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c#cpinvokemarshalling

Converting C Headers to C# - ByValArray versus ByValTStr for Fixed Char Array inside a Structure

I have a structure defined in C as:

typedef struct {
    char struct_id[4];
    int struct_version;
    int keepAliveInterval;
    ……
} MQTTClient_connectOptions

I create a corresponding structure in C# like this:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MQTTClient_connectOptions {
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 4)]
    public string struct_id;
    public int struct_version;
    public int keepAliveInterval;
    ……
}

This C# structure is what is generated by the P/Invoke Interop Assistant, and is the C# snippet that is recommended by multiple posts across my google search.

The same DLL / source code that defines the C structure also defines the function:

DLLExport  int MQTTClient_connect(MQTTClient handle, MQTTClient_connectOptions* options);

Which I have defined in my C# code as:

[DllImport("PahoMqttC", EntryPoint = "MQTTClient_connect", CharSet = CharSet.Ansi)]
public static extern int MQTTClient_connect(IntPtr handle, ref MQTTClient_connectOptions options);

In my C# code, I can set 

MQTTClient_connectOptions.struct_id = "MQTC"

and when I inspect the object while debugging I can see these 4 characters in that field. However, when I use this structure to call MQTTClient_connect(), the "MQTC" is truncated down to "MQT".

When I step through the code, as soon as I step into MQTTClient_connect, the struct_id field changes from "MQTC" to "MQT\0" in the C# object inspector, and MQTTClient_connect fails because the struct_id is not what is expected.

If I instead define the structure in C# like this:

 [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4)]
public byte[] struct_id;

and set its value like this:

struct_id = Encoding.ASCII.GetBytes("MQTC");

then everything works correctly ???

My goal is to understand Marshaling and P/Invoke and converting C/C++ headers to C# code, and so I would really like to know:

1 - Why does using "byte[]" work while using "string" causes the value of struct_id changing when I step into the MQTTClient_connect() routine?

2 - Is there a way to define the C# structure using "string", which would make the rest of my C# code simpler?

Thanks!

Solution

  • The reason for this is that the p/invoke marshaller has to decide whether or not the char[4] field is text or binary (i.e. bytes). And the convention in play here is that a string marshalled as UnmanagedType.ByValTStr is assumed to be text. In which case it can have variable length, which is determined by the null terminator, which must be present. That is the convention which matches the typical use of fixed length arrays of char to hold C strings. C strings are null terminated.

    In reality though, I suspect that your data isn't really text. I would suspect that the field would be better declared in C as unsigned char struct_id[4] to indicate that this contains 4 bytes. All this is a little subjective though, and of course we aren't party to all the rationale behind the design of the library. Perhaps there is some good reason for declaring it as a char array that I cannot see.

    No matter what, your C# code cannot use string for this field. A byte[] with SizeConst = 4 is the correct way to marshal this. Some helper methods on your record type could help transform between the byte array and a string. However, I wonder how many different IDs you are going to encounter. It may be that you just need to declare a handful of byte array constants and you can use those rather than string literals.