Question

In C code, the function is defined like:

INT WINAPI myFunction(LPCTSTR str1, LPCTSTR str2, INT iNumber,
    LPSTRUCT *lpStruct);

*lpStruct is an array of pointers of a struct type:

typedef struct myStruct
{
CHAR                    m_s1[64];       
UINT                    m_nS;       
CHAR                    m_s2[8][64];    
UINT                    m_nP;
CHAR                    m_s3[512];

} SomeStruct, *LPSTRUCT;

I need to call this external myFunction in C#, I defined SomeStruct as:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct SomeStruct
{

   [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 64)]
   public string m_s1;             
   public uint m_nS;             
   [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 512)]
   public string m_s2;             
   public uint m_nP;
   [MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 512)]
   public string m_s3;
 }

myFunction as:

[DllImport("some.dll")]
public static extern int myFunction(
             string str1,
             string str2,
             int iNumber,
             IntPtr[] lpStruct);

I initialize IntPtr[] in C#:

IntPtr[] lpptr = new IntPtr[iNumber]; 

I know the struct array of pointer has iNumber elements.

There is no error to call this function (lpStruct[i] has number). But when I tried to Marshal the pointer to the struct using:

SomeStruct st = (SomeStruct )Marshal.PtrToStructure(lpStruct[i], 
    typeof(SomeStruct ));

I got error message: try to write read-only memory. I don’t know what’s wrong here. Is the external function definition in C# wrong or the definition of struct wrong, or both.

Answer 1

The question lacks important details, but I would expect that you need to allocate memory for each IntPtr in the array.

IntPtr[] lpptr = new IntPtr[iNumber];
for (int i=0; i<iNumber; i++)
    lpptr[i] = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(SomeStruct)));

And obviously you should free the memory once you are done with it.