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Marshaling struct with dynamic array size (incorrect size) [duplicate]

How do I marshal this C++ type?
The ABS_DATA structure is used to associate an arbitrarily long data block with the length information. The declared length of the Data array is 1, but the actual length is given by the Length member.
typedef struct abs_data {
ABS_DWORD Length;
ABS_BYTE Data[ABS_VARLEN];
} ABS_DATA;
I tried the following code, but it's not working. The data variable is always empty and I'm sure it has data in there.
[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet = System.Runtime.InteropServices.CharSet.Ansi)]
public struct abs_data
{
/// ABS_DWORD->unsigned int
public uint Length;
/// ABS_BYTE[1]
[System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst = 1)]
public string Data;
}

Old question, but I recently had to do this myself and all the existing answers are poor, so...
The best solution for marshaling a variable-length array in a struct is to use a custom marshaler. This lets you control the code that the runtime uses to convert between managed and unmanaged data. Unfortunately, custom marshaling is poorly-documented and has a few bizarre limitations. I'll cover those quickly, then go over the solution.
Annoyingly, you can't use custom marshaling on an array element of a struct or class. There's no documented or logical reason for this limitation, and the compiler won't complain, but you'll get an exception at runtime. Also, there's a function that custom marshalers must implement, int GetNativeDataSize(), which is obviously impossible to implement accurately (it doesn't pass you an instance of the object to ask its size, so you can only go off the type, which is of course variable size!) Fortunately, this function doesn't matter. I've never seen it get called, and it the custom marshaler works fine even if it returns a bogus value (one MSDN example has it return -1).
First of all, here's what I think your native prototype might look like (I'm using P/Invoke here, but it works for COM too):
// Unmanaged C/C++ code prototype (guess)
//void DoThing (ABS_DATA *pData);
// Guess at your managed call with the "marshal one-byte ByValArray" version
//[DllImport("libname.dll")] public extern void DoThing (ref abs_data pData);
Here's the naïve version of how you might have used a custom marshaler (which really ought to have worked). I'll get to the marshaler itself in a bit...
[StructLayout(LayoutKind.Sequential)]
public struct abs_data
{
// Don't need the length as a separate filed; managed arrays know it.
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<byte>))]
public byte[] Data;
}
// Now you can just pass the struct but it takes arbitrary sizes!
[DllImport("libname.dll")] public extern void DoThing (ref abs_data pData);
Unfortunately, at runtime, you apparently can't marshal arrays inside data structures as anything except SafeArray or ByValArray. SafeArrays are counted, but they look nothing like the (extremely common) format that you're looking for here. So that won't work. ByValArray, of course, requires that the length be known at compile time, so that doesn't work either (as you ran into). Bizarrely, though, you can use custom marshaling on array parameters, This is annoying because you have to put the MarshalAsAttribute on every parameter that uses this type, instead of just putting it on one field and having that apply everywhere you use the type containing that field, but c'est la vie. It looks like this:
[StructLayout(LayoutKind.Sequential)]
public struct abs_data
{
// Don't need the length as a separate filed; managed arrays know it.
// This isn't an array anymore; we pass an array of this instead.
public byte Data;
}
// Now you pass an arbitrary-sized array of the struct
[DllImport("libname.dll")] public extern void DoThing (
// Have to put this huge stupid attribute on every parameter of this type
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<abs_data>))]
// Don't need to use "ref" anymore; arrays are ref types and pass as pointer-to
abs_data[] pData);
In that example, I preserved the abs_data type, in case you want to do something special with it (constructors, static functions, properties, inheritance, whatever). If your array elements consisted of a complex type, you would modify the struct to represent that complex type. However, in this case, abs_data is basically just a renamed byte - it's not even "wrapping" the byte; as far as the native code is concerned it's more like a typedef - so you can just pass an array of bytes and skip the struct entirely:
