How to Avoid System.AccessViolationException When Passing Strings Between C# and a C DLL?

Passing Strings between C# and C DLL: A Minimal Example

Problem Statement

Despite following a seemingly straightforward approach, passing strings between C# and a C DLL via interop is causing a System.AccessViolationException. The code in both languages appears correct, raising confusion as to the root cause of the issue.

C Code

The C code defines a function, concat, which concatenates two strings and returns the result. This function is marked for export to the DLL using __declspec(dllexport).

extern "C" {
    string concat(string a, string b){
        return a + b;
    }
}

C# Code

In C#, the code imports the concat function using DllImport. The CallingConvention is set to Cdecl to match the C convention.

[DllImport("*****.dll", CallingConvention = CallingConvention.Cdecl)]
static extern string concat(string a, string b);

Exception and Analysis

Calling concat("a", "b") in C# throws a System.AccessViolationException. This exception indicates that invalid memory access is occurring within the native code.

Solution: Interop-Compatible Types

The culprit of the issue lies in the use of C std::string across the interop boundary. C# cannot create or handle std::string objects directly. Instead, interop-compatible types must be employed for data exchange.

Interop-Friendly Data Types

Suitable interop types include null-terminated arrays of characters for passing strings from C# to C . When passing strings from C to C#, a StringBuilder can be used, with the caller allocating the buffer and the callee writing into it.

Modified Code

In C , the concat function can be declared with char* and int parameters to receive a buffer and its length.

extern "C" {
    void concat(char *str, int len)
    {
        // Concatenate strings and write to buffer
    }
}

In C#, the StringBuilder can be utilized.

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

....

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

By using interop-compatible string types, the exchange of strings between C# and the C DLL can be facilitated effectively.

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