How to Solve the System.AccessViolationException When Passing Strings Between C# and C DLLs?

String Interoperability Challenge in C# and C DLLs

Passing strings seamlessly between C# and C DLLs has proven to be a stumbling block for many. Let's delve into the intricacies of this issue and provide a solution that eliminates the persistent System.AccessViolationException.

The Problem

The crux of the problem lies in the inability to pass C std::string objects across the interop boundary. Attempting to do so, as in the given C# code, will result in an access violation exception.

The Solution: Using Interop-Friendly Types

To prevent this error, we must employ interop-friendly data types at the interop boundary. A prime choice is null-terminated character arrays. They excel when memory allocation and deallocation occur within the same module, simplifying data passing from C# to C .

C Code

void foo(const char *str) {
    // Perform some action on the string
}

C# Code

[DllImport("...", CallingConvention = CallingConvention.Cdecl)]
static extern void foo(string str);

// ...

foo("bar");

Passing Data from C to C#

In cases where the callee requires allocating the buffer, the following approach is recommended:

C Code

void foo(char *str, int len) {
    // Write up to len characters into str
}

C# Code

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

// ...

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

By embracing interop-friendly data types, we can effectively overcome the string exchange hurdle between C# and C DLLs, paving the way for seamless communication and error-free data flow.

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