Is it mandatory for all reference data types to be stored in the heap?

In C#, not all reference types are necessarily allocated on the heap. Typically, objects of reference types are created on the managed heap, but there are exceptions:

• Structures containing reference types as fields, if they are fields of a class, are allocated on the heap along with the class.
• Boxing — when a value type is boxed into an object, it is allocated on the heap.
• Span<T> and stackalloc — although Span<T> is a reference type, it can refer to data allocated on the stack.

Thus, a reference points to an object on the heap, but the reference itself can be stored on the stack or in another object. There are also optimizations, such as stack-allocated objects and escape analysis in .NET Core and .NET 5+.

Example:

class MyClass { int x; }

void Method() {
    MyClass obj = new MyClass(); // obj is a reference on the stack, object is on the heap
}

Here, the reference obj is stored on the stack, and the object is on the heap.