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I am having a bit of an issue grasping the concept of the struct and no one can answer my question. I know it is "like" an object, you can have methods and fields. What I do not understand is why they are sealed. Why is there a lack of inheritance and polymorphism which limits their abilities. Is there really much real world use for a struct, I know they are smaller than an object, I just cannot find any real examples that help me understand the benefits. Thanks.

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    See [Why a C# struct cannot be inherited?](http://stackoverflow.com/questions/2310103/), [Why are .NET value types sealed?](http://stackoverflow.com/questions/1769306/), [Inherit from struct](http://stackoverflow.com/questions/15408667/), [Why don't structs support inheritance?](http://stackoverflow.com/questions/1222935/), [Google: C# struct sealed site:stackoverflow.com](https://www.google.com/search?q=C%23+struct+sealed+site%3Astackoverflow.com). Please show some research effort. :) – CodeCaster Dec 04 '13 at 09:34

2 Answers2

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Because a struct is a value; a struct that contains a float is exactly 4 bytes long. There is no object-header that indicates what the concrete type is: the thing that knows the type is the compiler (and the resulting IL) - the type information is not self-contained in the value. So, regular polymorphism cannot work. If you do:

MyType type = new MySubType(); // if this was a struct and if this compiled

then from the compiler's perspective, the type of that is MyType. There is no notion, after the assignment, that this is or ever was a MySubType. Indeed, you can use unsafe code to force the bits of one value-type into another.

Additionally, this same facet of structs means that you would not be able to add fields in sub-types: the size has to be fixed, which makes this impossible.

Basically, if you are thinking in terms of inheritance, you are using structs incorrectly; actually, most times that I see people using structs they are using them incorrectly. structs should represent values (ideally immutable etc).

Is there really much real world use for a struct

Yes; for representing values - a ComplexNumber type for example; and for some highly optimized scenarios where GC is prohibitive, but which only affects a small number of developers. Here's one example of this; another common example is "games", but: switching between class and struct is a much bigger change than changing the keyword. You need to know what you are doing, and why you are doing it.

What you should not do is things like:

// ***BAD CODE: DO NOT DO***
public struct Customer
{ // I don't need inheritance; a struct will be faster and smaller
  // yay me, I'm awesome

    public int Id;
    public string Name;
}

The above is clearly best represented as a class. Also: properties.

I know they are smaller than an object

In some ways, this is not true; for example, most useful structs are larger than a reference - which can be problematic if you are passing super-sized structs around on the stack without thinking what you are doing.

Marc Gravell
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  • +1 "switching between class and struct is a much bigger change than changing the keyword". Coming from gamedev - could not agree more. – Engineer Feb 22 '19 at 15:51
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It is simple. It is a value type and all value types are sealed. Though you can inherit it from interface.

Ehsan
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