Can I use pointers like in C?

[htrada]

Hi,

I´m currently developing an application wich transmit floiting point numbers (trough a serial port). The numbers, wich are 32 bits long, are transmited in 4 bytes. Is there any way to use something like pointers in C????

 

what I want to do is get those 4 bytes and asign them to the 32 bit single variable.

[IngisKahn]

Sure:

 

byte[] floatBytes = new byte[4];
float val;

// assign values

unsafe // remember to enable unsafe mode in the build options
{
   fixed (byte* pfloatBytes = floatBytes) // all managed types must be fixed to use a pointer
       val = *(float*)pfloatBytes;
}

Edited May 2, 2005 by IngisKahn

[htrada]

thanks, but I think that is C# code. What would be the visual basic equivalent??

 

Sure:

 

byte[] floatBytes = new byte[4];
float val;

// assign values

unsafe // remember to enable unsafe mode in the build options
{
   fixed (byte* pfloatBytes = floatBytes) // all managed types must be fixed to use a pointer
       val = *(float*)pfloatBytes;
}

[IngisKahn]

Hehe.

There is none.

 

 

Embrace .NET! You can have VB access C# dlls, you can even have VB and C# in the same assembly.

[mark007]

Hehe.

There is none.

 

 

Embrace .NET! You can have VB access C# dlls, you can even have VB and C# in the same assembly.

 

 

Not quite true. You can use the system.runtime.interopservices marshall class. For example this code takes apart an integer into bytes and puts it back together again. You can of course do similar with a floating point number.

 

   Sub test()
       Dim x As System.Int32 = 32400
       Dim b(3) As System.Byte
       Dim i As New System.IntPtr

       'take apart
       i = Marshal.AllocHGlobal(4)
       Marshal.WriteInt32(i, x)
       For j As Integer = 0 To 3
           b(j) = Marshal.ReadByte(i, j)
           System.Windows.Forms.MessageBox.Show("Part: " & b(j))
       Next

       'put back together
       Dim y As System.Int32
       Dim py As System.IntPtr
       py = Marshal.AllocHGlobal(4)
       For j As Integer = 0 To 3
           Marshal.WriteByte(py, j, b(j))
       Next
       y = Marshal.ReadInt32(py)
       System.Windows.Forms.MessageBox.Show("Result: " & y)
   End Sub

 

HTH

 

:)

[IngisKahn]

Ya, but I'd suggest to stay away from Marshal wherever possible.

[htrada]

ok, I´ll see wich option suits my applicacion. Thanks both of you for your help.

 

 

Ya' date=' but I'd suggest to stay away from Marshal wherever possible.[/quote']

[mark007]

Ya' date=' but I'd suggest to stay away from Marshal wherever possible.[/quote']

 

 

I assume you have a reason for this?

[IngisKahn]

Take a look at the MSIL for each version.

[mark007]

Which will reveal?

[PlausiblyDamp]

Do you have a sample of what the values and their 4 byte representaion would be? Or would a .Net single or double be suitable anyway?

[IngisKahn]

Along those lines, would you be better served by a BinaryReader?

[mark007]

Not wanting to antagonise in any way, just would like to know...

 

Take a look at the MSIL for each version.

 

Which will reveal?

[IngisKahn]

It would reveal some of the overhead involved. But anyway, I don't think you should be using InteropServices when a simple stream would do.

[snarfblam]

If you want to get the bytes that make up most any numeric format, try using the System.BitConverter class. Alternatively you could create a "union" of sorts using the <System.Runtime.InteropServices.FieldOffset> and <System.Runtime.InteropServices.StructLayout> attributes on a struct.

 

<System.Runtime.InteropServices.StructLayout(Runtime.InteropServices.LayoutKind.Explicit)> _
Structure Union_SingleByte
   <System.Runtime.InteropServices.FieldOffset(0)> _
   Dim [single] As Single
   <System.Runtime.InteropServices.FieldOffset(0)> _
   Dim Byte0 As Single
   <System.Runtime.InteropServices.FieldOffset(1)> _
   Dim Byte1 As Single
   <System.Runtime.InteropServices.FieldOffset(2)> _
   Dim Byte2 As Single
   <System.Runtime.InteropServices.FieldOffset(3)> _
   Dim Byte3 As Single
 
   'You can assign the bytes and read the single, or
   'assign the single and read the bytes
End Structure
 
'Even easier 
Dim Bytes As Byte() = BitConverter.GetBytes(MySingle)

[mark007]

So your reasoning is it's slow? What sort of proportion compared to using c# pointers?

 

Any other issues?

 

Admittedly, this may not matter for this problem particularly but would be useful for others searching on the subject, and myself ;)

[htrada]

thanks, I think Ill go for te union if it works right it seems to be more flexible

[htrada]

the bit converter class is great!, I´ve tried it and has all conversions included