How to Efficiently Pass Strings Between C# and a C DLL?

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

In this query, a developer struggles to pass strings between C# and a C DLL. Analysis of their code reveals a fundamental issue that hinders their effort: incompatibility between C std::string and C# strings at the interop boundary.

The C DLL function they define:

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

Cannot be directly accessed from C#. This is because std::strings are not interop-friendly data types and should not be passed across the interop boundary.

To resolve this issue, the developer must use alternative interop-friendly data types. A common approach is to leverage null-terminated arrays of characters, as these can be easily marshaled between different programming languages. Here's a modified version of their C code:

extern "C" {
    void concat(char* str1, char* str2, char** result)
    {
        int len1 = strlen(str1);
        int len2 = strlen(str2);
        int resultSize = len1 + len2 + 1; //null-terminator included
        *result = (char*)malloc(resultSize); //allocate memory for the result string
        strcpy(*result, str1);
        strcat(*result, str2); //append str2 to str1
    }
}

This function takes three parameters: two input character arrays (str1 and str2) and a result character array (*result) that will be allocated and returned to the caller.

In C#, the code can be adjusted accordingly:

[DllImport("*****.dll", CallingConvention = CallingConvention.Cdecl)]
    static extern void concat(string str1, string str2, StringBuilder result);

...

StringBuilder result = new StringBuilder(10);
concat("a", "b", result);
Console.WriteLine(result.ToString());

This code allocates a StringBuilder with a capacity of 10 characters and passes it to the C function as the result parameter. After the function executes, the concatenated string will be available in the StringBuilder.

By utilizing this approach, the developer ensures that strings are passed and returned using interop-compatible data types, resolving the System.AccessViolationException that occurred previously.

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