// Actually, you can just pass an arbitrary-length byte array!
[DllImport("libname.dll")] public extern void DoThing (
// Have to put this huge stupid attribute on every parameter of this type
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<byte>))]
byte[] pData);
OK, so now you can see how to declare the array element type (if needed), and how to pass the array to an unmanaged function. However, we still need that custom marshaler. You should read "Implementing the ICustomMarshaler Interface" but I'll cover this here, with inline comments. Note that I use some shorthand conventions (like Marshal.SizeOf<T>()) that require .NET 4.5.1 or higher.
// The class that does the marshaling. Making it generic is not required, but
// will make it easier to use the same custom marshaler for multiple array types.
public class ArrayMarshaler<T> : ICustomMarshaler
{
// All custom marshalers require a static factory method with this signature.
public static ICustomMarshaler GetInstance (String cookie)
{
return new ArrayMarshaler<T>();
}
// This is the function that builds the managed type - in this case, the managed
// array - from a pointer. You can just return null here if only sending the
// array as an in-parameter.
public Object MarshalNativeToManaged (IntPtr pNativeData)
{
// First, sanity check...
if (IntPtr.Zero == pNativeData) return null;
// Start by reading the size of the array ("Length" from your ABS_DATA struct)
int length = Marshal.ReadInt32(pNativeData);
// Create the managed array that will be returned
T[] array = new T[length];
// For efficiency, only compute the element size once
int elSiz = Marshal.SizeOf<T>();
// Populate the array
for (int i = 0; i < length; i++)
{
array[i] = Marshal.PtrToStructure<T>(pNativeData + sizeof(int) + (elSiz * i));
}
// Alternate method, for arrays of primitive types only:
// Marshal.Copy(pNativeData + sizeof(int), array, 0, length);
return array;
}
// This is the function that marshals your managed array to unmanaged memory.
// If you only ever marshal the array out, not in, you can return IntPtr.Zero
public IntPtr MarshalManagedToNative (Object ManagedObject)
{
if (null == ManagedObject) return IntPtr.Zero;
T[] array = (T[])ManagedObj;
int elSiz = Marshal.SizeOf<T>();
// Get the total size of unmanaged memory that is needed (length + elements)
int size = sizeof(int) + (elSiz * array.Length);
// Allocate unmanaged space. For COM, use Marshal.AllocCoTaskMem instead.
IntPtr ptr = Marshal.AllocHGlobal(size);
// Write the "Length" field first
Marshal.WriteInt32(ptr, array.Length);
// Write the array data
for (int i = 0; i < array.Length; i++)
{ // Newly-allocated space has no existing object, so the last param is false
Marshal.StructureToPtr<T>(array[i], ptr + sizeof(int) + (elSiz * i), false);
}
// If you're only using arrays of primitive types, you could use this instead:
//Marshal.Copy(array, 0, ptr + sizeof(int), array.Length);
return ptr;
}
// This function is called after completing the call that required marshaling to
// unmanaged memory. You should use it to free any unmanaged memory you allocated.
// If you never consume unmanaged memory or other resources, do nothing here.
public void CleanUpNativeData (IntPtr pNativeData)
{
// Free the unmanaged memory. Use Marshal.FreeCoTaskMem if using COM.
Marshal.FreeHGlobal(pNativeData);
}
// If, after marshaling from unmanaged to managed, you have anything that needs
// to be taken care of when you're done with the object, put it here. Garbage
// collection will free the managed object, so I've left this function empty.
public void CleanUpManagedData (Object ManagedObj)
{ }
// This function is a lie. It looks like it should be impossible to get the right
// value - the whole problem is that the size of each array is variable!
// - but in practice the runtime doesn't rely on this and may not even call it.
// The MSDN example returns -1; I'll try to be a little more realistic.
public int GetNativeDataSize ()
{
return sizeof(int) + Marshal.SizeOf<T>();
}
}
Whew, that was long! Well, there you have it. I hope people see this, because there's a lot of bad answers and misunderstanding out there...

