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

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.