![]() ![]() "Reference types (classes, interfaces, delegates) are always allocated on the heap and never on the stack." Q: What issue may happen due to allocation and de-allocation of memory on the heap?Īs the memory on the heap is allocated and de-allocated, it becomes fragmented. ![]() When an object stored on the heap no longer has any references pointing to it, it's considered eligible for garbage collection.Īt a certain point, garbage collector kicks in, interrupts all running threads, invokes the finalizers of the objects it's trying to get rid of (on a special finalizer thread), and then marks the memory as free to use. While the objects stored on the stack are gone when the containing stack frame is popped, memory used by objects stored on the heap needs to be freed up by the garbage collector. This means that instead of copying the value, the consuming code will receive a reference to the value instead, be it on a stack or on a heap, as long as the lifetime of that value type is longer than that of consuming code. Keywords such as ref and out, ref return and ref local (C#7.0), in (C#7.2) allow accessing value types by reference. Q: Can we use value types with reference semantics? In such cases, the value is copied and stored on the heap wrapped as a reference type, in an operation known as boxing. Since every type in C# derives from System.Object, value types can be assigned to variables or passed to methods that expect an object. ![]() This means that every time a value type is assigned to a variable or passed as parameter, the value is copied.īecause copying value types can get expensive depending on the size of the object, it's not recommended to declare memory-heavy objects as value types. Instances of value types are passed by copy (unless used with reference semantics, see below). Starting with C#7.2, a struct can be declared as ref struct, in which case it will always be allocated on the stack, preventing it from being declared inside reference types. If the value type was declared as a member of a struct then it's stored wherever that struct is stored.If the value type was declared as a member of a class then it's stored on the heap, along with its parent.If the value type was declared as a method parameter then it's stored on the stack.If the value type was declared as a variable inside a method then it's stored on the stack.int, bool, char, enum and any struct) can be allocated on the heap or on the stack, depending on where they were declared. Value types (derived from System.ValueType, e.g. A reference is lightweight and is always constant size (32 bit or 64 bit depending on OS bitness) so copying it (and thus passing around reference types) is considered cheap. This means that you can change the reference to point to a different object without affecting the previous object itself or other references pointing to it. The reference (not the referenced object) can be allocated both on the stack or on the heap.īy passing a reference to an object, you're telling where that object is located on the heap so that your code can access it.Įvery time an object is passed as a reference, the reference itself is copied. When you pass a reference object as a parameter or assign it to a variable, you're in fact passing its reference. Reference types (classes, interfaces, delegates) are always allocated on the heap. Note: you should never say "reference types are allocated on the heap while value types are allocated on the stack", this is a commonly repeated mistake and sets off a red flag for an experienced interviewer. Q: Which objects are allocated on the stack and which objects are allocated on the heap? Objects allocated on the stack are available only inside of a stack frame (execution of a method), while objects allocated on the heap can be accessed from anywhere. In C# there are two places where an object can be stored - the heap and the stack. Note: if you find a mistake, please let me know in the comments and I will correct it. I will try to write these regularly and I hope that it can be helpful to someone. So instead I welcome you to follow this series, in which I will try to cover different interview topics in each post. When you're in-between jobs, getting ready for interviews may be tedious. I've decided to keep track of the most common interview questions I get and write down my answers to them. Despite each project being unique, interviewers tend to ask the same questions from time to time. Recently I've made myself unemployed and for a few weeks now I'm frequently doing technical interviews as part of looking for a new job. ![]()
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