How can I declare a pointer to a struct for P/Invoke?

Question

I'm trying to use P/Invoke Interop Assistant to call a C++ Dll in C#. Most of the header is converted fine but I'm having trouble with this:

#define FULLOCTAVE_BINS                12
#define THIRDOCTAVE_BINS               36

typedef struct tagTimeHistory
{
    UINT m_nAction;
    int m_nFlag;
    int m_nRecordNum; 
    int m_nTimeStamp;
    int m_nMiscStartIndex;
    float m_pfTHFloatVals[256]; // Number of valid values given by m_nNumFloatVals in Settings.
    float m_pfTH11OBAVals[4][FULLOCTAVE_BINS];  // 0-4 spectra given by m_nNumOBA11Vals in Settings
    float m_pfTH13OBAVals[4][THIRDOCTAVE_BINS]; // 0-4 spectra given by m_nNumOBA13Vals in Settings
    float m_fDuration;
} stTimeHistory_t;

typedef struct tagSlmBulkRecords
{
    int nRecType;
    union
    {
        stTimeHistory_t *m_ThRecs;
        stInterval_t    *m_Interval;
        stExceedence_t  *m_Exceedences;
        stRunRecord_t   *m_RunRecord;
        stSpeechData_t  *m_VoiceRecord;
        stSpeechData_t  *m_AudioRecord;
    };
} stSlmBulkRecord_t;

This is being converted to:

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Explicit)]
public struct Anonymous_d2bf9406_c664_4664_9196_800cc23f445a {

    /// stTimeHistory_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_ThRecs;

    /// stInterval_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_Interval;

    /// stExceedence_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_Exceedences;

    /// stRunRecord_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_RunRecord;

    /// stSpeechData_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_VoiceRecord;

    /// stSpeechData_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_AudioRecord;
}

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
public struct tagSlmBulkRecords {

    /// int
    public int nRecType;

    /// Anonymous_d2bf9406_c664_4664_9196_800cc23f445a
    public Anonymous_d2bf9406_c664_4664_9196_800cc23f445a Union1;
}

But how do I use m_ThRecs when it's just a System.IntPtr? Is there some way of explicitly declaring it to be a pointer to stTimeHistory_t? The C++ code I'm porting to C# uses it like this:

stSlmBulkRecord_t   bulkRecord;
bulkRecord.m_ThRecs = new stTimeHistory_t[dataCounts.m_nNumTH];

but if I try this in C#:

tagSlmBulkRecords bulkRecord;
bulkRecord.Union1.m_ThRecs = new tagTimeHistory[dataCounts.m_nNumTH];

I get:

Error 1 Cannot implicitly convert type 'SlmTest.Program.tagTimeHistory[]' to 'SlmTest.Program.tagTimeHistory'"

If I try an unsafe definition:

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
public struct tagTimeHistory
{
    /// UINT->unsigned int
    public uint m_nAction;

    /// int
    public int m_nFlag;

    /// int
    public int m_nRecordNum;

    /// int
    public int m_nTimeStamp;

    /// int
    public int m_nMiscStartIndex;

    /// float[256]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst = 256, ArraySubType = System.Runtime.InteropServices.UnmanagedType.R4)]
    public float[] m_pfTHFloatVals;

    /// float[48]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst = 48, ArraySubType = System.Runtime.InteropServices.UnmanagedType.R4)]
    public float[] m_pfTH11OBAVals;

    /// float[144]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst = 144, ArraySubType = System.Runtime.InteropServices.UnmanagedType.R4)]
    public float[] m_pfTH13OBAVals;

    /// float
    public float m_fDuration;
}

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Explicit)]
public unsafe struct Anonymous_d2bf9406_c664_4664_9196_800cc23f445a
{
    /// stTimeHistory_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public tagTimeHistory *m_ThRecs;
    /// stInterval_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr *m_Interval;
    /// stExceedence_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_Exceedences;
    /// stRunRecord_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_RunRecord;
    /// stSpeechData_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_VoiceRecord;
    /// stSpeechData_t*
    [System.Runtime.InteropServices.FieldOffsetAttribute(0)]
    public System.IntPtr m_AudioRecord;
}

