// 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