how to call a C# function, foo(out byte [])? -- help passing a byte pointer

Asked by daniel rasnow on 5 Jun 2014

Latest activity Commented on by daniel rasnow on 28 Jul 2014

Accepted Answer by Friedrich

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Been struggling to call the following function in a C# dll:

public bool GetLatestBuffer(out byte[] buffer)

I've allocated the buffer in Matlab and tried various castings with libpointer, but keep getting the result:

No method 'GetLatestBuffer' with matching signature found for class ...

I could write a wrapper in C# that allocates the buffer ... but would rather do the memory management in Matlab.

My pseudocode is basically:

   NET.addAssembly(dllLoc)
   ls = luma_ns.Luma(initParams);
   ls.function1(params) % --> it works passing parameters by value
   buf = zeros(10000,'uint8'); % tried uint16, int16, etc.
   pbuf = libpointer('uint8Ptr',buf); 
   ls.GetLatestBuffer(pbuf) % doesn't match the prototype of out byte []
No method 'GetLatestBuffer' with matching signature found for class 'Luma_ns.Luma'.

Thanks!!

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3 Answers

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Answer by Friedrich on 21 Jul 2014

Edited by Friedrich on 22 Jul 2014

Accepted Answer

Hi,

have you tried calling it like this

[bool_val, byte_array] = obj.GetLatestBuffer();

This works fine for me in 14a with the following example

            public bool GetLatestBuffer(out byte[] Buffer)
            {
                Buffer = new byte[10];
                for (int i = 0; i < 10;i++)
                {
                    Buffer[i] = (byte)i;
                }
                return true;
            }

ML:

>> a = ClassLibrary1.Class1
a = 
Class1 with no properties.
>> [bool,array] = a.GetLatestBuffer()
bool =
       1
array = 
Byte[] with properties:
            Length: 10
        LongLength: 10
              Rank: 1
          SyncRoot: [1x1 System.Byte[]]
        IsReadOnly: 0
       IsFixedSize: 1
    IsSynchronized: 0

SideNote: The out keyword forces the routine GetLatestBuffer to assign Buffer to some value. So you do NOT need to pass down a preallocated array and since you dont need to do this it appears as output in MATLAB directly.

Also in C# when using OUT you never pass down a preallocated array. So what MATLAB does is correct here and matches the .NET programming.

1 Comment

daniel rasnow on 28 Jul 2014

AWESOME!! That did it!! Followed by

img = shiftdim(reshape(uint8(array.SyncRoot),3,numH,numV),1);
imagesc(img) % rbg image, which would have taken pages of C#

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Answer by James Tursa on 5 Jun 2014

Have you tried changing the header signature to something more friendly to MATLAB, but have the same basic I/O? E.g.,

bool GetLatestBuffer(char *buffer);

3 Comments

daniel rasnow on 6 Jun 2014

Thanks! I was hoping to avoid modifying the dll source, with has other consequences. But I tried using a wrapper function:

  public bool GetLatestMLBuffer(char *buffer)
  {
           GetLatestBuffer(buffer);
  }

and C#is "too smart" for that, it wouldn't compile:

Error: Pointers and fixed size buffers may only be used in an unsafe context

James Tursa on 6 Jun 2014

Don't recompile. Just create a new header file off to the side for MATLAB to use. I.e., copy the actual header h file to a new file, modify that new file by changing the signature of the function in question, then use that new header h file for calllib in MATLAB.

Friedrich on 22 Jul 2014

This wont work because that is the .NET interface he is using and not loadlibrary!!!!

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Answer by Anthony on 17 Jul 2014

Hi,

You can create a byte[] like this:

A = NET.createArray('System.Byte',4).

Then you just have to pass it to your function.

Hope it helps,

Anthony

1 Comment

daniel rasnow on 20 Jul 2014

Thanks Anthony! That looks so logical, but it still doesn't work, generating the error in Matlab: No method 'GetLatestBuffer' with matching signature found for class ..." I have the source (but would rather not change it). GetLatestBuffer is defined as

public bool GetLatestBuffer(out byte[] buffer)

I'm wondering if C# tries to be so smart to check the sizes of the buffers -- but I'm pretty sure I'm passing the exact size needed. Also tried >> buf = NET.createArray('System.Byte',1200,1199,3) vs. a single argument equal to the product, but all permutations give the same error msg. I'm very stumped!! |

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