Integrated crosspatch device with C# form

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C#4.0

I'm trying to integrate with crossmatch device (PATROL ID) using C#
I have a problem when I want to Get Device count , all time I have( -1) value " it's means invalid parameter"

What I have tried:

My code In Load Form :

int devices = 0;
                int stat = VeriferNativeMethods.LSCAN_Main_GetDeviceCount(devices);
                MessageBox.Show(stat.ToString());

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load ddl in another Class (

VeriferNativeMethods

) :

const string DllName = "LScanEssentials.dll";

[DllImport(DllName, EntryPoint = "LSCAN_Main_GetDeviceCount")]
        public static extern int LSCAN_Main_GetDeviceCount(int DCount);