Heap or Stack: Where Does `new` Allocate Memory for a C# Struct?

When using the "new" keyword to create a structure in C#, is the memory allocated on the heap or the stack?

When creating a class instance using the "new" operator, memory is allocated on the heap. But when using the "new" operator to create a structure instance, where is the memory allocated?

Stack memory and heap memory

The stack is a data structure used to store local variables and method calls during program execution. It is a last-in-first-out (LIFO) structure, which means that the most recently allocated memory is removed first.

The heap is a dynamic memory space where objects are created and allocated as needed. Unlike the stack, it does not follow a specific order of memory allocation.

Use the "new" keyword to create memory allocation for the structure

For structures, when using the "new" operator, you need to consider two situations:

1. Parameterless constructor (new Guid();):

   • Using the "new" operator and the parameterless constructor allocates memory for the structure on the heap.
   • This is because the C# compiler treats parameterless constructors as a special case, treating them as zero-initialization operations.
   • Therefore, a new memory location is created on the heap and the structure is constructed in it.

2. Constructor with parameters (new Guid(someString);):

   • Using the "new" operator and the parameterized constructor allocates memory on the stack for temporary storage.
   • After the constructor initializes the structure, the allocated stack memory will be discarded.
   • This process is different from assigning an instantiated structure to a local variable, which allocates memory on the heap, just like a parameterless constructor.

IL code generation

To understand what’s going on behind the scenes, let’s examine the intermediate language (IL) code generated by the C# compiler:

The
• newobj directive allocates space on the stack and calls a parameterized constructor for intermediate values ​​(e.g., method parameters).
The
• call instance directive initializes an allocated storage location (stack or heap) using a parameterized constructor.
The
• initobj instruction initializes an allocated storage location (stack or heap), clearing its contents to zero (for parameterless constructor calls).

Conclusion

In summary, unlike class instances which always allocate memory on the heap, using the "new" operator on a struct will allocate memory on the heap in the case of a parameterless constructor and in the case of a parameterized constructor. Allocate memory on the stack (for temporary storage). This behavior is reflected in the generated IL code and provides insight into the underlying memory management process.

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