which unmanaged type should be used...??

Question

Hi,

for my graduation thesis, i need to call a native DLL into C#..the DLL has struct like this

typedef struct
{
   unsigned char TagType;
   unsigned char AntNum;
   unsigned char Ids[12];
}TagIds;

i have converted the struct into C# like this

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]

public struct TagIds

{

public byte TagType;

public byte AntNum;

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]

public string[] Ids;

}

am i wrong or is there any mistake..??

thanks in advance

Answer 1

you converted char[] to string[], which is not supported by the default marshaller.  If you want to continue using an arrary to store the ids, i suggest you to use BYTE in your declaration, which is typedefed into unsigned char.

//Generated by P/invoke Interop Assistant version 1.0
    /// BYTE[12]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValArray, SizeConst=12, ArraySubType=System.Runtime.InteropServices.UnmanagedType.I1)]
    public byte[] Ids;

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Answer 2

P/invoke Interop Assistant  tool converts the struct

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]

public struct TagIds

{
....

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)]

public string Ids;
}

i don't have enough practise with interoperability and don't know all marshalling rules..which one should i choose??

Answer 3

Depends on how you plan to use the data. Do you want an array of bytes, or a string?

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Answer 4

Depends on how you plan to use the data. Do you want an array of bytes, or a string?

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so i want to use array of bytes, i need to do

public struct TagIds

{
...

[MarshalAs(UnmanagedType.ByValArray, SizeConst = 12,ArraySubType=UnmanagedType.I1)]

public byte[] Ids;

}


if i want to use a string, i need to do

public struct TagIds

{
...

[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 12)] public string Ids;

}

is it true??

Answer 5

Yes, when your structure's charset setting is ANSI.

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Answer 6

i read an article and it was said althouhg string is a reference type, when a string parameter is passed to a method it behaves like a value type..but Arrays behave like reference types..?? if so, for this structure if i use string,does callee see the changes a caller makes to string type..??

otherwise we have done this using [in,out] directional attribute,ref or out keyword for the parameter which is defined with this structure type when the imported function is declared??

Answer 7

Both string and byte array are reference types.  It does not matter how data is stored in memory on the managed heap. The way the default marshaller copy data to/from the native heap matters.

The default marshaller uses the COM heap, so you need to allocate memory on the COM heap if you return a block of memory in ref. 

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