Develop Reference

Equivalent of Marshal.PtrToStructure in Unsafe class?

None of the methods in Unsafe class do work when trying to read a structure from a pointer.
They all display Unable to read memory in the debugger:
var info1 = Marshal.PtrToStructure<DbgHelp.SYMBOL_INFO_V>(pSymInfo); // works
var info2 = Unsafe.AsRef<DbgHelp.SYMBOL_INFO_V>((void*)pSymInfo); // unable to read memory
var info3 = Unsafe.Read<DbgHelp.SYMBOL_INFO_V>((void*)pSymInfo); // unable to read memory
var info4 = Unsafe.ReadUnaligned<DbgHelp.SYMBOL_INFO_V>((void*)pSymInfo); // unable to read memory
Is it possible to read a structure with Unsafe class just as Marshal.PtrToStructure does?
About:
pSymInfo is an IntPtr.
SYMBOL_INFO_V:
public struct SYMBOL_INFO_V
{
public uint SizeOfStruct;
public uint TypeIndex;
public ulong Reserved0;
private ulong Reserved1;
public uint Index;
public uint Size;
public ulong ModBase;
public SYMFLAG Flags; // enum, uint
public ulong Value;
public ulong Address;
public uint Register;
public uint Scope;
public uint Tag;
public uint NameLen;
public uint MaxNameLen;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 1)]
public string Name;
}

The Unsafe class has no knowledge of P/Invoke rules, only the Marshal class does.
For example, if you take this little program:
unsafe internal class Program
{
static void Main()
{
var info = new MyStruct();
Console.WriteLine(Marshal.SizeOf<MyStruct>()); // 8
Console.WriteLine(Unsafe.SizeOf<MyStruct>()); // 16
info.SomeValue = 0x12345678;
info.Name = "Test";
var p = Marshal.AllocCoTaskMem(100);
var ptr = p.ToPointer();
Unsafe.Write(ptr, info);
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyStruct
{
public uint SomeValue;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 4)] // includes terminating zero
public string Name;
}
}
And debug it with a breakpoint after the Unsafe.Write method, open a memory window and use ptr as the start address, and you'll see this:
The SomeValue member is visible, but the Name is not, because ptr points to the managed object (with a probable leading pointer to something internal), not to it's P/invoke "projection".
Also, note in this case Marshal.SizeOf (P/Invoke size = 8) and Unsafe.SizeOf (Managed size = 16) results are different.

What type of string you're trying to read?
You can use the fixed modifier to read raw byte/char array in a struct:
public fixed byte Name[5];
Note that in .NET, all char/string is in Unicode, with each character taking 2 bytes. If you're trying to read an ascii/ansi string you should read them as bytes then use Encoding.ASCII.GetString to convert it to managed string. You should read them as char array if you're using wchar_t in unmanged code.
If the struct does not contain the actual string, but a pointer(char*/wchar_t*). You can read it as IntPtr then convert it with Marshal.PtrToString*
P.S.
Instead of using Unsafe, you can just read the struct with pointers:
var myStruct = *(SYMBOL_INFO_V*)pSymInfo;

C# <-> C++ : Marshaling an [In,Out] struct containing Char[]

I have a C++ .dll (unable to make changes to the code) from which I am trying to call a function that takes a reference parameter of a Struct type (also defined in the .dll).
The function requires the Struct to have the 'paramName' and 'groupName' fields populated, and based on those fields, it will return the Struct with the remaining fields populated.
I am not getting any marshaling errors, however, the library call returns an error code and a debug log shows that it is receiving an empty string for the two string fields (see below for how fields are set). My assumption is that the size of this struct is not aligning between the managed and unmanaged representation, and thus the struct is not blittable.
Here is the C++ method signature:
int GetConfigs(int contextHandle, Configs* configs);
And the C++ Configs struct:
struct Configs {
int myInt;
float myFloat;
bool flag;
char name[64];
char group[64];
}
The C# function wrapper:
[DllImport(DLLNAME, CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern int GetConfigs(int contextHandle, [MarshalAs(UnmanagedType.Struct), In, Out] ref Configs configs);
The C# struct definition:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Configs
{
public int myInt;
public float myFloat;
[MarshalAs(UnmanagedType.U1)]
public bool flag;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string group;
}
As per the Microsoft documentation I have declared the C++ char[]'s to be represented in C# by strings and marshaled as ByValTStr with a set size.
Also from Microsoft documentation:
bool is not blittable, so I mark it with an explicit MarshalAs.
float is also not blittable, but declaring these fields as a float or a double made no difference in the original issue.
Lastly, the C# calling code:
var configs = new Library.Configs
{
name = "testName",
group = "testGroup"
};
var returnCode = Library.GetConfigs(GetContextId(), ref configs);
The return code comes back as a failure code and the library debug output file shows that the struct argument had:
name = []
(name is clearly set to what I expect it to be at the time of the call when I debug through the C# code)
Instead of declaring the string fields as
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
I tried:
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 64)]
public byte[] name;
but that also made no difference.

