如何将字符串从本机代码返回到 Windows Azure 中的托管代码？

Question

I have found this tutorial about how to interop C# code and unmanaged C++ code on Windows Azure very helpful http://msdn.microsoft.com/en-us/hh351534

However when I modify it, I run into some unforeseen errors.

Instead of returning and displaying an int from unmanaged code, I would like it to return a string an display that string in C# code. So in my experiment, I changed the function AddNumbers() from

NATIVECALCULATOR_API int AddNumbers(int left, int right){
    return left+right;
}

into

NATIVECALCULATOR_API char *AddNumbers(int left, int right){
    return "a string";
}

so that when I hit the button on ASP.NET page, it will return "a string".

However, in default.aspx.cs, the web page codebehind file, it tells me I can't change

static extern int AddNumbers(int left, int right);

into

static extern char *AddNumbers(int left, int right);

because "pointers can only be used in unsafe context"

And from there I have tried many methods, but I am unable to convert AddNumbers() into a string to display.

So basically, can someone show me how to modify the tutorial to display a string instead of a int.

Answer 1

What C# is trying to tell you is that it won't let you use raw pointers willingly. Thankfully we can still pass strings (even raw char strings) from unmanaged code to managed code through Data Marshalling. In the case of strings, the system translates a char* in C++ to a string in C#.

For your specific question, change AddNumbers in default.aspx.cs from

static extern int AddNumbers(int left, int right);

to

static extern string AddNumbers(int left, int right);

Likewise, change the C++ code to read like so

NATIVECALCULATOR_API char* AddNumbers(int left, int right)
{
    return "a string";
}

The data marshalling should correctly pass the string from one to the other.

Here's the MSDN article that explains data marshalling in (MUCH) more detail.