += (int)mixedList[j] * (int)mixedList[j]; } // The sum displayed is 30, the sum of 1 + 4 + 9 + 16. Console.WriteLine("Sum: " + sum); // Output: // Answer42True // First Group: // 1 // 2 // 3 // 4 // Second Group: // 5 // 6 // 7 // 8 // 9 // Sum: 30
int i = 123;
// Boxing copies the value of i into object o. object o = i;
int i = 123; object o = (object)i; // explicit boxing
class TestBoxing { static void Main() { int i = 123; // Boxing copies the value of i into object o. object o = i; // Change the value of i. i = 456; // The change in i does not effect the value stored in o. System.Console.WriteLine("The value-type value = {0}", i); System.Console.WriteLine("The object-type value = {0}", o); } } /* Output: The value-type value = 456 The object-type value = 123 */
class TestUnboxing { static void Main() { int i = 123; object o = i; // implicit boxing try { int j = (short)o; // attempt to unbox System.Console.WriteLine("Unboxing OK."); } catch (System.InvalidCastException e) { System.Console.WriteLine("{0} Error: Incorrect unboxing.", e.Message); } } }
Specified cast is not valid. Error: Incorrect unboxing.
int j = (short) o;
int j = (int) o;
Unboxing OK.
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Überprüfen der Objektinstanz, um sicherzustellen, dass es sich um einen mittels Boxing "verpackten" Wert des jeweiligen Werttyps handelt. -
Kopieren des Werts aus der Instanz in die Werttypvariable.
int i = 123; // a value type object o = i; // boxing int j = (int)o; // unboxing
Weitere Informationen finden Sie in der C#-Sprachspezifikation. Die Sprachspezifikation ist die verbindliche Quelle für die Syntax und Verwendung von C#.
Weitere Informationen finden Sie in der C#-Sprachspezifikation. Die Sprachspezifikation ist die verbindliche Quelle für die Syntax und Verwendung von C#.
