C# unsafe code
When a code block is marked with the unsafe modifier,C# allows the use of pointer variables in functions.Unsafe or unmanaged code refers to code blocks that use pointer variables.
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:
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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#:
Example
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();
}
}
}
When the above code is compiled and executed, it produces the following results:
Data is: 20
Address is: 99215364
You can also not declare the entire method as unsafe code, but only declare part of the method as unsafe code. The following example illustrates this.
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:
Example
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();
}
}
}
When the above code is compiled and executed, it produces the following results:
Data is: 20
Data is: 20
Address is: 77128984
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:
Example
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();
}
}
}
When the above code is compiled and executed, it produces the following results:
Before Swap: var1: 10, var2: 20
After Swap: var1: 20, var2: 10
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:
Example
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();
}
}
}
When the above code is compiled and executed, it produces the following results:
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
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
Buildtabs.Select option “Allow unsafe code”.