Chapter141:Pointers&UnsafeCode

Section141.1:Introductiontounsafecode

C# allows using pointer variables in a function of code block when it is marked by the unsafemodifier. The unsafe code or the unmanaged code is a code block that uses a pointer variable.

A pointer is a variable whose value is the address of another variable i.e., the direct address of the memory location. similar to any variable or constant, you must declare a pointer before you can use it to store any variable address.

Thegeneralformofapointerdeclarationis:

type*var-name;

Following arevalidpointerdeclarations:

int *ip; /*pointertoaninteger*/

double*dp; /*pointertoadouble*/

float*fp; /*pointertoafloat*/

char*ch /*pointertoacharacter*/

ThefollowingexampleillustratesuseofpointersinC#,usingtheunsafemodifier:

usingSystem;

namespaceUnsafeCodeApplication

{

classProgram

{

staticunsafevoidMain(string[]args)

{

intvar=20;

int*p=&var;

Console.WriteLine(“Data is:{0}”,      var);

Console.WriteLine(“Addressis:{0}”,           (int)p);

Console.ReadKey();

}

}

}

Whentheabovecodewasscompiledandexecuted,itproducesthefollowingresult:

Data is: 20

Address is: 99215364

Insteadofdeclaringanentiremethodasunsafe,youcanalsodeclareapartofthecodeasunsafe:

//safecode

unsafe

{

//youcanusepointershere

}

//safecode

Section141.2:AccessingArrayElementsUsingaPointer

InC#,anarraynameandapointertoadatatypesameasthearraydata,arenotthesamevariabletype.For example,int*pandint[]p,arenotsametype.Youcanincrementthepointervariablepbecauseitisnotfixedin memorybutanarrayaddressisfixedinmemory,andyoucan’tincrementthat.

Therefore,ifyouneedtoaccessanarraydatausingapointervariable,aswetraditionallydoinC,orC++,youneed to fix the pointer using the fixed keyword.

The following example demonstrates this:

usingSystem;

namespaceUnsafeCodeApplication

{

classTestPointer

{

publicunsafestaticvoidMain()

{

int[]     list={10,100,200};

fixed(int*ptr=list)

/*letushavearrayaddressinpointer*/

for(inti=0;i<3;i++)

{

Console.WriteLine(“Addressoflist[{0}]={1}”,i,(int)(ptr+i));

Console.WriteLine(“Valueoflist[{0}]={1}”,i,*(ptr+i));

}

Console.ReadKey();

}

}

}

Whentheabovecodewascompiledandexecuted,itproducesthefollowingresult:

Addressoflist[0]=31627168 Value

of list[0] = 10

Addressoflist[1]=31627172

Value of list[1] = 100

Addressoflist[2]=31627176

Value of list[2] = 200

Section141.3:CompilingUnsafeCode

Forcompilingunsafecode,youhavetospecifythe/unsafecommand-lineswitchwithcommand-linecompiler.

For example, to compile a program named prog1.cs containing unsafe code, from command line, give the command:

csc/unsafeprog1.cs

IfyouareusingVisualStudioIDEthenyouneedtoenableuseofunsafecodeintheprojectproperties.

Todothis:

• Open project properties by double clicking the properties node in the Solution Explorer.
• Click on the Build tab.
• Select the option “Allow unsafe code”

Section141.4:RetrievingtheDataValueUsingaPointer

You can retrieve the data stored at the located referenced by the pointer variable, using the ToString() method. The following example demonstrates this:

usingSystem;

namespaceUnsafeCodeApplication

{

classProgram

{

publicstaticvoidMain()

{

unsafe

{

intvar=20;

int*p=&var;

Console.WriteLine(“Datais:{0}”,var);

Console.WriteLine(“Datais:{0}”,p->ToString());

Console.WriteLine(“Addressis:{0}”,(int)p);

}

Console.ReadKey();

}

}

}

Whentheabovecodewascompiledandexecuted,itproducesthefollowingresult:

Data is: 20

Data is: 20

Address is: 77128984

Section141.5:PassingPointersasParameterstoMethods

Youcanpassapointervariabletoamethodasparameter.Thefollowingexampleillustratesthis:

usingSystem;

namespaceUnsafeCodeApplication

{

classTestPointer

{

publicunsafevoidswap(int*p,int*q)

{

inttemp=*p;

*p=*q;

*q=temp;

}

publicunsafestaticvoidMain()

{

TestPointerp=newTestPointer();

intvar1=10;

intvar2=20;

int*x=&var1;

int*y=&var2;

Console.WriteLine(“BeforeSwap:var1:{0},var2:{1}”,var1,var2); p.swap(x,y);

Console.WriteLine(“AfterSwap:var1:{0},var2:{1}”,var1,var2); Console.ReadKey();

}

}

}

Whentheabovecodeiscompiledandexecuted,itproducesthefollowingresult:

Before Swap: var1: 10, var2: 20

After Swap: var1: 20, var2: 10