How Can I Seamlessly Pass Strings Between C# and a C DLL?

Interfacing Strings between C# and C DLL

Encapsulating string manipulation within a C DLL and invoking it from C# can seem straightforward; however, passing strings across the interoperability boundary presents a challenge. This article explores the complexities involved in such exchanges and provides a workaround for seamless string transfer.

The Pitfall

Directly passing C standard library strings (std::string) across the C#/.NET boundary is incompatible. This is because C# does not provide a counterpart for std::string. As a result, attempts to perform interoperability fail with a System.AccessViolationException.

The Solution: Interoperable Strings

To overcome this obstacle, we must employ interoperable string types at the boundary. One such option is null-terminated character arrays, which can be conveniently handled in both C and C#. This approach entails allocating and deallocating memory within the same module.

Passing Strings from C# to C

Consider the following C function that accepts a null-terminated character array:

void foo(const char *str) {
  // ...
}

In C#, we can declare an interop function as follows:

[DllImport("...")]
static extern void foo(string str);

// ...

foo("bar");

In this scenario, the C# compiler automatically marshals the managed string ("bar") into a null-terminated character array, which is passed to the C function.

Passing Strings from C to C#

On the other hand, C functions can write strings into buffers allocated by the C# caller:

void foo(char *str, int len) {
  // Perform operations within len characters of str
}

And in C#:

[DllImport("...")]
static extern void foo(StringBuilder str, int len);

// ...

StringBuilder sb = new StringBuilder(10);
foo(sb, sb.Capacity);

Here, C# allocates a StringBuilder object and exposes its internal buffer to the C function via the StringBuilder and Capacity parameters. The C function can then populate the buffer with up to Capacity characters.

Conclusion

By adopting interoperable string types, strings can be seamlessly passed between C# and C DLLs, providing a convenient mechanism for exchanging textual data across different languages and platforms.

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