Question Converting nested C structure to C# Is this possible?

Ranandar

I have a struct written in C that I want to use in C#. It uses an array of structures nested inside of another structure. This is very straightforward in C but I have been unsuccessful in duplicating this simple example. Is this even possible using C#? If it is not possible what other options do I have?

Here is the C code simplified. The actual structure is much more complex.

// -- Message File Structure -- 
        struct message_struct{       // Message Structure 
           char mname[20];             // Message Name 
           char mattr;                 // Message Slot Used flag 
           struct {    // Page             // Page Structure 
              int  duration;         // Duration 
              char stext[201];       // Scroll Buffer 
              struct { // Line         // Line Structure 
                 char csize;         // Char Size 
                 char mchar[31];      // Line Characters 
              } mline[8];            // # of Lines 
           } mpage[10];                 // # of Pages 
        } message;   

Then to access the structure
message.mpage[0].mline[0].mchar[0] = 'A';

7 Replies

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Alan18y ago
Accepted answer
When you 'new' a struct in C#, it initializes all value type fields to their default values (zero for numeric types, false for bool etc). However, reference type fields such as strings and arrays are set to null. You therefore need to explicitly create objects of those types:

       message myMessage = new message();
       myMessage.mname = new char[20];
       myMessage.mname[0] = 'A';
       Console.WriteLine(myMessage.mname[0]); // A
+1
Ranandar18y ago
I looked into using unsafe but it did not seem like a good option even for storing the message data. In the C program the message is stored and retrieved by finding the start and length of the structure and dumping to or reading from memory. In C# I will need to pick values from the C memory dump and plug them into the C# structure. A rather lengthy process, but not difficult to code.
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Alan18y ago
Yes, you have to create the arrays in each struct so your code looks good now.

I see you've also removed the attributes which you only need if you're going to use the struct(s) for interop with C code.

Incidentally, the reason we have to go through this rigmarole is that array fields contained within a struct are not stored inline in C# as they are in C. Instead the C# struct contains a pointer to where the array is stored on the heap.

Just as a matter of interest, you can use inline C-style arrays in unsafe code (see link below) but this wouldn't have helped here as this feature can only be used for arrays of primitive types, not arrays of user defined structs.

http://msdn.microsoft.com/en-us/library/zycewsya(VS.80).aspx
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Ranandar18y ago
Thank you Alan, I may have something working now. At least I am no longer stuck.
This is what I have come up with...

        public struct message_struct
        {
            public string name; // Message Name
            public int pattr2;
            public message_page[] pages;
        }

        public struct message_page
        {
            public int duration;        // Duration
            public string stext; // Scroll Buffer
            public message_line[] lines;
        }

        public struct message_line
        {
            public int csize;       // Char Size
            public string mchar;     // Line Characters
        }

        public message_struct myMessage = new message_struct();

        ...

       public Form1()
        {
            InitializeComponent();
            myMessage.pages = new message_page[10];
            for (int i = 0; i < 10; i++) myMessage.pages[i].lines = new message_line[8];
        }
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Ranandar18y ago
Sorry, but this still does not work. I cannot access anything beyond the message structure.

myMessage.mpage[0] = new page();
// Object reference not set to an instance of an object

myMessage.mpage[0].stext = new char[20];
// Object reference not set to an instance of an object
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Ranandar18y ago
Thank you Alan. Interesting approach, but I cannot get it to work.

When I try:
message myMessage = new message();
myMessage.mname[0] = 'A';

I get:
"Object reference not set to an instance of an object."
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Alan18y ago
It should be possible to represent that struct in C# by building it up in layers.

Without any other info, I'd have come up with this for interop purposes:

[StructLayout(LayoutKind. Sequential, CharSet=CharSet.Ansi)]
public struct message
{
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
   public string mname;
   public char mattr;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
   public page[] mpage;
}

[StructLayout(LayoutKind. Sequential, CharSet=CharSet.Ansi)]
public struct page
{
   public int duration;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 201)]
   public string stext;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
   public line[] mline;
}

[StructLayout(LayoutKind. Sequential, CharSet=CharSet.Ansi)]
public struct line
{
   public char csize;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 31)]
   public string mchar;
}

However, your example suggests you might want the individual characters of the strings to be writable from C#. If that's the case, you'll need to use char arrays rather than strings:

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct message
{
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 20)]
   public char[] mname;
   public char mattr;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
   public page[] mpage;
}

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct page
{
   public int duration;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 201)]
   public char[] stext;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
   public line[] mline;
}

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct line
{
   public char csize;
   [MarshalAs(UnmanagedType.ByValArray, SizeConst = 31)]
   public char[] mchar;
}
+1