使用 C# 在结构中使用结构的不安全固定大小数组访问旧版 C-dll

Question

I have a legacy C-dll to access a Hardware Device. The dll uses structures containing structure Arrays as function arguments. And I am having a hard time to get this working together with C# (in unsafe mode which is ok since access Speed is an issue here).

The original declaration from DeviceDll.h of the legacy C-dll Looks like:

    typedef struct tag_RESULT_JUDGEMENT
    {
        short nJudgementResult;             
    } struc_RESULT_JUDGEMENT;

    typedef struct tag_RESULT_FORMULA
    {
        float fFormulaResult[3];            
        float fAnalogResult;                
    } struc_RESULT_FORMULA;


    typedef struct tag_RESULT_SCRIPT
    {
        short nScriptNo;                    
        float fTime;                        
        short nFormulaCount;                
        struc_RESULT_FORMULA sFormula[32];
        short nJudgementCount;              
        struc_RESULT_JUDGEMENT sJudgement[8];
    } struc_RESULT_SCRIPT;

    WORD PASCAL GetResult(LPCTSTR pDeviceCode, struc_RESULT_SCRIPT_NO_LIST* pList, struc_RESULT_SCRIPT* pResult[]);

Declaring the C-dll functions in C# is done in the following way:

[DllImport("ExternalDevice.dll")]   
public unsafe static extern int GetResult(StringBuilder pDeviceCode, struc_RESULT_SCRIPT_NO_LIST* pList, struc_RESULT_SCRIPT **ppResult);

and the structure which I want to access I have declared in a way which unfortunatly gives an error:

[StructLayout(LayoutKind.Explicit)]
public unsafe struct struc_RESULT_SCRIPT {
    [FieldOffset(0)]
    public Int16 nScriptNo;

    [FieldOffset(2)]
    public float fTime;

    [FieldOffset(6)]
    public Int16 iFormulaCount;              

    [FieldOffset(8)
    public fixed struc_RESULT_FORMULA[32] oFormula;
}

Error: Buffer of a fixed size must be of "bool", "Byte", . . . or "double"

Is there a way to declare a fixed structure Array within a structure so that I can use a structure-type variable afterwards as an Argument for calling the legacy DLL?

What I have tried so far:

Avoiding a structure Array within a structure (rather clumsy but working)

    [StructLayout(LayoutKind.Explicit)]
    public unsafe struct struc_RESULT_SCRIPT {
        [FieldOffset(0)]
        public Int16 nScriptNo;

        [FieldOffset(2)]
        public float fTime;

        [FieldOffset(6)]
        public Int16 iFormulaCount;              

        [FieldOffset(8)
        public struc_RESULT_FORMULA oFormula01;

        [FieldOffset(8 + 16)
        public struc_RESULT_FORMULA oFormula02;

        . . . 

        [FieldOffset(8 + 31*16)
        public struc_RESULT_FORMULA oFormula32;
    }

    . . . 


    struc_RESULT_SCRIPT** ppResult;       //local variable => allocated on the stack => so it's already fixed

    . . .

    int iRv = GetResult(sbMpmIp, &oScriptList, ppResult);
    struc_RESULT_FORMULA oFormulaResult = ppResult[0]->oFormula01;

This is working - but accessing oFormula01 … oFormula32 for 32 structure-variables is rather clumsy. I would strongly prefer to get the result as an Array so that I can Access it like oFormula = arrayFormula[xx]; within a for - loop.

Is there a way to declare an unsafe, fixed structure Array within a structure in C# - or a feasable work-around?

Thank you very much in advance!

Answer 1

According to the documentation for fixed sized buffers there is no way use structs in them.

The best option seem to do as you have done and declare 1..n fields. The anwer to this related question suggest the same thing. If the problem is simply not being able to loop over the fields, then consider adding a iterator block, like:

public IEnumerable<struc_RESULT_FORMULA> GetResultFormulas(){
    yield return oFormula01;
    ...
    yield return oFormula32;
}

An alternative would be a large switch statement/expression if you need random indexing.