C# pinvoke C char array

Question

Marshalling char array.

The C typedef of the function is:

typedef bool (*get_VariableInfoFunc)(int *, char * *, int *, int *);

It`s usage is like:

pnVariableIdArray = new (nothrow) int[numVars];
pcVariableName = new (nothrow) char*[numVars];
for (int i = 0; i < numVars; i++)
{
    pcVariableName[i] = new char[100];
}
pnUnVariableIdArray = new (nothrow) int[numVars];
pnVariableType = new (nothrow) int[numVars];
res = get_VariableInfo(pnVariableIdArray, pcVariableName, pnUnVariableIdArray, pnVariableType);

My C# Signature is:

[DllImport(AUTODYNResultsApiDll, EntryPoint = "get_VariableInfo", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool GetVariableInfo(
    int[] pnVariableIdArray,
    IntPtr[] pcVariableName,
    int[] pnUnVariableIdArray,
    int[] pnVariableType);

I use it like:

var stringPointers= new IntPtr[numVars];
for (int i = 0; i < numVars; i++)
{
    by[i] = Marshal.AllocHGlobal(100);
}
GetVariableInfo(pnVariableIdArray, stringPointers, pnUnVariableIdArray, pnVariableType);

and then back to string with:

var strings = new string[numVars];
for (int i = 0; i < numVars; i++)
{
    strings[i] = Marshal.PtrToStringAnsi(by[i]);
}

Is there a nicer way doing this without freeing the Memory of the pointer. Just copy the char array as string array?

David Heffernan · Accepted Answer · 2017-08-18 08:12:22Z

1

First of all, a few problems with your code as it stands:

The calling convention is cdecl, you need to include CallingConvention = CallingConvention.Cdecl in your attribute.
You set SetLastError = true, but the unmanaged function surely does not call SetLastError. If that is so, remove SetLastError = true.
You call AllocHGlobal but there is no matching call to FreeHGlobal, and so the memory is leaked.
Your assumption that no string will exceed a length of 100 (including null terminator) seems brittle, and at risk of buffer overflow.

As far as the main body of your question goes, the marshaller won't marshal an array of strings for you. Your current approach, subject to the provisos above, is the correct way to tackle the problem.

answered Aug 18, 2017 at 8:12

David Heffernan

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