When using PInvoke, I noticed that we need to use IntPtr to refer to Windows handles. I am wondering why not just use int for the handle? My understanding of a handle is that it is just an integer value.
A windows handle is defined as an integer of the native machine pointer size. That's so that they can secretly be a pointer if they need to be. (A handle probably is not a pointer, but it is permitted to be one if the operating system implementers deem it necessary. They typically are not actually pointers for security reasons; it makes it too easy for people to abuse the system if they are actually pointers.)
An int in C# defined as a 32 bit integer, which will be too small on a 64 bit machine. An IntPtr is defined as an integer that can hold a pointer of the machine size. That's why you always use IntPtr when interoperating with handles.