Inherit from struct

Question

I am try to figure out what is the problem whit my code. I have this code:

public struct MyStructA
{
    public MyStructA(string str)
    {
        myString=         


        

Answer 1

Struct does not support inheritance, if you need you have to use class, see msdn

There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class Object. A struct can implement interfaces, and it does that exactly as classes do.

Answer 2

Structs can implement an interface but they cannot inherit from another struct. For that reason, struct members cannot be declared as protected.

Answer 3

From MSDN;

There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class Object. A struct can implement interfaces, and it does that exactly as classes do.

But remember, since structs are a value type and they inherit System.ValueType

Answer 4

There are actually a few good reasons:

1. Structs have no 'Type'

   ...unless they are 'boxed' into an object.

   An object on the other hand has two "header" fields in the normal CLR where the type is stored (and some GC- and locking info). Adding that would change the size of the structs, and make their size unpredictable (because some runtimes might chose to add that information differently, for example the mono runtime adds more "header" information to its objects than the .net framework runtime, or at least did so in the past)

   This boxing is actually what happens when you try to assign a struct to an interface field it implements. So it would be possible in theory, but then all your structs would be boxed, and that'd be really bad for performance reasons.

2. Typing and fixed size

   To show why specifically inheriting structs would be a huge problem lets make a simple example.

   Consider two structs: struct MyBaseStruct { public int A; } and a hypothetical struct MyDerivedStruct : MyBaseStruct { public int B; }.

   Now what would happen when I call var array = new MyBaseStruct[10]; ?? How much size would the runtime allocate for that?

   The assignment array[0] = new MyDerivedStruct(); would be troublesome, on 32bit systems it would probably write to the first AND the second slot as well.

   Even if you'd try to "collect" all derived types it wouldn't work, what if you load another dll that defines yet another struct that derives from your base-struct?

I personally find it pretty important to know the actual issues that probably led the designers to the decision in the first place. But of course a one could also just say "because the designers of the language made it so!" or "because that's what the C# language specification says" :P

Answer 5

Inheritance isn't alloweded between structs but structs can implement interfaces.

Answer 6

A struct Is Implicitly Sealed

According to this link:

Every struct in C#, whether it is user-defined or defined in the .NET Framework, is sealed–meaning that you can’t inherit from it. A struct is sealed because it is a value type and all value types are sealed.

A struct can implement an interface, so it’s possible to see another type name following a colon, after the name of the struct.

In the example below, we get a compile-time error when we try to define a new struct that inherits from the one defined above.

public struct PersonName
{
    public PersonName(string first, string last)
    {
        First = first;
        Last = last;
    }

    public string First;
    public string Last;
}

// Error at compile time: Type 'PersonName' in interface list is not an interface
public struct AngryPersonName : PersonName
{
    public string AngryNickname;
}