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Asked: 2020-05-12 05:04:31 +0800 CST 2020-05-12 05:04:31 +0800 CST

Why can't I get the value of sizeof used in a class?

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I am "optimizing" the code of some interpreter that I am doing and for this I need to know the size in bytes of my class Token, which is made up of two variables: Type (enum), Value (object)and an override of the method ToString():

enum TokenType { String, Keyword, Ident, Number, Symbol }

class Token
{
    TokenType Type;
    object Value;
    public override string ToString() { return "Type: " + Type.ToString() + "\t\tValue: " + Value.ToString(); }
}

When I use a class for definition, it doesn't allow me to do sizeofin said class, however, when I define Tokenas a struct:

struct Token
{
    TokenType Type;
    object Value;
    public override string ToString() { return "Type: " + Type.ToString() + "\t\tValue: " + Value.ToString(); }
}

The sizeofdefault function doesn't let me know the size, so I managed structto get it to give me the result:

Console.WriteLine(Marshal.SizeOf(typeof(Token))); // Resultado: 16

I have read some answers from SO but I still have the same question about this function, why can't sizeofde System.Runtime.InteropServicesor the original function be used in a class, but in a struct?

c#

2 Answers

  1. Best Answer

    I don't have much experience with c# but maybe this is what you are looking for, if asking why I can't get the value you want some alternative:

    using System;
using System.Runtime.InteropServices;

enum TokenType { String, Keyword, Ident, Number, Symbol }

public class Test
{
    public static void Main()
    {
        // your code goes here
        Console.WriteLine(Marshal.SizeOf(typeof(Token0)));
        Console.WriteLine(Marshal.SizeOf(typeof(Token1)));
        Console.WriteLine(Marshal.SizeOf(typeof(Token2)));
    }
}

[StructLayout(LayoutKind.Sequential)]
class Token2
{
    TokenType Type;
    object Value;
    int Value1;
    public override string ToString() { return "Type: " + Type.ToString() + "\t\tValue: " + Value.ToString(); }
}

[StructLayout(LayoutKind.Sequential)]
class Token1
{
    TokenType Type;
    object Value;
    public override string ToString() { return "Type: " + Type.ToString() + "\t\tValue: " + Value.ToString(); }
}

[StructLayout(LayoutKind.Sequential)]
struct Token0
{
    TokenType Type;
    object Value;
    public override string ToString() { return "Type: " + Type.ToString() + "\t\tValue: " + Value.ToString(); }
}

    Test

    https://msdn.microsoft.com/es-es/library/5s4920fa(v=vs.100).aspx

    https://msdn.microsoft.com/es-es/library/system.runtime.interopservices.structlayoutattribute(v=vs.110).aspx

    • 1

  2. The Marshal.SizeOf() documentation says (emphasis mine ):

    Returns the size, expressed in bytes, of an unmanaged type .

    But what is an unmanaged type ? The C# 3.5 specification says, in section 18.2 - Pointer types:

    An unmanaged type is any type that is not a reference type and does not contain reference type fields at any level of nesting. In other words:

    • sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, or bool.
    • Any enum-type.
    • Any pointer-type.
    • Any type of user - defined structure that contains only fields of unmanaged types .

    With the above, it could be concluded that:

    • Marshal.SizeOf(typeof(T)), being Ta struct containing a String field should fail
    • Marshal.SizeOf(typeof(T)), being Tan enumerator it should work

    When executing the code we see that the opposite happens, why? The C# compiler by default decorates structures with the StructLayout attribute and the Sequential layout value as the documentation says . And what about the Enums? Well , it seems that it treats them with LayoutKind.Auto

    With this information, and seeing what (I think) you want to do, I would say that this is not the solution to your problem and you could better use a profiler.

    • 1