Getting System.ExecutionEngineException Error Calling a C Function That Passes an Array of Structs in C#

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?

Hans Passant · Accepted Answer · 2012-01-10 18:23:07Z

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; }

Thanks Hans, replacing the ref with In,out has fixed this issue

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