Understanding Values and References in Microsoft Visual C#

  • 11/18/2015
Contents

Casting data safely

By using a cast, you can specify that, in your opinion, the data referenced by an object has a specific type and that it is safe to reference the object by using that type. The key phrase here is “in your opinion.” The C# compiler will not check that this is the case, but the runtime will. If the type of object in memory does not match the cast, the runtime will throw an InvalidCastException, as described in the preceding section. You should be prepared to catch this exception and handle it appropriately if it occurs.

However, catching an exception and attempting to recover in the event that the type of an object is not what you expected it to be is a rather cumbersome approach. C# provides two more very useful operators that can help you perform casting in a much more elegant manner: the is and as operators.

The is operator

You can use the is operator to verify that the type of an object is what you expect it to be, like this:

WrappedInt wi = new WrappedInt();
...
object o = wi;
if (o is WrappedInt)
{
    WrappedInt temp = (WrappedInt)o; // This is safe; o is a WrappedInt
    ...
}

The is operator takes two operands: a reference to an object on the left, and the name of a type on the right. If the type of the object referenced on the heap has the specified type, is evaluates to true; otherwise, is evaluates to false. The preceding code attempts to cast the reference to the object variable o only if it knows that the cast will succeed.

The as operator

The as operator fulfills a similar role to is but in a slightly truncated manner. You use the as operator like this:

WrappedInt wi = new WrappedInt();
...
object o = wi;
WrappedInt temp = o as WrappedInt;
if (temp != null)
{
    ...  // Cast was successful
}

Like the is operator, the as operator takes an object and a type as its operands. The runtime attempts to cast the object to the specified type. If the cast is successful, the result is returned and, in this example, is assigned to the WrappedInt variable temp. If the cast is unsuccessful, the as operator evaluates to the null value and assigns that to temp instead.

There is a little more to the is and as operators than is described here, and Chapter 12 discusses them in greater detail.

Pointers and unsafe code

This section is purely for your information and is aimed at developers who are familiar with C or C++. If you are new to programming, feel free to skip this section.

If you have already written programs in languages such as C or C++, much of the discussion in this chapter concerning object references might be familiar in that both languages have a construct that provides similar functionality: a pointer.

A pointer is a variable that holds the address of, or a reference to, an item in memory (on the heap or on the stack). A special syntax is used to identify a variable as a pointer. For example, the following statement declares the variable pi as a pointer to an integer:

int *pi;

Although the variable pi is declared as a pointer, it does not actually point anywhere until you initialize it. For example, to use pi to point to the integer variable i, you can use the following statements and the address-of operator (&), which returns the address of a variable:

int *pi;
int i = 99;
...
pi = &i;

You can access and modify the value held in the variable i through the pointer variable pi like this:

*pi = 100;

This code updates the value of the variable i to 100 because pi points to the same memory location as the variable i.

One of the main problems that developers learning C and C++ encounter is understanding the syntax used by pointers. The * operator has at least two meanings (in addition to being the arithmetic multiplication operator), and there is often great confusion about when to use & rather than *. The other issue with pointers is that it is easy to point somewhere invalid, or to forget to point somewhere at all, and then try to reference the data pointed to. The result will be either garbage or a program that fails with an error because the operating system detects an attempt to access an illegal address in memory. There is also a whole range of security flaws in many existing systems resulting from the mismanagement of pointers; some environments (not Windows) fail to enforce checks that a pointer does not refer to memory that belongs to another process, opening up the possibility that confidential data could be compromised.

Reference variables were added to C# to avoid all these problems. If you really want to, you can continue to use pointers in C#, but you must mark the code as unsafe. The unsafe keyword can be used to mark a block of code or an entire method, as shown here:

public static void Main(string [] args)
{
    int x = 99, y = 100;
    unsafe
    {
        swap (&x, &y);
    }
    Console.WriteLine($"x is now {x}, y is now {y}");
}

public static unsafe void swap(int *a, int *b)
{
    int temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

When you compile programs containing unsafe code, you must specify the Allow Unsafe Code option when building the project. To do this, right-click the project in Solution Explorer and then click Properties. In the Properties window, click the Build tab, select Allow Unsafe Code, and then, on the File menu, click Save All.

Unsafe code also bears on how memory is managed. Objects created in unsafe code are said to be unmanaged. Although situations that require you to access memory in this way are not common, you might encounter some, especially if you are writing code that needs to perform some low-level Windows operations.

You will learn about the implications of using code that accesses unmanaged memory in more detail in Chapter 14.

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