It is not possible to marshal structs containing variable-length arrays (but it is possible to marshal variable-length arrays as function parameters). You will have to read your data manually:
IntPtr nativeData = ... ;
var length = Marshal.ReadUInt32 (nativeData) ;
var bytes = new byte[length] ;
Marshal.Copy (new IntPtr ((long)nativeData + 4), bytes, 0, length) ;

If the data being saved isn't a string, you don't have to store it in a string. I usually do not marshal to a string unless the original data type was a char*. Otherwise a byte[] should do.
Try:
[MarshalAs(UnmanagedType.ByValArray, SizeConst=[whatever your size is]]
byte[] Data;
If you need to convert this to a string later, use:
System.Text.Encoding.UTF8.GetString(your byte array here).
Obviously, you need to vary the encoding to what you need, though UTF-8 usually is sufficient.
I see the problem now, you have to marshal a VARIABLE length array. The MarshalAs does not allow this and the array will have to be sent by reference.
If the array length is variable, your byte[] needs to be an IntPtr, so you would use,
IntPtr Data;
Instead of
[MarshalAs(UnmanagedType.ByValArray, SizeConst=[whatever your size is]]
byte[] Data;
You can then use the Marshal class to access the underlying data.
Something like:
uint length = yourABSObject.Length;
byte[] buffer = new byte[length];
Marshal.Copy(buffer, 0, yourABSObject.Data, length);
You may need to clean up your memory when you are finished to avoid a leak, though I suspect the GC will clean it up when yourABSObject goes out of scope. Anyway, here is the cleanup code:
Marshal.FreeHGlobal(yourABSObject.Data);

You are trying to marshal something that is a byte[ABS_VARLEN] as if it were a string of length 1. You'll need to figure out what the ABS_VARLEN constant is and marshal the array as:
[MarshalAs(UnmanagedType.LPArray, SizeConst = 1024)]
public byte[] Data;
(The 1024 there is a placeholder; fill in whatever the actual value of ASB_VARLEN is.)

In my opinion, it's simpler and more efficient to pin the array and take its address.
Assuming you need to pass abs_data to myNativeFunction(abs_data*):
public struct abs_data
{
public uint Length;
public IntPtr Data;
}
[DllImport("myDll.dll")]
static extern void myNativeFunction(ref abs_data data);
void CallNativeFunc(byte[] data)
{
GCHandle pin = GCHandle.Alloc(data, GCHandleType.Pinned);
abs_data tmp;
tmp.Length = data.Length;
tmp.Data = pin.AddrOfPinnedObject();
myNativeFunction(ref tmp);
pin.Free();
}

Converting a structure to an intptr

I have a class definition as follows:
[StructLayout(LayoutKind.Sequential)]
public class OUR_MEM_STR
{
public byte[] p;
public int len;
};
This is an equivalent defintion of the C structure below:
typedef struct
{
void *p;
int len;
} OUR_MEM_STR;
I used byte[] instead of IntPtr type for member p becuase of the way it was being used thorughout c# project.
I have defined an object obj with len = 10 and p = new byte[10]
I want to make it an intptr. How do I get the size of the object for that?
IntPtr pObj = Marshal.AllocHGlobal(obj.len + sizeof(int));
Marshal.StructureToPtr(obj, pObj, true);
See what I did there. It seems too hard coded. If I do the below snippet;
IntPtr pObj = Marshal.AllocHGlobal(Marshal.SizeOf(obj));
Doing this returns the wrong size because obj.p returns a size of 4 and not 10. Because of the memory taken by the pointer pointing to the byte array is 4 bytes.
Is there a better way?

The return value is correct, p is a pointer, it takes 4 bytes.
You cannot leave it this way, there are two memory allocations. The marshaller allocated the memory for the array. It created a SAFEARRAY, a COM array type. Pretty unlikely that your C code is going to be happy with that. Declare it like this instead:
[StructLayout(LayoutKind.Sequential)]
public class OUR_MEM_STR {
public IntPtr p;
public int len;
};
And use Marshal.AllocHGlobal(10) to assign p. Don't forget to clean-up again.
Don't pass true to StructureToPtr(), the memory allocated by AllocHGlobal() isn't initialized. That's going to randomly crash your program. You must pass false.

Read non-byte array from file without having to use a loop?

Is there a way to read binary data from file into an array like in C where I can pass a pointer of any type to the i/o functions? I am thinking of something like BinaryReader::ReadBytes(), but that returns a byte[] which I cannot cast to the desired array pointer type.

