1.62. C# unsafe code

1.62.1. Pointer variable #

A pointer is a variable whose value is the address of another variable, thatis, the direct address of the memory location. Like other variables or constants, you must declare pointers before using pointers to store the addresses of other variables.

The general form of a pointer variable declaration is:

type* var-name; 

Here is an example of a pointer type declaration:

When multiple pointers are declared in the same declaration, the asterisk * write only with the underlying type; it is not used as a prefix for each pointer name. For example:

int* p1, p2, p3; // correct int *p1, *p2, *p3; // error 

The following example illustrates the use of pointers whenthe unsafe modifier is used in C#:

using System; namespace UnsafeCodeApplication { class Program { static unsafe void Main(string[] args) { int var = 20; int* p = &var; Console.WriteLine("Data is: {0} ", var); Console.WriteLine("Address is: {0}", (int)p); Console.ReadKey(); } } } 

Data is: 20 Address is: 99215364 

1.62.2. Use pointers to retrieve data values #

You can use the ToString() method to retrieve data stored at the location referenced by the pointer variable. The following example demonstrates this:

using System; namespace UnsafeCodeApplication { class Program { public static void Main() { unsafe { int var = 20; int* p = &var; Console.WriteLine("Data is: {0} " , var); Console.WriteLine("Data is: {0} " , p->ToString()); Console.WriteLine("Address is: {0} " , (int)p); } Console.ReadKey(); } } } 

Data is: 20 Data is: 20 Address is: 77128984 

1.62.3. Pass the pointer as a parameter to the method #

You can pass a pointer variable as a parameter to a method. The following example illustrates this:

using System; namespace UnsafeCodeApplication { class TestPointer { public unsafe void swap(int* p, int *q) { int temp = *p; *p = *q; *q = temp; } public unsafe static void Main() { TestPointer p = new TestPointer(); int var1 = 10; int var2 = 20; int* x = &var1; int* y = &var2; Console.WriteLine("Before Swap: var1:{0}, var2: {1}", var1, var2); p.swap(x, y); Console.WriteLine("After Swap: var1:{0}, var2: {1}", var1, var2); Console.ReadKey(); } } } 

Before Swap: var1: 10, var2: 20 After Swap: var1: 20, var2: 10 

1.62.4. Use pointers to access array elements #

In C#, the array name and a pointer to the same data type as the array dataare different variable types. For example, int*p and int[] p aredifferent types. You can increase the pointer variable p, because it is notfixed in memory, but the array address is fixed in memory, so you cannot increase the array p.

So, if you need to access array data using pointer variables, as we usually do in C or C++, use fixed keyword to fix the pointer.

The following example demonstrates this:

using System; namespace UnsafeCodeApplication { class TestPointer { public unsafe static void Main() { int[] list = {10, 100, 200}; fixed(int *ptr = list) /* Show array addresses in pointers */ for ( int i = 0; i < 3; i++) { Console.WriteLine("Address of list[{0}]={1}",i,(int)(ptr + i)); Console.WriteLine("Value of list[{0}]={1}", i, *(ptr + i)); } Console.ReadKey(); } } } 

Address of list[0] = 31627168 Value of list[0] = 10 Address of list[1] = 31627172 Value of list[1] = 100 Address of list[2] = 31627176 Value of list[2] = 200 

1.62.5. Compile unsafe code #

You must switch to the /unsafe command line specified bythe command-line compiler.

For example, to compile an unsafe code named prog1.cs , which need to entera command on the command line:

csc /unsafe prog1.cs 

If you are using a Visual Studio IDE , then you need to enable unsafe code in the project properties.

The steps are as follows:

• Open the project properties by double-clicking the properties node in Explorer (Solution Explorer).

• Click Build tabs.

• Select option “Allow unsafe code”.