Just what is an IntPtr exactly?

Question

Through using IntelliSense and looking at other people's code, I have come across this IntPtr type; every time it has needed to be used I have simply put null or IntPtr.Zero and found most functions to work. What exactly is it and when/why is it used?

Answer 1

It's a "native (platform-specific) size integer." It's internally represented as void* but exposed as an integer. You can use it whenever you need to store an unmanaged pointer and don't want to use unsafe code. IntPtr.Zero is effectively NULL (a null pointer).

Answer 2

It's a value type large enough to store a memory address as used in native or unsafe code, but not directly usable as a memory address in safe managed code.

You can use IntPtr.Size to find out whether you're running in a 32-bit or 64-bit process, as it will be 4 or 8 bytes respectively.

Answer 3

Here's an example:

I'm writing a C# program that interfaces with a high-speed camera. The camera has its own driver that acquires images and loads them into the computer's memory for me automatically.

So when I'm ready to bring the latest image into my program to work with, the camera driver provides me with an IntPtr to where the image is ALREADY stored in physical memory, so I don't have to waste time/resources creating another block of memory to store an image that's in memory already. The IntPtr just shows me where the image already is.

Answer 4

A direct interpretation

An IntPtr is an integer which is the same size as a pointer.

You can use IntPtr to store a pointer value in a non-pointer type. This feature is important in .NET since using pointers is highly error prone and therefore illegal in most contexts. By allowing the pointer value to be stored in a "safe" data type, plumbing between unsafe code segments may be implemented in safer high-level code -- or even in a .NET language that doesn't directly support pointers.

The size of IntPtr is platform-specific, but this detail rarely needs to be considered, since the system will automatically use the correct size.

The name "IntPtr" is confusing -- something like Handle might have been more appropriate. My initial guess was that "IntPtr" was a pointer to an integer. The MSDN documentation of IntPtr goes into somewhat cryptic detail without ever providing much insight about the meaning of the name.

An alternative perspective

An IntPtr is a pointer with two limitations:

1. It cannot be directly dereferenced
2. It doesn't know the type of the data that it points to.

In other words, an IntPtr is just like a void* -- but with the extra feature that it can (but shouldn't) be used for basic pointer arithmetic.

In order to dereference an IntPtr, you can either cast it to a true pointer (an operation which can only be performed in "unsafe" contexts) or you can pass it to a helper routine such as those provided by the InteropServices.Marshal class. Using the Marshal class gives the illusion of safety since it doesn't require you to be in an explicit "unsafe" context. However, it doesn't remove the risk of crashing which is inherent in using pointers.

Answer 5

What is a Pointer?

In all languages, a pointer is a type of variable that stores a memory address, and you can either ask them to tell you the address they are pointing at or the value at the address they are pointing at.

A pointer can be thought of as a sort-of book mark. Except, instead of being used to jump quickly to a page in a book, a pointer is used to keep track of or map blocks of memory.

Imagine your program's memory precisely like one big array of 65535 bytes.

Pointers point obediently

Pointers remember one memory address each, and therefore they each point to a single address in memory.

As a group, pointers remember and recall memory addresses, obeying your every command ad nauseum.

You are their king.

Pointers in C#

Specifically in C#, a pointer is an integer variable that stores a memory address between 0 and 65534.

Also specific to C#, pointers are of type int and therefore signed.

You can't use negatively numbered addresses though, neither can you access an address above 65534. Any attempt to do so will throw a System.AccessViolationException.

A pointer called MyPointer is declared like so:

int *MyPointer;

A pointer in C# is an int, but memory addresses in C# begin at 0 and extend as far as 65534.

Pointy things should be handled with extra special care

The word unsafe is intended to scare you, and for a very good reason: Pointers are pointy things, and pointy things e.g. swords, axes, pointers, etc. should be handled with extra special care.

Pointers give the programmer tight control of a system. Therefore mistakes made are likely to have more serious consequences.

In order to use pointers, unsafe code has to be enabled in your program's properties, and pointers have to be used exclusively in methods or blocks marked as unsafe.

Example of an unsafe block

unsafe
{
    // Place code carefully and responsibly here.

}

How to use Pointers

When variables or objects are declared or instantiated, they are stored in memory.

• Declare a pointer by using the * symbol prefix.

int *MyPointer;

• To get the address of a variable, you use the & symbol prefix.

MyPointer = &MyVariable;

Once an address is assigned to a pointer, the following applies:

• Without * prefix to refer to the memory address being pointed to as an int.

MyPointer = &MyVariable; // Set MyPointer to point at MyVariable

• With * prefix to get the value stored at the memory address being pointed to.

"MyPointer is pointing at " + *MyPointer;

Since a pointer is a variable that holds a memory address, this memory address can be stored in a pointer variable.

Example of pointers being used carefully and responsibly

    public unsafe void PointerTest()
    {
        int x = 100; // Create a variable named x

        int *MyPointer = &x; // Store the address of variable named x into the pointer named MyPointer

        textBox1.Text = ((int)MyPointer).ToString(); // Displays the memory address stored in pointer named MyPointer

        textBox2.Text = (*MyPointer).ToString(); // Displays the value of the variable named x via the pointer named MyPointer.

    }

Notice the type of the pointer is an int. This is because C# interprets memory addresses as integer numbers (int).

Why is it int instead of uint?

There is no good reason.

Why use pointers?

Pointers are a lot of fun. With so much of the computer being controlled by memory, pointers empower a programmer with more control of their program's memory.

Memory monitoring.

Use pointers to read blocks of memory and monitor how the values being pointed at change over time.

Change these values responsibly and keep track of how your changes affect your computer.

Answer 6

MSDN tells us:

The IntPtr type is designed to be an integer whose size is platform-specific. That is, an instance of this type is expected to be 32-bits on 32-bit hardware and operating systems, and 64-bits on 64-bit hardware and operating systems.

The IntPtr type can be used by languages that support pointers, and as a common means of referring to data between languages that do and do not support pointers.

IntPtr objects can also be used to hold handles. For example, instances of IntPtr are used extensively in the System.IO.FileStream class to hold file handles.

The IntPtr type is CLS-compliant, while the UIntPtr type is not. Only the IntPtr type is used in the common language runtime. The UIntPtr type is provided mostly to maintain architectural symmetry with the IntPtr type.

http://msdn.microsoft.com/en-us/library/system.intptr(VS.71).aspx

Answer 7

Well this is the MSDN page that deals with IntPtr.

The first line reads:

A platform-specific type that is used to represent a pointer or a handle.

As to what a pointer or handle is the page goes on to state:

The IntPtr type can be used by languages that support pointers, and as a common means of referring to data between languages that do and do not support pointers.

IntPtr objects can also be used to hold handles. For example, instances of IntPtr are used extensively in the System.IO.FileStream class to hold file handles.

A pointer is a reference to an area of memory that holds some data you are interested in.

A handle can be an identifier for an object and is passed between methods/classes when both sides need to access that object.

Answer 8

An IntPtr is a value type that is primarily used to hold memory addresses or handles. A pointer is a memory address. A pointer can be typed (e.g. int*) or untyped (e.g. void*). A Windows handle is a value that is usually the same size (or smaller) than a memory address and represents a system resource (like a file or window).