If you have a fixed size struct
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
struct MyFixedStruct
{
//..
}
You can then read it in in one go using this:
public static T ReadStruct<T>(Stream stream)
{
byte[] buffer = new byte[Marshal.SizeOf(typeof(T))];
stream.Read(buffer, 0, Marshal.SizeOf(typeof(T)));
GCHandle handle = GCHandle.Alloc(buffer, GCHandleType.Pinned);
T typedStruct = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T));
handle.Free();
return typedStruct;
}
This reads in a byte array covering the size of the struct and then marshals the byte array into the structure. You can use it like this:
MyFixedStruct fixedStruct = ReadStruct<MyFixedStruct>(stream);
The struct may include array types as long as the array length is specified, i.e:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct MyFixedStruct
{
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)]
public int[] someInts; // 5 int's
//..
};
Edit:
I see you just want to read in a short array - In this case just read in the byte array and use Buffer.BlockCopy() to convert to the array you want:
byte[] someBytes = ..
short[] someShorts = new short[someBytes.Length/2];
Buffer.BlockCopy(someBytes, 0, someShorts, 0, someBytes.Length);
This is quite efficient, equivalent to a memcpy in C++ under the hood. The only other overhead you have of course is that the original byte array will be allocated and later garbage collected. This approach would work for any other primitive array type as well.

How about storing a serialized array of your struct in the file? You can build the array of structs easily. Not sure how to stream through the file though, as you would do in C.

How about using a Stream Class as it provides a generic view for sequence of bytes.

How do I marshal a struct that contains a variable-sized array to C#?

How do I marshal this C++ type?
The ABS_DATA structure is used to associate an arbitrarily long data block with the length information. The declared length of the Data array is 1, but the actual length is given by the Length member.
typedef struct abs_data {
ABS_DWORD Length;
ABS_BYTE Data[ABS_VARLEN];
} ABS_DATA;
I tried the following code, but it's not working. The data variable is always empty and I'm sure it has data in there.
[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet = System.Runtime.InteropServices.CharSet.Ansi)]
public struct abs_data
{
/// ABS_DWORD->unsigned int
public uint Length;
/// ABS_BYTE[1]
[System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst = 1)]
public string Data;
}