The float/double was what was throwing off the struct byte size- the library was expecting a 4 byte floating point number but the data marshaler by default keeps those as 8 bytes. Fixing that along with the boolean as mentioned in the comments above solved the problem:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Configs
{
public int myInt;
[MarshalAs(UnmanagedType.R4)]
public float myFloat;
[MarshalAs(UnmanagedType.U1)]
public bool flag;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string name;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
public string group;
}

char[] array equivalent in c#

i have a c++ struct which has char[20] like below, and it is packed.
#pragma pack(push, temp_aion_packed, 1)
struct temp
{
char x[20];
char y[20];
};
#pragma pack(pop, temp_aion_packed)
now how can i write this struct in c# so that both are same. i have written like this in c#
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public class temp
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}
below is the pinvoke declaration in c#
[DllImport("rmsCAPI.dll", CallingConvention = CallingConvention.StdCall, ExactSpelling = true, EntryPoint = "OrderRequirement")]
public static extern int OrderRequirement(ref temp tmp);
c++ function that calls this struct as parameter
long __stdcall OrderRequirement(struct temp *tmp)
{
string p="";
string q="";
p=temp->x;
q=temp->y;
char buff[2048];
sprintf(buff,"p: %s\n q: %s\n x: %s\n y: %s\n",p,q,temp->x,temp->y);
}
but when i do like this in c# it is giving me the junk data in c++, when i assign values for them in c#. Can any one please assist.
Thank you all for help on the above one, but now i got a new problem which is an extension of this, i am providing everything in detail below.
My structures in c++
#pragma pack(push, temp_aion_packed, 1)
struct temp
{
long req;
struct type m_type;
short id;
char x[20];
char y[20];
};
#pragma pack(pop, temp_aion_packed)
#pragma pack(push, type_aion_packed, 1)
struct type
{
short i;
};
#pragma pack(pop, type_aion_packed)
i have written equivalent c# structs like this
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct temp
{
[DataMember]
public long req;
[DataMember]
[MarshalAs(UnmanagedType.Struct)]
public type m_type;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}
[DataContract]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct type
{
[DataMember]
public short i;
}
below is my c# pinvoke
[DllImport("rmsCAPI.dll", CallingConvention = CallingConvention.StdCall, ExactSpelling = true, EntryPoint = "OrderRequirement")]
public static extern int OrderRequirement(ref temp tmp);
below is my c++ method that calls struct as parameter
long __stdcall OrderRequirement(struct temp *tmp)
{
char buff[2048];
sprintf(buff,"req: %ld \n id: %d \n x: %s\n",tmp->req,tmp->id,tmp->x);
}
now the problem i am getting is as i have structure variable m_type(of struct "type") declared in struct temp, the variables declared before (long req)that are printing fine in my c++ program but the variable declared after that are not giving me any output. So i think the declaration of structures in c# is messing up and i am unable to figure it out so can any one please help.

You declared the struct as a class in C#. That's fine, but it means that any variable of that type is already a reference. So you don't need to pass by ref. When you pass a class by ref you end up passing a pointer to a pointer to the object. That's one level of indirection too many.
The P/invoke in the C# code should therefore be like this:
public static extern int OrderRequirement(temp tmp);
The other way to fix it is to leave the ref in the function declaration, but to declare the temp type as a struct rather than a class. That's because a struct is a value type. A variable whose type is a struct is a value rather than a reference.
Both solutions work, it's up to you which you choose.
There is another problem in your C++ code.
sprintf(buff,"p: %s\n q: %s\n x: %s\n y: %s\n",p,q,temp->x,temp->y);
You are passing p and q, which are of type std::string to printf and expecting the %s format string to print them. That's an error. You need to call c_str() on the strings. Like this:
sprintf(
buff,
"p: %s\n q: %s\n x: %s\n y: %s\n",
p.c_str(),q.c_str(),temp->x,temp->y
);
The problem with your update is that long is 32 bits in Windows C++ and 64 bits in C#. You need to declare it as int in C#. And you missed the id field altogether.
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1),Serializable]
public struct temp
{
[DataMember]
public int req;
[DataMember]
public type m_type;
[DataMember]
public short id;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string x;
[DataMember]
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
public string y;
}

How to convert a C++ Struct with Union into C#?

