Pointer types (C# Programming Guide)


Updated: July 20, 2015

In an unsafe context, a type may be a pointer type, a value type, or a reference type. A pointer type declaration takes one of the following forms:

type* identifier;  
void* identifier; //allowed but not recommended  

Any of the following types may be a pointer type:

Pointer types do not inherit from object and no conversions exist between pointer types and object. Also, boxing and unboxing do not support pointers. However, you can convert between different pointer types and between pointer types and integral types.

When you declare multiple pointers in the same declaration, the asterisk (*) is written together with the underlying type only; it is not used as a prefix to each pointer name. For example:

int* p1, p2, p3;   // Ok  
int *p1, *p2, *p3;   // Invalid in C#  

A pointer cannot point to a reference or to a struct that contains references, because an object reference can be garbage collected even if a pointer is pointing to it. The garbage collector does not keep track of whether an object is being pointed to by any pointer types.

The value of the pointer variable of type myType* is the address of a variable of type myType. The following are examples of pointer type declarations:

int* pp is a pointer to an integer.
int** pp is a pointer to a pointer to an integer.
int*[] pp is a single-dimensional array of pointers to integers.
char* pp is a pointer to a char.
void* pp is a pointer to an unknown type.

The pointer indirection operator * can be used to access the contents at the location pointed to by the pointer variable. For example, consider the following declaration:

int* myVariable;  

The expression *myVariable denotes the int variable found at the address contained in myVariable.

There are several examples of pointers in the topics fixed Statement and Pointer Conversions. The following example shows the need for the unsafe keyword and the fixed statement, and how to increment an interior pointer. You can paste this code into the Main function of a console application to run it. (Remember to enable unsafe code in the Project Designer; choose Project, Properties on the menu bar, and then select Allow unsafe code in the Build tab.)

// Normal pointer to an object.  
int[] a = new int[5] {10, 20, 30, 40, 50};  
// Must be in unsafe code to use interior pointers.  
    // Must pin object on heap so that it doesn't move while using interior pointers.  
    fixed (int* p = &a[0])  
        // p is pinned as well as object, so create another pointer to show incrementing it.  
        int* p2 = p;  
        // Incrementing p2 bumps the pointer by four bytes due to its type ...  
        p2 += 1;  
        p2 += 1;  
        // Deferencing p and incrementing changes the value of a[0] ...  
        *p += 1;  
        *p += 1;  
// Output:  

You cannot apply the indirection operator to a pointer of type void*. However, you can use a cast to convert a void pointer to any other pointer type, and vice versa.

A pointer can be null. Applying the indirection operator to a null pointer causes an implementation-defined behavior.

Be aware that passing pointers between methods can cause undefined behavior. Examples are returning a pointer to a local variable through an Out or Ref parameter or as the function result. If the pointer was set in a fixed block, the variable to which it points may no longer be fixed.

The following table lists the operators and statements that can operate on pointers in an unsafe context:

*Performs pointer indirection.
->Accesses a member of a struct through a pointer.
[]Indexes a pointer.
&Obtains the address of a variable.
++ and --Increments and decrements pointers.
+ and -Performs pointer arithmetic.
==, !=, <, >, <=, and >=Compares pointers.
stackallocAllocates memory on the stack.
fixed statementTemporarily fixes a variable so that its address may be found.

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

C# Programming Guide
Unsafe Code and Pointers
Pointer Conversions
Pointer Expressions
fixed Statement
Boxing and Unboxing