Old question, but I recently had to do this myself and all the existing answers are poor, so...
The best solution for marshaling a variable-length array in a struct is to use a custom marshaler. This lets you control the code that the runtime uses to convert between managed and unmanaged data. Unfortunately, custom marshaling is poorly-documented and has a few bizarre limitations. I'll cover those quickly, then go over the solution.
Annoyingly, you can't use custom marshaling on an array element of a struct or class. There's no documented or logical reason for this limitation, and the compiler won't complain, but you'll get an exception at runtime. Also, there's a function that custom marshalers must implement, int GetNativeDataSize(), which is obviously impossible to implement accurately (it doesn't pass you an instance of the object to ask its size, so you can only go off the type, which is of course variable size!) Fortunately, this function doesn't matter. I've never seen it get called, and it the custom marshaler works fine even if it returns a bogus value (one MSDN example has it return -1).
First of all, here's what I think your native prototype might look like (I'm using P/Invoke here, but it works for COM too):
// Unmanaged C/C++ code prototype (guess)
//void DoThing (ABS_DATA *pData);
// Guess at your managed call with the "marshal one-byte ByValArray" version
//[DllImport("libname.dll")] public extern void DoThing (ref abs_data pData);
Here's the naïve version of how you might have used a custom marshaler (which really ought to have worked). I'll get to the marshaler itself in a bit...
[StructLayout(LayoutKind.Sequential)]
public struct abs_data
{
// Don't need the length as a separate filed; managed arrays know it.
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<byte>))]
public byte[] Data;
}
// Now you can just pass the struct but it takes arbitrary sizes!
[DllImport("libname.dll")] public extern void DoThing (ref abs_data pData);
Unfortunately, at runtime, you apparently can't marshal arrays inside data structures as anything except SafeArray or ByValArray. SafeArrays are counted, but they look nothing like the (extremely common) format that you're looking for here. So that won't work. ByValArray, of course, requires that the length be known at compile time, so that doesn't work either (as you ran into). Bizarrely, though, you can use custom marshaling on array parameters, This is annoying because you have to put the MarshalAsAttribute on every parameter that uses this type, instead of just putting it on one field and having that apply everywhere you use the type containing that field, but c'est la vie. It looks like this:
[StructLayout(LayoutKind.Sequential)]
public struct abs_data
{
// Don't need the length as a separate filed; managed arrays know it.
// This isn't an array anymore; we pass an array of this instead.
public byte Data;
}
// Now you pass an arbitrary-sized array of the struct
[DllImport("libname.dll")] public extern void DoThing (
// Have to put this huge stupid attribute on every parameter of this type
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<abs_data>))]
// Don't need to use "ref" anymore; arrays are ref types and pass as pointer-to
abs_data[] pData);
In that example, I preserved the abs_data type, in case you want to do something special with it (constructors, static functions, properties, inheritance, whatever). If your array elements consisted of a complex type, you would modify the struct to represent that complex type. However, in this case, abs_data is basically just a renamed byte - it's not even "wrapping" the byte; as far as the native code is concerned it's more like a typedef - so you can just pass an array of bytes and skip the struct entirely:
// Actually, you can just pass an arbitrary-length byte array!
[DllImport("libname.dll")] public extern void DoThing (
// Have to put this huge stupid attribute on every parameter of this type
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef=typeof(ArrayMarshaler<byte>))]
byte[] pData);
OK, so now you can see how to declare the array element type (if needed), and how to pass the array to an unmanaged function. However, we still need that custom marshaler. You should read "Implementing the ICustomMarshaler Interface" but I'll cover this here, with inline comments. Note that I use some shorthand conventions (like Marshal.SizeOf<T>()) that require .NET 4.5.1 or higher.
// The class that does the marshaling. Making it generic is not required, but
// will make it easier to use the same custom marshaler for multiple array types.
public class ArrayMarshaler<T> : ICustomMarshaler
{
// All custom marshalers require a static factory method with this signature.
public static ICustomMarshaler GetInstance (String cookie)
{
return new ArrayMarshaler<T>();
}
// This is the function that builds the managed type - in this case, the managed
// array - from a pointer. You can just return null here if only sending the
// array as an in-parameter.
public Object MarshalNativeToManaged (IntPtr pNativeData)
{
// First, sanity check...
if (IntPtr.Zero == pNativeData) return null;
// Start by reading the size of the array ("Length" from your ABS_DATA struct)
int length = Marshal.ReadInt32(pNativeData);
// Create the managed array that will be returned
T[] array = new T[length];
// For efficiency, only compute the element size once
int elSiz = Marshal.SizeOf<T>();
// Populate the array
for (int i = 0; i < length; i++)
{
array[i] = Marshal.PtrToStructure<T>(pNativeData + sizeof(int) + (elSiz * i));
}
// Alternate method, for arrays of primitive types only:
// Marshal.Copy(pNativeData + sizeof(int), array, 0, length);
return array;
}
// This is the function that marshals your managed array to unmanaged memory.
// If you only ever marshal the array out, not in, you can return IntPtr.Zero
public IntPtr MarshalManagedToNative (Object ManagedObject)
{
if (null == ManagedObject) return IntPtr.Zero;
T[] array = (T[])ManagedObj;
int elSiz = Marshal.SizeOf<T>();
// Get the total size of unmanaged memory that is needed (length + elements)
int size = sizeof(int) + (elSiz * array.Length);
// Allocate unmanaged space. For COM, use Marshal.AllocCoTaskMem instead.
IntPtr ptr = Marshal.AllocHGlobal(size);
// Write the "Length" field first
Marshal.WriteInt32(ptr, array.Length);
// Write the array data
for (int i = 0; i < array.Length; i++)
{ // Newly-allocated space has no existing object, so the last param is false
Marshal.StructureToPtr<T>(array[i], ptr + sizeof(int) + (elSiz * i), false);
}
// If you're only using arrays of primitive types, you could use this instead:
//Marshal.Copy(array, 0, ptr + sizeof(int), array.Length);
return ptr;
}
// This function is called after completing the call that required marshaling to
// unmanaged memory. You should use it to free any unmanaged memory you allocated.
// If you never consume unmanaged memory or other resources, do nothing here.
public void CleanUpNativeData (IntPtr pNativeData)
{
// Free the unmanaged memory. Use Marshal.FreeCoTaskMem if using COM.
Marshal.FreeHGlobal(pNativeData);
}
// If, after marshaling from unmanaged to managed, you have anything that needs
// to be taken care of when you're done with the object, put it here. Garbage
// collection will free the managed object, so I've left this function empty.
public void CleanUpManagedData (Object ManagedObj)
{ }
// This function is a lie. It looks like it should be impossible to get the right
// value - the whole problem is that the size of each array is variable!
// - but in practice the runtime doesn't rely on this and may not even call it.
// The MSDN example returns -1; I'll try to be a little more realistic.
public int GetNativeDataSize ()
{
return sizeof(int) + Marshal.SizeOf<T>();
}
}
Whew, that was long! Well, there you have it. I hope people see this, because there's a lot of bad answers and misunderstanding out there...