Guys I am having difficulties on retrieving struct member values after calling a function in the DLL. I tried to convert the C++ codes into C# but I’m not sure if it is correct or not. Please help me understand my mistakes here (if there is) and how to correct.
My problem here is I can’t correctly retrieved the values of the INNER STRUCTS (Union) after I called the ReceiveMessage function from the DLL. Like for example m_objMsg.MsgData.StartReq.MsgID is always 0.
But when I try to use the C++ .exe program, the MsgID has a correct value. (not 0)
C++ Code:
extern int ReceiveMessage(SESSION, int, Msg*);
typedef struct
{
char SubsId[15];
int Level;
char Options[12];
} ConxReq;
typedef struct
{
char MsgId[25];
} StartReq;
typedef struct
{
long Length;
short Type;
union
{
ConxReq oConxReq;
StartReq oStartReq;
} Data;
} Msg;
/////////////////////////////////////////////////////
Msg oMsg;
int rc=ReceiveMessage(Session, 0, &oMsg);
switch(rc)
{
case 0:
switch(oMsg.Type)
{
case 0: // ConxReq
…
break;
case 1: // StartReq
…
break;
…
}
And here is my attempt to convert this into c#:
[DllImport("MyDLL.dll",
CallingConvention = CallingConvention.Cdecl,
CharSet = CharSet.Ansi)]
protected static extern Int32 ReceiveMessage(IntPtr session,
Int32 nTimeOut,
[MarshalAs(UnmanagedType.Struct)] ref Msg ptrMsg);
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct ConxReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 15)]
public string SubsId;
public Int32 Level;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]
public string Options;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct StartReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 25)]
public string MsgId;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
protected struct Msg
{
public int Length;
public Int16 Type;
public Data MsgData;
}
StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct Data
{
[FieldOffset(0)]
public ConxReq oConxReq;
[FieldOffset(0)]
public StartReq oStartReq;
}
Msg m_objMsg = new Msg();
m_objMsg.MsgData = new Data();
m_objMsg.MsgData.oConxReq = new ConxReq();
m_objMsg.MsgData.oStartReq = new StartReq();
int rc = ReceiveMessage(m_Session, nTimeOut, ref m_objMsg);
then the SWITCH Condition
And If I add this struct inside the UNION for c++ and c#...
I've got an error stating the "... incorrectly align" or "...overlapped..."
c++
ConxNack oConxNack;
typedef struct
{
int Reason;
} ConxNack;
[StructLayout(LayoutKind.Sequential)]
public struct ConxNack
{
public int nReason;
}
[FieldOffset(0)]
public ConxNack oConxNack;
Thank you so much in advance for your time and help...

Akash is right, have a look here: http://social.msdn.microsoft.com/Forums/en/csharplanguage/thread/60150e7b-665a-49a2-8e2e-2097986142f3
Another option is create two structs and use an appropriate cast once you know which type it is.
hth
Mario

