Cannot Generate Delegate for Public Type RustCallback = extern "C" fn()

[Yongle-Fu]

I’m encountering an issue where I cannot properly generate a delegate in C# for a Rust function pointer type defined as pub type BatteryLevelCallback = extern "C" fn(id: *const c_char, battery_level: u8). The Rust code exposes this type and a corresponding function, but the C# interop seems to fail when defining the delegate.

#[no_mangle]
pub extern "C" fn callback_test(cb: extern "C" fn(a: i32) -> i32) -> i32 {
    cb(100)
}

pub type BatteryLevelCallback = extern "C" fn(id: *const c_char, battery_level: u8);

#[no_mangle]
pub extern "C" fn set_battery_level_callback(callback: BatteryLevelCallback) {
    // Placeholder for callback usage
}

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
internal delegate int callback_test_cb_delegate(int a);

[DllImport(__DllName, EntryPoint = "callback_test", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
internal static extern int callback_test(callback_test_cb_delegate cb);

// Attempted delegate for BatteryLevelCallback - this is where the issue arises
[DllImport(__DllName, EntryPoint = "set_battery_level_callback", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
internal static extern void set_battery_level_callback(set_battery_level_callback_callback_delegate callback);

Problem
The callback_test function works as expected with its delegate callback_test_cb_delegate.
However, defining a delegate for set_battery_level_callback (e.g., set_battery_level_callback_callback_delegate) fails. The C# compiler or runtime does not recognize or properly bind to the BatteryLevelCallback type

Expected Behavior
I expect to define a delegate in C# that matches the BatteryLevelCallback type, allowing me to pass a callback function from C# to Rust via set_battery_level_callback.

Additional Notes
The callback_test example with a simple i32 parameter and return type works fine, suggesting the issue is specific to the BatteryLevelCallback signature.
Is there a correct way to define the delegate for extern "C" fn(id: *const c_char, battery_level: u8) in C#?