Question

I am building a COM wrapper written in C# around a C dll and am getting an error when I call one particular function which takes a reference to a pre-initialized array of structs.

The error is: “An unhandled exception of type ‘System.ExecutionEngineException occurred in sXEposCOM.dll”

The C function has a signature as follows:

UINT16 sXGetSupportedSchemes( SXEposScheme_t *pSchemes );

And the SXEposScheme_t struct in the C dll is declared as:

typedef struct _SXEposScheme_t
{
BYTE    schemeID[SX_EPOS_SID_LENGTH];
char    schemeName[SX_EPOS_MAX_SCHEME_NAME_LENGTH];
} SXEposScheme_t;

In my C# wrapper (sXEposCOM) I have the following:

private const int SX_EPOS_SID_LENGTH    = 2;
private const int SX_EPOS_MAX_NUM_SCHEMES = 10;

[DllImport("SXEposDll.dll")]
private static extern UInt16 sXGetSupportedSchemes(ref SXEposScheme_t [] pSchemes);

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct SXEposScheme_t
{
    [MarshalAs(
    UnmanagedType.ByValArray,
    SizeConst = SX_EPOS_SID_LENGTH)]
    public byte[] schemeID;
    [MarshalAs(
    UnmanagedType.ByValArray,
    SizeConst = SX_EPOS_MAX_SCHEME_NAME_LENGTH)]
public char [] schemeName;
}

public int GetSupportedSchemes()
{
    uint result = 0;
    uint schemeCnt = 0;
    GetNumSchemes(ref schemeCnt); //This call to the C dll succeeds and returns 2 to   schemeCnt

    SquidEposScheme_t[] schemes = new SXEposScheme_t[schemeCnt];    
    result = sXGetSupportedSchemes(ref schemes); //Errors on this line with An unhandled exception of type 'System.ExecutionEngineException occurred in sXEposCOM.dll

...
...
return result;
}       

Can anyone tell me why I am getting this error?

Answer 1

This exception is triggered when the native code corrupts the garbage collected heap. Plenty of opportunity here, your C# declarations look wrong. An array is already passed as a pointer, adding ref to the argument makes it a pointer to a pointer. Possible fix:

[DllImport("SXEposDll.dll")]
private static extern UInt16 sXGetSupportedSchemes([In,Out] SXEposScheme_t [] pSchemes);

Pack=1 is very rarely appropriate, use ByValTStr to marshal a string. Possible fix:

[StructLayout(LayoutKind.Sequential)]
public struct SXEposScheme_t
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = SX_EPOS_SID_LENGTH)]
    public byte[] schemeID;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = SX_EPOS_MAX_SCHEME_NAME_LENGTH)]
    public string schemeName;
}