C Union -> C# struct LayoutKind.Explicit FieldOffset - how?

[Guest]

Hi there,

I had a union in C:

typedef union TagMessage
{
struct {
unsigned char avalue;
unsigned char bvalue;
} a;
struct {
unsigned short cvalue;
} b;
} Message;

From these few links, there is a possible way is to declare a struct with
LayoutKind.Explict and use FieldOffset.

http://www.dotnet247.com/247reference/msgs/18/93017.aspx
http://www.hanselman.com/blog/PermaLink.aspx?guid=187

The problem is how do i relate the FieldOffset in my scenarios. I had 2
structs in my C union. How do i achieve it with the help of that?

Any idea please? Thanks.

 

[Sijin Joseph]

Try this

[StructLayout(LayoutKind.Explicit)]
public struct Message
{
[StructLayout(LayoutKind.Explicit)]
public struct AStruct
{
[FieldOffset(0)] public System.Byte avalue;
[FieldOffset(1)] public System.Byte bvalue;
}

[FieldOffset(0)]public AStruct a;

[StructLayout(LayoutKind.Explicit)]
public struct BStruct
{
[FieldOffset(0)] public System.UInt16 cvalue;
}

[FieldOffset(0)]public BStruct b;
}

 

[Guest]

Thanks Sijin, i had 1 question.

How do i know what argument to be past in here:

FieldOffset(0) or (1) or (16)

Hmm, can you give me some examples? I can't find proper explanation even on
msdn.

Thanks a lot. Really appreciate it.

Try this

[StructLayout(LayoutKind.Explicit)]
public struct Message
{
[StructLayout(LayoutKind.Explicit)]
public struct AStruct
{
[FieldOffset(0)] public System.Byte avalue;
[FieldOffset(1)] public System.Byte bvalue;
}

[FieldOffset(0)]public AStruct a;

[StructLayout(LayoutKind.Explicit)]
public struct BStruct
{
[FieldOffset(0)] public System.UInt16 cvalue;
}

[FieldOffset(0)]public BStruct b;
}

 

[Sijin Joseph]

The FieldOffset attribute defines the number of bytes from the start of the
structure where this field will be layed out in memory.

In your case avalue and b value are unsigned chars which take 1 byte so they
will be layed out at mem locations 0 and 1. cvalue is a unsigned short since
it is a union that will also be layed out at offset 0 from the start of the
structure and take 2 bytes.

Basically FieldOffset defines the number of bytes from the start of the
structure where the field will be placed.


Let me know if you need more hlp.
--
Sijin Joseph
http://www.indiangeek.net
http://weblogs.asp.net/sjoseph

Thanks Sijin, i had 1 question.

How do i know what argument to be past in here:

FieldOffset(0) or (1) or (16)

Hmm, can you give me some examples? I can't find proper explanation even on
msdn.

Thanks a lot. Really appreciate it.

Try this

[StructLayout(LayoutKind.Explicit)]
public struct Message
{
[StructLayout(LayoutKind.Explicit)]
public struct AStruct
{
[FieldOffset(0)] public System.Byte avalue;
[FieldOffset(1)] public System.Byte bvalue;
}

[FieldOffset(0)]public AStruct a;

[StructLayout(LayoutKind.Explicit)]
public struct BStruct
{
[FieldOffset(0)] public System.UInt16 cvalue;
}

[FieldOffset(0)]public BStruct b;
}

--
Sijin Joseph
http://www.indiangeek.net
http://weblogs.asp.net/sjoseph

Hi there,

I had a union in C:

typedef union TagMessage
{
struct {
unsigned char avalue;
unsigned char bvalue;
} a;
struct {
unsigned short cvalue;
} b;
} Message;

From these few links, there is a possible way is to declare a struct with
LayoutKind.Explict and use FieldOffset.

http://www.dotnet247.com/247reference/msgs/18/93017.aspx
http://www.hanselman.com/blog/PermaLink.aspx?guid=187

The problem is how do i relate the FieldOffset in my scenarios. I had 2
structs in my C union. How do i achieve it with the help of that?

Any idea please? Thanks.

 

[Guest]

Hi Sijin,

Thanks a lot for the explanation. Ok, i will give it a try when i get back
to office, i hope it will work.

Thanks again.

The FieldOffset attribute defines the number of bytes from the start of the
structure where this field will be layed out in memory.

In your case avalue and b value are unsigned chars which take 1 byte so they
will be layed out at mem locations 0 and 1. cvalue is a unsigned short since
it is a union that will also be layed out at offset 0 from the start of the
structure and take 2 bytes.

Basically FieldOffset defines the number of bytes from the start of the
structure where the field will be placed.


Let me know if you need more hlp.
--
Sijin Joseph
http://www.indiangeek.net
http://weblogs.asp.net/sjoseph

Thanks Sijin, i had 1 question.

How do i know what argument to be past in here:

FieldOffset(0) or (1) or (16)

Hmm, can you give me some examples? I can't find proper explanation even on
msdn.

Thanks a lot. Really appreciate it.

Try this

[StructLayout(LayoutKind.Explicit)]
public struct Message
{
[StructLayout(LayoutKind.Explicit)]
public struct AStruct
{
[FieldOffset(0)] public System.Byte avalue;
[FieldOffset(1)] public System.Byte bvalue;
}

[FieldOffset(0)]public AStruct a;

[StructLayout(LayoutKind.Explicit)]
public struct BStruct
{
[FieldOffset(0)] public System.UInt16 cvalue;
}

[FieldOffset(0)]public BStruct b;
}

--
Sijin Joseph
http://www.indiangeek.net
http://weblogs.asp.net/sjoseph


Hi there,

I had a union in C:

typedef union TagMessage
{
struct {
unsigned char avalue;
unsigned char bvalue;
} a;
struct {
unsigned short cvalue;
} b;
} Message;

From these few links, there is a possible way is to declare a struct with
LayoutKind.Explict and use FieldOffset.

http://www.dotnet247.com/247reference/msgs/18/93017.aspx
http://www.hanselman.com/blog/PermaLink.aspx?guid=187

The problem is how do i relate the FieldOffset in my scenarios. I had 2
structs in my C union. How do i achieve it with the help of that?

Any idea please? Thanks.