How to Seamlessly Pass Strings Between C# and C DLLs?

Passing Strings from C# to C DLL and Back

When attempting to pass strings between C# and C , developers often encounter difficulties. In this article, we delve into this issue and provide a concise solution.

The Problem

Consider the following code, where C defines a function to concatenate strings:

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

In C#, the function is called using interop:

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

However, this approach results in a System.AccessViolationException.

The Solution

The problem lies in the incompatibility of C std::string with the interoperability boundary. To resolve this issue, interop-friendly types must be used at the boundary, such as null-terminated arrays of characters.

Passing Strings from C# to C

For strings passed from C# to C , the C function can be modified:

void foo(const char *str)
{
    // do something with str
}

And in C#:

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

Passing Strings from C to C#

For strings passed from C to C#, the C function becomes:

void foo(char *str, int len)
{
    // write no more than len characters into str
}

In C#:

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

By adhering to these guidelines, developers can pass strings seamlessly between C# and C DLLs.

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