How Can I Access C Classes from C# Using P/Invoke?

Accessing C Classes in C# Code via DLL

When interfacing with a C DLL from C# code, limitations arise when attempting to directly access classes defined within the DLL. Despite P/Invoke's ability to expose functions, functions within classes rely on non-static member variables, requiring instance creation.

P/Invoke and Indirect Access

While direct class access is not feasible, P/Invoke can be utilized indirectly to access class members. The approach involves creating non-member functions that call into the class member functions.

Example Code

Consider the following C code:

class Foo {
public:
  int Bar();
};

extern "C" Foo* Foo_Create() { return new Foo(); }
extern "C" int Foo_Bar(Foo* pFoo) { return pFoo->Bar(); }
extern "C" void Foo_Delete(Foo* pFoo) { delete pFoo; }

Here, the C code exposes the class Foo through non-member functions.

C# Interfacing

In C#, P/Invoke can access these functions:

[DllImport("Foo.dll")]
public static extern IntPtr Foo_Create();

[DllImport("Foo.dll")]
public static extern int Foo_Bar(IntPtr value);

[DllImport("Foo.dll")]
public static extern void Foo_Delete(IntPtr value);

C# code must now manage IntPtr values, which can be wrapped in a wrapper class for convenience.

Alternative Approach

If direct ownership of the C code is not possible, a separate DLL can be created to wrap the original DLL and provide a P/Invoke layer, simplifying C# code interaction with the C class.

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