C# 3.0 Cookbook

Problem

You need to know whether two pointers point to the same memory location. If they don't, you need to know which of the two pointers points to a higher or lower element in the same block of memory.

Solution

Using the == and != operators, we can determine if two pointers point to the same memory location. For example, the code:

unsafe { int[] arr = new int[5] {1,2,3,4,5}; fixed(int* ptrArr = &arr[0]) { int* p1 = (ptrArr + 1); int* p2 = (ptrArr + 3); Console.WriteLine("p2 > p1"); Console.WriteLine("(p2 == p1) = " + (p2 == p1)); Console.WriteLine("(p2 != p1) = " + (p2 != p1)); p2 = p1; Console.WriteLine("p2 == p1"); Console.WriteLine("(p2 == p1) = " + (p2 == p1)); Console.WriteLine("(p2 != p1) = " + (p2 != p1)); } }

displays the following:

p2 > p1 (p2 == p1) = False (p2 != p1) = True p2 == p1 (p2 == p1) = True (p2 != p1) = False

Using the > , < , >= , or <= comparison operators, we can determine whether two pointers are pointing to a higher, lower, or the same element in an array. For example, the code:

unsafe { int[] arr = new int[5] {1,2,3,4,5}; fixed(int* ptrArr = &arr[0]) { int* p1 = (ptrArr + 1); int* p2 = (ptrArr + 3); Console.WriteLine("p2 > p1"); Console.WriteLine("(p2 > p1) = " + (p2 > p1)); Console.WriteLine("(p2 < p1) = " + (p2 < p1)); Console.WriteLine("(p2 >= p1) = " + (p2 >= p1)); Console.WriteLine("(p2 <= p1) = " + (p2 <= p1)); p2 = p1; Console.WriteLine("p2 == p1"); Console.WriteLine("(p2 > p1) = " + (p2 > p1)); Console.WriteLine("(p2 < p1) = " + (p2 < p1)); Console.WriteLine("(p2 >= p1) = " + (p2 >= p1)); Console.WriteLine("(p2 <= p1) = " + (p2 <= p1)); } }

displays the following:

p2 > p1 (p2 > p1) = True (p2 < p1) = False (p2 >= p1) = True (p2 <= p1) = False p2 == p1 (p2 > p1) = False (p2 < p1) = False (p2 >= p1) = True (p2 <= p1) = True

Discussion

When manipulating the addresses that pointers point to, it is sometimes necessary to compare their addresses. The == , != , > , < , >= , and <= operators have been overloaded to operate on pointer type variables . These comparison operators do not compare the value pointed to by the pointers; instead, they compare the addresses pointed to by the pointers.

To compare the values pointed to by two pointers, dereference the pointers and then use a comparison operator on them. For example:

*intPtr == *intPtr2

or:

structPtr1->value1 != structPtr2->value1

will compare the values pointed to by these pointers, rather than their addresses.

See Also

See the "C# Operators," "= = Operator," and "! = Operator" topics in the MSDN documentation.