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.
Note − To execute the programs mentioned in this chapter at codingground, please set compilation option in Project >> Compile Options >> Compilation Command to
mcs *.cs -out:main.exe -unsafe"
Pointers
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. The example in the following section shows this.
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
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++ ( please check: C Pointers), 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.
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“.