C # and the sizeof Marshal.SizeOf

In C #, sizeof to calculate unmanaged type (type value) of the size, it can not be used to calculate a managed type (type reference) of the size, in bytes.

When a reference to the types of sizeof, the compiler will complain, as the code:

Console.WriteLine(sizeof(string));

Will be thrown

A, sizeof

1, except struct, the general value types may be used as the sizeof () to calculate its size, it does not involve a context security issues, such as:

Console.WriteLine(sizeof(int));//4

DETAILED sizeof is calculated as follows:

2, as for struct, if its use the sizeof () relates to the security context, such as code

public struct TestStruct
{
    public bool TestA;
    public int TestInt;
    public bool TestB;
}

// use sizeof must be hung in unsafe under the code, but also to be set up to support the project properties unsafe context mode
unsafe
{
    Console.WriteLine(sizeof(TestStruct));
}

At this time, the output byte size is 12 , not 1 + 4 + 1 = 6.

Why is it 12, which relates to memory alignment .

Memory Alignment rules can click here

TestStruct corresponding memory is: 0xxx | 0000 | 0xxx

If TestB and TestInt two lines about the swap, the following code

public struct TestStruct2
{
    public bool TestA;
    public bool TestB;
    public int TestInt;
}

At this time TestStruct2 byte size 8 is occupied, the corresponding memory should be: 00xx | 0000

To TestStruct and share the same memory size TestStruct2, you can use TestStruct field layout [ StructLayout ( LayoutKind . Auto ) ]

[StructLayout(LayoutKind.Auto)]
public struct TestStruct
{
    public bool TestA;
    public int TestInt;
    public bool TestB;
}

As for field layout , can be seen here

二、Marshal.SizeOf

Marshal.SizeOf returns the class size unmanaged

Marshal.SizeOf document, see the official website

To facilitate understanding, on marshaling can reference herein

I tested the code is as follows:

using System;
using System.Runtime.InteropServices;

namespace TestCSharp
{
    [StructLayout(LayoutKind.Sequential)]
    public class TestClass
    {
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct TestStruct2
    {
        public byte by;
        public sbyte sb;
        public bool b;
        public char c;
        public short s;
        public ushort us;
        public int i;
        public uint ui;
        public long l;
        public ulong ul;
        public float f;
        public double d;
        public decimal dec;
        public TestClass tc;
    }

    internal class Program
    {
        public static void Main(string[] args)
        {
            TestStruct2 t = new TestStruct2();
            t.tc = new TestClass();
            Console.WriteLine(Marshal.SizeOf(t.by));//1
            Console.WriteLine(Marshal.SizeOf(t.sb));//1
            Console.WriteLine(Marshal.SizeOf(t.b));//4
            Console.WriteLine(Marshal.SizeOf(t.c));//1
            Console.WriteLine(Marshal.SizeOf(t.s));//2
            Console.WriteLine(Marshal.SizeOf(t.us));//2
            Console.WriteLine(Marshal.SizeOf(t.i));//4
            Console.WriteLine(Marshal.SizeOf(t.ui));//4
            Console.WriteLine(Marshal.SizeOf(t.l));//8
            Console.WriteLine(Marshal.SizeOf(t.ul));//8
            Console.WriteLine(Marshal.SizeOf(t.f));//4
            Console.WriteLine(Marshal.SizeOf(t.d));//8
            Console.WriteLine(Marshal.SizeOf(t.dec));//16
            Console.WriteLine(Marshal.SizeOf(t.tc));//1
            Console.WriteLine(Marshal.SizeOf(t));//80，#pragma pack(n) 表示设置为n字节对齐。 C#默认应该也是8字节对齐（没有查证），所以t.dec应该是按照8而不是16对齐，这个需要注意
        }
    }
}

这个是测试输出的字节大小

其中类的封送不能用[StructLayout(LayoutKind.Auto)]，可以改成[StructLayout(LayoutKind.Sequential)]

Marshal.SizeOf所得大小也是按内存对齐规则来的

另：如果sizeof(TestStruct)没有放在unsafe下，如

Console.WriteLine(sizeof(TestStruct));

会有编译错误：

解决办法是：

unsafe
{
　　Console.WriteLine(sizeof(TestStruct));//用sizeof必须要挂在unsafe 代码下，而且工程属性要设置为支持上下文不安全模式
}

别忘记工程属性要设置为支持上下文不安全模式，否则会报如下编译错误：

C # and the sizeof Marshal.SizeOf

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