Equivalent c# code of this C++ code

Question

Hi everyone,

I have to create a desktop application in C# that gets connected to a Bluetooth low energy device and send values to the device using Set characteristic value function. For this I need get Characteristic buffer using get characteristics function. (http://msdn.microsoft.com/en-us/library/windows/hardware/hh450795(v=vs.85).aspx). 

I have C++ equivalent code to do this:

USHORT charBufferSize  = 0;
HRESULT hres = BluetoothGATTGetCharacteristics( BLE_Service,NULL, 0,NULL,
                                             &charBufferSize,BLUETOOTH_GATT_FLAG_NONE);
 if(hres==HRESULT_FROM_WIN32(ERROR_MORE_DATA)) // if 5
   {
      Characteristic_Buffer = new BTH_LE_GATT_CHARACTERISTIC [charBufferSize ];														  
   } // end of if 5													   
USHORT val = 0;
// get characteristics after allocating required size 
hres = BluetoothGATTGetCharacteristics(BLE_Service,NULL,charBufferSize,
                                 Characteristic_Buffer,&val,BLUETOOTH_GATT_FLAG_NONE);

I have to do this same thing in C#

So, I tried something like this

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
        public struct BTH_LE_GATT_CHARACTERISTIC
        {
            public UInt16 ServiceHandle;
            public BTH_LE_UUID CharacteristicUuid;
            public UInt16 AttributeHandle;
            public UInt16 CharacteristicValueHandle;
            public Boolean IsBroadcastable;
            public Boolean IsReadable;
            public Boolean IsWritable;
            public Boolean IsWritableWithoutResponse;
            public Boolean IsSignedWritable;
            public Boolean IsNotifiable;
            public Boolean IsIndicatable;
            public Boolean HasExtendedProperties;
        } 
[DllImport(@"BluetoothApis.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern Boolean BluetoothGATTGetCharacteristics(
                                                             IntPtr Device_Handle,
                                                             IntPtr Service,
                                                             UInt16 Char_Buffer_Count,
                                         ref BTH_LE_GATT_CHARACTERISTIC CharBuffer,
                                                             out UInt16 RequiredSize,
                                                             uint Flags
                                                             );

UInt16 Size_Required = 0;
UInt16 NBytes = 0;
BTH_LE_GATT_CHARACTERISTIC gattChar = new BTH_LE_GATT_CHARACTERISTIC();
Success = BluetoothGATTGetCharacteristics(Ble_Device, IntPtr.Zero, 0, IntPtr.Zero, 
                      out Size_Required, (uint)BTH_LE_FLAGS.BLUETOOTH_GATT_FLAG_NONE);

I am stuck here. Now, to make second call to the function, I should allocate memory to the characteristic buffer as done in c++. 

Any suggestions as how to allocate memory of required size to the characteristic structure??

Or is there any other way to use bluetooth low energy functions in C#?

Thanks in advance

phanitk23

Answer 1

In the C++ code, it looks like the fourth parameter of BluetoothGATTGetCharacteristics is a pointer to an array of BTH_LE_GATT_CHARACTERISTICs. (You are allocating Characteristic_Buffer so that it can hold charBufferSize items.)

In the C# code, however, you have declared the fourth parameter of the function to be a single item, and you are also allocating a single item.

Try changing the function declaration to

[DllImport(@"BluetoothApis.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern Boolean BluetoothGATTGetCharacteristics(
    IntPtr Device_Handle,
    IntPtr Service,
    UInt16 Char_Buffer_Count,
    BTH_LE_GATT_CHARACTERISTIC[] CharBuffer,
    out UInt16 RequiredSize,
    uint Flags
);

and the allocation to

BTH_LE_GATT_CHARACTERISTIC[] gattChar = new BTH_LE_GATT_CHARACTERISTIC[Size_Required]();

You also need to put the allocation of gattChar after the initial call to BluetoothGATTGetCharacteristics.

Then hopefully you will be able to make your second call to BluetoothGATTGetCharacteristics like this:

BluetoothGATTGetCharacteristics(Ble_Device, 
    IntPtr.Zero, Size_Required, gattChar, 
    out Size_Required,
    (uint)BTH_LE_FLAGS.BLUETOOTH_GATT_FLAG_NONE);

Answer 2

The equivalent C# code is shown below

C++

    byte Characteristic_Buffer[] = new BTH_LE_GATT_CHARACTERISTIC [charBufferSize ];               

           or

    byte *Characteristic_Buffer = new BTH_LE_GATT_CHARACTERISTIC [charBufferSize ];               

C#

    byte[] Characteristic_Buffer = new BTH_LE_GATT_CHARACTERISTIC [charBufferSize ];               

---

jdweng

Answer 3

Hey 

Thanks for the answers.. I ll try them and get back to you..

Answer 4

Hey mbj79,

I tried what you suggested.

It is getting executed well without any errors, but the structure is null after the second call also..

And the GetlastWin32error() gives 234 i.e, more data is available..

Any suggestions??

Thanks

Answer 5

Hi joel,

How can i give byte array as characteristic stucture?? do i need to typecast it??

Answer 6

Don't laugh when you see the solution.  I don't understand why you need to use 'new' with a char[] and don't need to use 'new' it with a byte[].

        static void Main(string[] args)
        {
            byte[] bytearray = { 0x31, 0x32, 0x33 };
            char[] chararrray = Encoding.UTF8.GetChars(bytearray);
            chararrray = new char[]{'A','B','B'};
            bytearray = Encoding.UTF8.GetBytes(chararrray);
        }

---

jdweng

Answer 7

Any chance you could share the whole code?

Also you should know that there already is a c# api for this, but in my case i need CharacteristicValueHandle and that isen't exposed in the c# api.

Example usage of the C# api https://github.com/AnderssonPeter/Home-Assistant-Windows-Tools/blob/master/gatttool/Program.cs