It is not possible to marshal structs containing variable-length arrays (but it is possible to marshal variable-length arrays as function parameters). You will have to read your data manually:
IntPtr nativeData = ... ;
var length = Marshal.ReadUInt32 (nativeData) ;
var bytes = new byte[length] ;
Marshal.Copy (new IntPtr ((long)nativeData + 4), bytes, 0, length) ;

If the data being saved isn't a string, you don't have to store it in a string. I usually do not marshal to a string unless the original data type was a char*. Otherwise a byte[] should do.
Try:
[MarshalAs(UnmanagedType.ByValArray, SizeConst=[whatever your size is]]
byte[] Data;
If you need to convert this to a string later, use:
System.Text.Encoding.UTF8.GetString(your byte array here).
Obviously, you need to vary the encoding to what you need, though UTF-8 usually is sufficient.
I see the problem now, you have to marshal a VARIABLE length array. The MarshalAs does not allow this and the array will have to be sent by reference.
If the array length is variable, your byte[] needs to be an IntPtr, so you would use,
IntPtr Data;
Instead of
[MarshalAs(UnmanagedType.ByValArray, SizeConst=[whatever your size is]]
byte[] Data;
You can then use the Marshal class to access the underlying data.
Something like:
uint length = yourABSObject.Length;
byte[] buffer = new byte[length];
Marshal.Copy(buffer, 0, yourABSObject.Data, length);
You may need to clean up your memory when you are finished to avoid a leak, though I suspect the GC will clean it up when yourABSObject goes out of scope. Anyway, here is the cleanup code:
Marshal.FreeHGlobal(yourABSObject.Data);

You are trying to marshal something that is a byte[ABS_VARLEN] as if it were a string of length 1. You'll need to figure out what the ABS_VARLEN constant is and marshal the array as:
[MarshalAs(UnmanagedType.LPArray, SizeConst = 1024)]
public byte[] Data;
(The 1024 there is a placeholder; fill in whatever the actual value of ASB_VARLEN is.)

In my opinion, it's simpler and more efficient to pin the array and take its address.
Assuming you need to pass abs_data to myNativeFunction(abs_data*):
public struct abs_data
{
public uint Length;
public IntPtr Data;
}
[DllImport("myDll.dll")]
static extern void myNativeFunction(ref abs_data data);
void CallNativeFunc(byte[] data)
{
GCHandle pin = GCHandle.Alloc(data, GCHandleType.Pinned);
abs_data tmp;
tmp.Length = data.Length;
tmp.Data = pin.AddrOfPinnedObject();
myNativeFunction(ref tmp);
pin.Free();
}

AccessViolationException when serializing a struct of arrays of structs?

I have a sequential struct that I'd like to serialize to a file, which seems trivial. However, this struct consists of, among other things, 2 arrays of other types of structs. The main struct is defined as follows...
[StructLayout(LayoutKind.Sequential)]
public struct ParentStruct
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
public const string prefix = "PRE";
public Int32 someInteger;
public DataLocater[] locater; //DataLocater is another struct
public Body[] body; //Body is another struct
};
I can create these structs exactly as intended. However, when trying to serialize with the following method (which seems popular online), I get an AccessViolationException:
public static byte[] RawSerialize(object structure)
{
int size = Marshal.SizeOf(structure);
IntPtr buffer = Marshal.AllocHGlobal(size);
Marshal.StructureToPtr(structure, buffer, true);
byte[] data = new byte[size];
Marshal.Copy(buffer, data, 0, size);
Marshal.FreeHGlobal(buffer);
return data;
}
I'm assuming this is because the structures don't define exactly how large the arrays are, so it cannot explicitly determine the size beforehand? It seems that since it cannot get that, it is not allocating the right amount of space for the structure and it ends up being too short when casting the structure to a pointer. I'm not sure on this. Why might this occur and what are possible alternatives?
Edit: The line throwing the error is
Marshal.StructureToPtr(structure, buffer, true);

Not possible because of the nested struct arrays. See When I try to use a structure containing an array of other structures, I get an exception. What's wrong?.

In C# it makes more sense to implement ISerializable and use the BinaryFormatter class to write the struct to disk.
ISerializable