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Writing Unsafe code using C#

, 10 Oct 2001
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A simple tutorial that shows how to write unsafe code using C#
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Introduction

In unsafe code or in other words unmanaged code it is possible to declare and use pointers. But the question is why do we write unmanaged code? If we want to write code that interfaces with the operating system, or want to access memory mapped device or want to implement a time critical algorithm then the use of pointer can give lots of advantages.

But there are some disadvantages of using pointers too. If pointers were chosen to be 32 bit quantities at compile time, the code would be restricted to 4gig of address space, even if it were run on a 64 bit machine. If pointers were chosen at compile time to be 64 bits, the code could not be run on a 32 bit machine.

In C# we can write unsafe code with the modifier unsafe. All the unsafe code must be clearly marked with the unsafe modifier. Writing unsafe code is much like writing C code in a C# program.

Now let's see the first program

Program 1

using System;

class MyClass {
	public static void Main() {
		int iData = 10;
		int* pData = &iData;
		Console.WriteLine("Data is " + iData);
		Console.WriteLine("Address is " + (int)pData );
	}
}

Here I use a pointer in this program. Now compile this program. The compiler gives the error

Microsoft (R) Visual C# Compiler Version 7.00.9030 [CLR version 1.00.2204.21]
Copyright (C) Microsoft Corp 2000. All rights reserved.

um1.cs(6,8): error CS0214: Pointers may only be used in an unsafe context
um1.cs(8,27): error CS0214: Pointers may only be used in an unsafe context

Now let's change the program a little bit and add unsafe modifier with the function.

Program 2

using System;

class MyClass {
	public unsafe static void Main() {
		int iData = 10;
		int* pData = &iData;
		Console.WriteLine("Data is " + iData);
		Console.WriteLine("Address is " + (int)pData );
	}
}

In this program the Main() function is defined as unsafe so we can use pointer in this function. The output of the program is

Data is 10
Address is 1244316

It is not necessary that we define the unsafe modifier with the function. We can define a block of unsafe code. Let's change a program little bit more.

Program 3

using System;

class MyClass {
	public static void Main() {
		unsafe {
			int iData = 10;
			int* pData = &iData;
			Console.WriteLine("Data is " + iData);
			Console.WriteLine("Address is " + (int)pData );
		}
	}
}

In this program a block is defined with unsafe modifier. So we can use pointers in that code. The output of this program is the same as previous one.

Now let's change the program a little bit to get a value from the pointer.

Program 4

using System;

class MyClass {
	public static void Main() {
		unsafe {
			int iData = 10;
			int* pData = &iData;
			Console.WriteLine("Data is " + iData);
			Console.WriteLine("Data is " + pData->ToString() );
			Console.WriteLine("Address is " + (int)pData );
		}
	}
}

We can use the ToString() member function to get data from pointer. Lets change the program a little bit to see it's behavior.

Program 5

using System;

class MyClass {
	public static void Main() {
		testFun();
	}

	public static unsafe void testFun() {
		int iData = 10;
		int* pData = &iData;
		Console.WriteLine("Data is " + iData);
		Console.WriteLine("Address is " + (int)pData );
	}
}

In this program a function with unsafe modifier is called from a normal function. This program shows that a managed code can call unmanaged functions. The output of the program is the same as previous program.

Now change the program little bit and make an unsafe function in another class.

Program 6

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.testFun();
	}
}

class TestClass {
	public unsafe void testFun() {
		int iData = 10;
		int* pData = &iData;
		Console.WriteLine("Data is " + iData);
		Console.WriteLine("Address is " + (int)pData );
	}
}

The output of the program is same as previous one.

Now try to pass pointer as a parameter. Let’s see this program.

Program 7

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.testFun();
	}
}

class TestClass {
	public unsafe void testFun() {
		int x = 10;
		int y = 20;
		Console.WriteLine("Before swap x = " + x + " y= " + y);
		swap(&x, &y);
		Console.WriteLine("After swap x = " + x + " y= " + y);
	}

	public unsafe void swap(int* p_x, int *p_y) {
		int temp = *p_x;
		*p_x = *p_y;
		*p_y = temp;
	}
}

In this program the unsafe function testFun() calls the classic swap() function to interchange the value of two variables passing by reference. Now change the program a little bit.

Program 8

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		unsafe {
			int x = 10;
			int y = 20;
			Console.WriteLine("Before swap x = " + x + " y= " + y);
			Obj.swap(&x, &y);
			Console.WriteLine("After swap x = " + x + " y= " + y);
		}
	}
}

class TestClass {
	public unsafe void swap(int* p_x, int* p_y) {
		int temp = *p_x;
		*p_x = *p_y;
		*p_y = temp;
	}
}

This program does the same job as previous one. But in this program we write only one unsafe function and call this function from the unsafe block in Main.

Now let's see another program which show the usage of array in C#

Program 9

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		// get value from array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			Console.WriteLine(iArray[iIndex]);
		}
	}
}

This program display the square of numbers from zero to 9.

Let's change the program a little bit and pass the array as a parameter to a function.

Program 10

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		testFun(iArray);
	}

	public unsafe void testFun(int [] p_iArray) {

		// get value from array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			Console.WriteLine(p_iArray[iIndex]);
		}
	}
}

The output of the program is same as previous one.

Now let's change the program a little bit and try to get the value of the array from a pointer rather than an index.

