how to use C# invoke C dll?

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C#

C code

// The cJSON structure:
typedef struct cJSON {
struct cJSON *next,*prev;
struct cJSON *child;
int type; // The type of the item, as above.

char *valuestring; // The item's string, if type==cJSON_String
int valueint; // The item's number, if type==cJSON_Number
double valuedouble; // The item's number, if type==cJSON_Number

char *string; // The item's name string, if this item is the child of, or is in the list of subitems of an object.
} cJSON;

typedef struct cJSON_Hooks {
  void *(*malloc_fn)(size_t sz);
  void (*free_fn)(void *ptr);
} cJSON_Hooks;

// Supply malloc, realloc and free functions to cJSON
__declspec(dllexport) void cJSON_InitHooks(cJSON_Hooks* hooks);

C# code

namespace DeviceApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

[DllImport("cJson.dll")]
public static extern void cJSON_InitHooks(cJSON_Hooks* hooks);
}
}


cJSON_Hooks and cJSON is a struct in cJson.h, but in C# code ,cJSON_Hooks and cJSON can't be incognizanced. If someone could walk me through it or point me in the right direction it would be greatly appreciated. Thanks