In what case is dynamic memory allocation on the heap used?

Dynamic memory in the heap is used when the size of data is unknown in advance or can change during program execution. For example, when creating objects of classes, collections (lists, dictionaries), or allocating large arrays whose size is determined at runtime.

In C#, reference type objects are always placed in the heap, which allows managing the lifetime of objects through the garbage collector. This differs from the stack, where local variables of value types with fixed lifetimes are stored.

Example:

class Person
{
    public string Name;
}

Person p = new Person(); // object is created in the heap
p.Name = "Ivan";

Thus, the heap is used for storing objects with dynamic lifetime and size.