Program 11

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		// get value from array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			Console.WriteLine(*(iArray + iIndex) );
		}
	}
}

Here in this program we try to access the value of the array from *(iArray + iIndex) rather than iArray[iIndex]. But the program gives the following error.

Microsoft (R) Visual C# Compiler Version 7.00.9030 [CLR version 1.00.2204.21]
Copyright (C) Microsoft Corp 2000. All rights reserved.

um11.cs(21,24): error CS0019: Operator '+' cannot be applied to operands of type 'int[]' and 'int'

In C# int* and in[] are not treated the same. To understand it more let's see one more program.

Program 12

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];
		iArray++;

		int* iPointer = (int*)0;
		iPointer++;

	}
}

There are two different types of variable in this program. First, the variable iArray is declared an array and the second variable iPointer is a pointer variable. Now I am going to increment both. We can increment the pointer variable because it is not fixed in memory but we can't increment the iArray, because the starting address of the array is stored in iArray and if we are allowed to increment this then we will lose starting address of array.

The output of the program is an error.

Microsoft (R) Visual C# Compiler Version 7.00.9030 [CLR version 1.00.2204.21]
Copyright (C) Microsoft Corp 2000. All rights reserved.

um12.cs(13,3): error CS0187: No such operator '++' defined for type 'int[]'

To access the element of the array via a pointer we have to fix the pointer so it can't be incremented. C# uses the fixed reserve word to do this.

Program 13

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		// get value from array
		fixed(int* pInt = iArray)
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			Console.WriteLine(*(pInt + iIndex) );
		}
	}
}

We can use the same technique to pass the array to a function which receives the pointer as a parameter.

Program 14

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		// get value from array
		fixed(int* pInt = iArray)
		testFun(pInt);
	}

	public unsafe void testFun(int* p_pInt) {

		for (int iIndex = 0; iIndex < 10; iIndex++) {
			Console.WriteLine(*(p_pInt + iIndex) );
		}
	}
}

The output of the program is the same as the previous one. If we try to access beyond the array limit then it will print garbage.

Program 15

using System;

class MyClass {
	public static void Main() {
		TestClass Obj = new TestClass();
		Obj.fun();
	}
}

class TestClass {
	public unsafe void fun() {
		int [] iArray = new int[10];

		// store value in array
		for (int iIndex = 0; iIndex < 10; iIndex++) {
			iArray[iIndex] = iIndex * iIndex;
		}

		// get value from array
		fixed(int* pInt = iArray)
		testFun(pInt);
	}

	public unsafe void testFun(int* p_pInt) {

		for (int iIndex = 0; iIndex &lt 20; iIndex++) {
			Console.WriteLine(*(p_pInt + iIndex) );
		}
	}
}

Here we try to read 20 elements from array but there are only 10 elements in the array so it will print garbage after printing the elements of array.

Program 16

using System;

struct Point {
	public int iX;
	public int iY;
}

class MyClass {
	public unsafe static void Main() {

		// reference of point
		Point refPoint = new Point();
		refPoint.iX = 10;
		refPoint.iY = 20;

		// Pointer of point
		Point* pPoint = &refPoint;

		Console.WriteLine("X = " + pPoint->iX);
		Console.WriteLine("Y = " + pPoint->iY);

		Console.WriteLine("X = " + (*pPoint).iX);
		Console.WriteLine("Y = " + (*pPoint).iY);

	}
}

Here pPoint is the pointer of Point class instance. We can access the element of it by using the -> Operator.

Change in Beta 2

When you want to compile program using command line switch you type the program name after the compiler name; for example if your program name is prog1.cs then you will compile this:

scs prog1.cs

This works fine for unsafe code while you are programming in beta 1. In beta 2 Microsft added one more switch to command line compiler of C# for writing unsafe code. Now if you want to write unsafe code then you have to specify the /unsafe command line switch with command line compiler otherwise the compiler gives an error. In beta 2 if you want to write unsafe code in your program then you compile your programas follows:

csc /unsafe prog1.cs

Here prog1.cs is the name of the program. If you compile your program which has unsafe code without using the /unsafe switch then compiler gives error.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Zeeshan Amjad
Software Developer (Senior) Bloomberg LP
United States United States
Working as a Sr C++ Developer at Bloomberg LP

Comments and Discussions

 
Questiongreat PinmemberMember 946260114-Mar-13 9:55 
Well explained , thanks alot
GeneralMy vote of 5 PinmemberVEMS24-Oct-12 6:16 
QuestionNice course PinmemberGophern Kwok27-Apr-12 23:02 
QuestionPointer to array without fixed PinmemberWilliam The Developer27-Mar-10 12:57 
QuestionI just need to compare ;if this code is the same in C++ and C# PinmemberMember 295931415-Jun-09 8:14 
QuestionCan I declare struct in C# Pinmemberrichmike326-May-09 20:26 
Questionpass by ref and unsafe pass by pointer [modified] PinmemberAsm4pic3-Aug-06 5:09 
AnswerRe: pass by ref and unsafe pass by pointer PinmemberLukas Fellechner10-Oct-06 3:21 
GeneralmyStruct[] to byte[] Pinmemberflopturnriver28-Jul-06 4:28 
QuestionRe: myStruct[] to byte[] Pinmemberking harold8-Nov-07 2:33 
Newsif you use .NET it'll help more Pinmembercorgan_hejijun20-Mar-06 14:04 
GeneralVery nice explanation Pinsussedho1-Jan-05 20:07 
GeneralUnsafe vs Unmanaged.... PinsussAnonymous18-Feb-03 22:21 

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