C# 0 The Complete Reference

val

 the value 10, which is the value of 

num

, which is pointed to by 

ip

. The 

operation of 

*

 can be remembered as “at address.” In this case, then, the statement could be 

read as “

val

 receives the value at address 

ip

.”

The

*

 can also be used on the left side of an assignment statement. In this usage, it sets 

the value pointed to by the pointer. For example,

*ip = 100;

This statement assigns 100 to the variable pointed to by 

ip

, which is 

num

 in this case. Thus, 

this statement can be read as “at address 

ip

, put the value 100.”

Using unsafe

Any code that uses pointers must be marked as unsafe by using the 

unsafe

 keyword. You 

can mark types (such as classes and structures), members (such as methods and operators), 

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or individual blocks of code as unsafe. For example, here is a program that uses pointers 

inside

Main( )

, which is marked unsafe:

// Demonstrate pointers and unsafe.

using System;

class UnsafeCode {

  // Mark Main as unsafe.

  unsafe static void Main() {

    int count = 99;

    int* p; // create an int pointer

    p = &count; // put address of count into p

    Console.WriteLine("Initial value of count is " + *p);

    *p = 10; // assign 10 to count via p

    Console.WriteLine("New value of count is " + *p);

  }

}

The output of this program is shown here:

Initial value of count is 99

New value of count is 10

Using fixed

The

fixed

 modifier is often used when working with pointers. It prevents a managed 

variable from being moved by the garbage collector. This is needed when a pointer refers to 

a field in a class object, for example. Because the pointer has no knowledge of the actions of 

the garbage collector, if the object is moved, the pointer will point to the wrong object. Here 

is the general form of 

fixed

:

fi xed (

type* p

 = &

fi xedObj

) {

    // use fi xed object

}

Here, 

p

 is a pointer that is being assigned the address of an object. The object will remain at 

its current memory location until the block of code has executed. You can also use a single 

statement for the target of a 

fixed

 statement. The 

fixed

 keyword can be used only in an 

unsafe context. You can declare more than one fixed pointer at a time using a comma-

separated list.

Here is an example of 

fixed

:

// Demonstrate fixed.

using System;

class Test {

  public int num;

  public Test(int i) { num = i; }

}

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