calling unmanaged dll function that returns an array of struct using C#

Question

I am having an unmanaged dll written using C++. I am able to call some functions easily from my C# application. But one function make me suffer :)

C++

The problem is in the log parameter. It should be reflected as an array of the Data_Struct type:

typedef struct
{
unsigned int    id;
unsigned short  year;
unsigned char   month;
unsigned char   day;
unsigned char   hour;
unsigned char   min;
unsigned char   sec;
unsigned char   status; 
}Data_Struct;

int Read_Stored_Data(HUNIT pUnitHandle, int option, int updateFlag, 
                     int maxEntries, unsigned char *log)

C# (my conversion)

public struct Data_Struct
{
    public uint id;
    public ushort year;
    public byte month;
    public byte day;
    public byte hour;
    public byte min;
    public byte sec;
    public byte status;

}

[DllImport("SData.dll", EntryPoint = "Read_Stored_Data")]
public static extern int Read_Stored_Data(int pUnitHandle, int option, 
    int updateFlag, int maxEntries, ref Data_Struct[] log);

Please assume that I am passing pUnitHandle, option, updateFlag, maxEntries with correct values. The problem is the last parameter (log):

Data_Struct[] logs = new Data_Struct[1000];
res = Read_Stored_Data(handle, 1, 0, 1000, ref logs); // This should work but it 
                                                      // causes the application 
                                                      // to terminate!

Any idea?

What happens when you call it? Unbalanced stack? AccessViolationException? Fails silently? — Polynomial
– Polynomial, Commented Dec 4, 2011 at 11:46

GSerg · Accepted Answer · 2011-12-04 12:13:28Z

1

Try playing with the PInvoke attributes.

Specifically, apply layout to the struct:

[StructLayout(LayoutKind.Sequential)]
public struct Data_Struct
{
    public uint id;
    public ushort year;
    public byte month;
    public byte day;
    public byte hour;
    public byte min;
    public byte sec;
    public byte status;
}

and apply a marshalling attribute to the parameter, while removing ref:

[DllImport("SData.dll", EntryPoint = "Read_Stored_Data")]
public static extern int Read_Stored_Data(int pUnitHandle, int option,
    int updateFlag, int maxEntries, [MarshalAs(UnmanagedType.LPArray), Out()] Data_Struct[] log);

See if that helps, adjust accordingly.

edited Dec 4, 2011 at 12:13

answered Dec 4, 2011 at 12:07

GSerg

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2 Comments

IoC Over a year ago

This worked perfectly. Even though [StructLayout(LayoutKind.Sequential)] is not required.

IoC Over a year ago

again got trapped after changing the struct as follows:

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