I get:

error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type

Is your `stSlmBulkRecord_t` in C# declared as a `struct` or a `class`? — Jim Mischel, Oct 12 '14 at 04:06
stSlmBulkRecord_t isn't declared in C#, only tagSlmBulkRecords which is a struct as shown above. — parsley72, Oct 12 '14 at 05:23

ForNeVeR · Accepted Answer · 2014-10-12T06:27:54.330

If you really want to interoperate with the native code, you can use the fixed operator:

var array = new tagTimeHistory[dataCounts.m_nNumTH];
fixed (tagTimeHistory* ptr = array)
{
    // do anything with the raw pointer
}

Note that fixed operator and pointer types in C# requires unsafe capability. And you may want to replace IntPtrs with the proper pointer types for more type safety (although there is a method for converting pointers to IntPtrs and back).

Another way of doing the same is through the methods of Marshal class.

EDIT. Here is a sample of the revised unsafe definition of your funny named union structure:

[StructLayout(LayoutKind.Explicit)]
public unsafe struct Anonymous_d2bf9406_c664_4664_9196_800cc23f445a
{
    [FieldOffset(0)]
    public stTimeHistory_t* m_ThRecs;

    [FieldOffset(0)]
    public stInterval_t* m_Interval;

    [FieldOffset(0)]
    public stExceedence_t* m_Exceedences;

    [FieldOffset(0)]
    public stRunRecord_t* m_RunRecord;

    [FieldOffset(0)]
    public stSpeechData_t* m_VoiceRecord;

    [FieldOffset(0)]
    public stSpeechData_t* m_AudioRecord;
}

You should define all structures such as stTimeHistory_t in your code (or replace the ones you don't care with generic IntPtrs).

And regarding creation of structure array with Marshal: native memory pool have no such concept as structure array; all it care is only bytes. So you may, for example, use the Marshal.AllocHGlobal method:

IntPtr myPtr = Marshal.AllocHGlobal(Marshal.SizeOf<tagTimeHistory>() * dataCounts.m_nNumTH);
// ... write something to an array, use it
// And don't forget to free it to prevent memory leaks!
Marshal.FreeHGlobal(myPtr);

EDIT 2. Regarding the "Cannot take the address of, get the size of, or declare a pointer to a managed type" error - your definition is not fully unmanaged. Unsafe definitions and the ones using the marshaling logic are not always equal; here it thinks your class is "managed" because of array references in it. Try fixed arrays:

[StructLayout(LayoutKind.Sequential)]
public unsafe struct tagTimeHistory
{
    public uint m_nAction;
    public int m_nFlag;
    public int m_nRecordNum;
    public int m_nTimeStamp;
    public int m_nMiscStartIndex;
    public fixed float m_pfTHFloatVals[256];
    public fixed float m_pfTH11OBAVals[48];
    public fixed float m_pfTH13OBAVals[144];
    public float m_fDuration;
}

When you say "you may want to replace 'IntPtr's with the proper pointer types for more type safety" how is this done? Alternatively, how do I use marshalling to create an array of structs? — parsley72, Oct 12 '14 at 05:48
I tried the unsafe definition but I get "error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type" — parsley72, Oct 12 '14 at 06:09
@parsley72, please show the definition of your `tagTimeHistory` in your question. I bet that's a `class` and not a `struct`. — ForNeVeR, Oct 12 '14 at 06:11
By the way, please note that my post here is discussing two alternatives: using the `Marshal` class and using the unsafe pointers. They can be combined not in every case. — ForNeVeR, Oct 12 '14 at 06:31
Ah, I assumed P/Invoke Interop Assistant knew what it was doing but your suggestion has fixed my problem. Thanks! — parsley72, Oct 12 '14 at 06:38
I lost a lot of time until I read your comment that says that having arrays without the fixed makes the compiler consider the struct as managed — orellabac, Jul 24 '19 at 00:56

How can I declare a pointer to a struct for P/Invoke?

1 Answers1

Linked