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前言

在C语言中，有一种类型名为共用体，关键字为union，它与结构体十分类似，但是它所有的数值全都指向同一个指针。也就是说，当你修改共用体内的某一个数据时，其他数据也会同时改变。
这个类型的好处，在于可以存储某种不确定的数据对象，等到稍后再来分析出该对象的用途。
这个对于数制的转换和传输非常有用。
在之前的VB6.0中，我使用的是CopyMemory的方法，但在现在的VB.net中这里有了新的变化，在这里记录一下

结构体和共用体的定义

union MyUnion
{char b; //单字节整数，在c语言中用char类型来表示单字节整数short s; //双字节整数int i; //四字节整数
}

在 .net 中使用联合体，我们只需要声明一个普通的结构体，然后在结构体上使用属性描述即可。

结构体：

    Structure MyStructDim b0 As ByteDim b1 As ByteDim b2 As ByteDim b3 As ByteDim s As ShortDim us As UShortDim i As IntegerDim ui As UIntegerEnd Structure

联合体：

Imports System.Runtime.InteropServices'引入运行时非托管数据管理服务<StructLayout(LayoutKind.Explicit)> _Structure MyUnion<FieldOffset(0)> Dim b0 As Byte<FieldOffset(1)> Dim b1 As Byte<FieldOffset(2)> Dim b2 As Byte<FieldOffset(3)> Dim b3 As Byte<FieldOffset(0)> Dim s As Short<FieldOffset(2)> Dim us As UShort<FieldOffset(0)> Dim i As Integer<FieldOffset(0)> Dim ui As UIntegerEnd Structure

可以看出 .NET 中的联合体更强大，因为它甚至可以指定某个数值的偏移位置。

一个完整的程序示例

Imports System.Runtime.InteropServices '引入运行时非托管数据管理服务Public Class Form1<StructLayout(LayoutKind.Explicit)> _Structure MyUnion<FieldOffset(0)> Dim b0 As Byte<FieldOffset(1)> Dim b1 As Byte<FieldOffset(2)> Dim b2 As Byte<FieldOffset(3)> Dim b3 As Byte<FieldOffset(0)> Dim s As Short<FieldOffset(2)> Dim us As UShort<FieldOffset(0)> Dim i As Integer<FieldOffset(0)> Dim ui As UInteger<FieldOffset(0)> Dim fd As SingleEnd StructureStructure MyStructDim b0 As ByteDim b1 As ByteDim b2 As ByteDim b3 As ByteDim s As ShortDim us As UShortDim i As IntegerDim ui As UIntegerEnd StructurePrivate Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.ClickDim mu As MyUnion' 一种安全的int与uint转换方法'mu.ui = UInteger.MaxValuemu.i = &H8234567Fmu.ui = &H7234567FMsgBox(mu.i) ' 一个存储与读取IP数据的方法mu.b0 = 128 : mu.b1 = 0 : mu.b2 = 0 : mu.b3 = 1 '127.0.0.1Dim ip As New Net.IPAddress({128, 0, 0, 1})MsgBox(CInt(ip.GetHashCode) = mu.i) 'ip数值相等' 其他用法...mu.ui = UInteger.MaxValueMsgBox(mu.s) '-1MsgBox(mu.us) '65535End SubEnd Class

下位机和上位机浮点传输示例

如果下位机和上位机是通过串口传输

下位机示例

union DoubleToBit 
{unsigned char  char_buffer[8];double   double_num;
};union FloatToBit 
{unsigned char  char_buffer[4];float   float_num;
};void transFloat(float f_ptr)
{int i;unsigned char data;Float_char_union.float_num = f_ptr;for(i = 0 ; i < 4 ; i++){data = Float_char_union.char_buffer[3-i];write_queue_tx(data);}}void transDouble(double d_ptr)
{int i;unsigned char data;Double_char_union.double_num = d_ptr;for(i = 0 ; i < 8 ; i++){data = Double_char_union.char_buffer[7-i];write_queue_tx(data);}}

上位机示例

Imports System.Runtime.InteropServices '引入运行时非托管数据管理服务Public Class Form1<StructLayout(LayoutKind.Explicit)> _Structure MyUnion<FieldOffset(0)> Dim b0 As Byte<FieldOffset(1)> Dim b1 As Byte<FieldOffset(2)> Dim b2 As Byte<FieldOffset(3)> Dim b3 As Byte<FieldOffset(0)> Dim fd As SingleEnd StructureDim mu As MyUnionmu.b0 = hex1 mu.b1 = hex2  mu.b2 = hex3  mu.b3 = hex4  MsgBox(mu.fd)

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