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C# - C# | Pointers & Unsafe Code
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  Introduction to unsafe code

C# allows using pointer variables in a function of code block when it is marked by the unsafe modifier. 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.

The general form of a pointer declaration is:

type *var-name;

Following are valid pointer declarations:

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The following example illustrates use of pointers in C#, using the unsafe modifier:

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 wass compiled and executed, it produces the following result:

Data is: 20
Address is: 99215364

Instead of declaring an entire method as unsafe, you can also declare a part of the code as unsafe:

// safe code
unsafe
{
    // you can use pointers here
}
// safe code
  Accessing Array Elements Using a Pointer

In C#, an array name and a pointer to a data type same as the array data, are not the same variable type. For example, int *p and int[] p, are not same type. You can increment the pointer variable p because it is not fixed in memory but an array address is fixed in memory, and you can't increment that.

Therefore, if you need to access an array data using a pointer variable, as we traditionally do in C, or C++, you need to fix the pointer using the fixed keyword.

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)
         
         /* let us have array address in pointer */
         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 was compiled and executed, it produces the following result:

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
  Compiling Unsafe Code

For compiling unsafe code, you have to specify the /unsafe command-line switch with command-line compiler.

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

csc /unsafe prog1.cs

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

enter image description here (opens new window)

To do this:

  • Open project properties by double clicking the properties node in the Solution Explorer.
    • - Click on the Build tab.
  • Select the option "Allow unsafe code"
    		•
      Retrieving the Data Value Using a Pointer

    You can retrieve the data stored at the located referenced by the pointer variable, using the ToString() method. 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();
      }
   }
}

    When the above code was compiled and executed, it produces the following result:

    Data is: 20
Data is: 20
Address is: 77128984
      Passing Pointers as Parameters to Methods

    You can pass a pointer variable to a method as parameter. 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();
      }
   }
}

    When the above code is compiled and executed, it produces the following result:

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



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