In C++, we know that all members of UNION shared the same memory chunk and can only have one member of an object at a time.
In order to implement this in C#, we need to use the LayoutKind to Explicit and set all the starting point of each member to 0.
In my previous example, An error message is displayed stating that the offset of an object type is incorrectly aligned or overlapped by a non-object type.
Answer is we cannot set all the members to FieldOffSet to 0 since it is not allowed to combine the reference type with the value type.
- Thanks to the explanation of Hans Passant
What I did is to create a copy of the UNION Member Structs and change the type of all the String Member Variables to bytes.
I used bytes since this is a value type so I can put this struct into FieldOffSet(0).
Take note, i adjust the FieldOffSet of the next member variable so i can still get the same size of my string variable.
And also for the struct size since i have byte member at the last.
Thanks to Akash Kava and Mario The Spoon for giving me an idea and providing me a useful link.
After calling the function in the DLL and passed this Struct Obj (ref m_objMsg) as a paramter, I need to extract the values.
One way is to have a pointer that points to the address of the struct in the unmanaged memory and convert this pointer a new
Struct with the corresponding member variables (my original structs).
NEW STRUCTS (BYTES)
////////////////////////////////////////////////////////////////
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Size = 31)]
public struct ConxReq
{
[FieldOffSet(0)]
public byteSubsId;
[FieldOffSet(15)]
public Int32 Level;
[FieldOffSet(19)]
public byte Options;
}
[StructLayout(LayoutKind.Explicit, Size = 4)]
public struct ConxNack
{
[FieldOffSet(0)]
public int nReason;
}
[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi, Size = 25)]
public struct StartReq
{
[FieldOffSet(0)]
public byte MsgId;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
protected struct Msg
{
public int Length;
public Int16 Type;
public Data MsgData;
}
StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct Data
{
[FieldOffset(0)]
public ConxReq oConxReq;
[FieldOffset(0)]
public ConxNack oConxNack;
[FieldOffset(0)]
public StartReq oStartReq;
}
////////////////////////////////////////////////////////////////
MY ORIGINAL STRUCTS
////////////////////////////////////////////////////////////////
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyConxReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 15)]
public string SubsId;
public Int32 Level;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]
public string Options;
}
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyStartReq
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 25)]
public string MsgId;
}
[StructLayout(LayoutKind.Sequential)]
public struct MyConxNack
{
public int nReason;
}
///////////////////////////////////////////////////////////////
Since I have a Msg.Type, i know what kind of struct (type) I could cast the object.
Like for example
ReceiveMessage(m_Session, nTimeOut, ref oMsg);
switch (oMsg.Type)
{
case 0: // ConxReq
IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(oMsg.MsgData.ConxReq); // use the new struct (bytes)
Marshal.StructureToPtr(oMsg.MsgData.ConxReq, ptr, false);
MyConxReq oMyConxReq = new MyConxReq;
oMyConxReq = (MyConxReq) Marshal.PtrToStructure(ptr, typeof(MyConxReq)); // convert it to the original struct
Marshal.FreeHGlobal(ptr);
Then you can use now the oMyConxReq object to acccess the member variables directly.
Please let me know if you have other or better way to do this...
Kindly advise if what I did is correct or if I missed something.
Thank you so much!!! :)

You have to use StructLayout(LayoutKind.Explicit) and FieldOffsets to make union.

Marshal C++ struct array into C#

I have the following struct in C++:
#define MAXCHARS 15
typedef struct
{
char data[MAXCHARS];
int prob[MAXCHARS];
} LPRData;
And a function that I'm p/invoking into to get an array of 3 of these structures:
void GetData(LPRData *data);
In C++ I would just do something like this:
LPRData *Results;
Results = (LPRData *)malloc(MAXRESULTS*sizeof(LPRData));
GetData( Results );
And it would work just fine, but in C# I can't seem to get it to work.
I've created a C# struct like this:
public struct LPRData
{
/// char[15]
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
/// int[15]
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}
And if I initialize an array of 3 of those (and all their sub-arrays) and pass it into this:
GetData(LPRData[] data);
It returns with success, but the data in the LPRData array has not changed.
I've even tried to create a raw byte array the size of 3 LPRData's and pass that into a function prototype like this:
GetData(byte[] data);
But in that case I will get the "data" string from the very first LPRData structure, but nothing after it, including the "prob" array from the same LPRData.
Any ideas of how to properly handle this?

I would try adding some attributes to your struct decloration
[StructLayout(LayoutKind.Sequential, Size=TotalBytesInStruct),Serializable]
public struct LPRData
{
/// char[15]
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
/// int[15]
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}
*Note TotalBytesInStruct is not intended to represent a variable
JaredPar is also correct that using the IntPtr class could be helpful, but it has been quite awhile since I have used PInvoke so I'm rusty.

One trick when dealing with pointers is to just use an IntPtr. You can then use Marshal.PtrToStructure on the pointer and increment based on the size of the structure to get your results.
static extern void GetData([Out] out IntPtr ptr);
LPRData[] GetData()
{
IntPtr value;
LPRData[] array = new LPRData[3];
GetData(out value);
for (int i = 0; i < array.Length; i++)
{
array[i] = Marshal.PtrToStructure(value, typeof(LPRData));
value += Marshal.SizeOf(typeof(LPRData));
}
return array;
}

A similar topic was discussed on this question, and the one of the conclusions was that the CharSet named parameter must be set to CharSet.Ansi. Otherwise, we would be making a wchar_t array instead of a char array. Thus, the correct code would be as follows:
[Serializable]
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct LPRData
{
[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst = 15)]
public string data;
[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 15)]
public int[] prob;
}

The PInvoke Interop Assistant may help. http://clrinterop.codeplex.com/releases/view/14120

Did you mark GetData parameter with OutAttribute?
Combining the InAttribute and
OutAttribute is particularly useful
when applied to arrays and formatted,
non-blittable types. Callers see the
changes a callee makes to these types
only when you apply both attributes.