CODEWITHSUNDEEP
c#cwinapi
xuyunhai
xuyunhai

Reputation: 27

Why can't c# get the correct parameter value from unmanaged dll

Win32 program as below:

 #ifdef A_EXPORTS
    #define DLL_API __declspec(dllexport)
 #else
    #define DLL_API __declspec(dllimport)
 #endif
extern "C" DLL_API  void EC_GetGin(int* icard);
void EC_GetGin(int* icard)
{
    icard = 1;
}

C# code as below:

[DllImport("test.dll", EntryPoint = "EC_GetGin", CharSet = CharSet.Auto, CallingConvention = CallingConvention.Cdecl)]
public static extern void EC_GetGin(ref int icard);

int icard = 0;
EC_GetGin(ref icard);

Why can't C# get 1 ？

Upvotes: 0

Views: 68

Answers (3)

Vivek Nuna
Vivek Nuna

Reputation: 1

You need to modify the value, use * with the pointer to get the value.

void EC_GetGin(int* icard)
{
    *icard = 1;
}

Upvotes: 1

David Heffernan
David Heffernan

Reputation: 613442

Your C code modifies the pointer rather than writing to the variable it points to. Change it to

void EC_GetGin(int* icard)
{
    *icard = 1;
}

The C# code would be cleaner if you declared the argument as out rather the ref.

Upvotes: 4

René Vogt
René Vogt

Reputation: 43916

In your C code you don't set the value in the memory location, but change the pointer in icard (you let icard point to memory location 1).

To change the value in the memory location icard points to use

*icard = 1;

Upvotes: 1

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