按值为 C++ 结构 PInvoke 类

Question

I've got a small issue regarding P/Invoke. Currently, I am implementing a wrapper for a C API for a hardware device whose original C# implementation sucks. The issue/inconvenience I am having is the following:

The API implements multiple structs for getting/setting device settings. These are of similar structure, thus I am giving an example implementation:

typedef struct _Struct1
{
    bool b1;
    unsigned int ui1;
    unsigned int ui2;
    unsigned int ui3;
    unsigned int ui4;
    unsigned int ui5;
} Struct1;

The getter and setter are implemented as follows:

unsigned int SetSetting(bool b1, Struct1 s1);
unsigned int GetSetting(bool b1, Struct1 &s1);

The C# DllImport is:

[DllImport("api.dll", EntryPoint = "GetSetting", CallingConvention = CallingConvention.StdCall]
public static extern uint GetStatus(bool b1, CStruct1 s1);

[DllImport("api.dll", EntryPoint = "SetSetting", CallingConvention = CallingConvention.StdCall]
public static extern uint SetStatus(bool b1, SStruct1 s1);

The C# struct implementation is done using LayoutKind.Sequential and represents the C struct exactly and the P/Invoke calls work quite well except for: As you may noticed, the getter and setter DllImports differ slightly in that the setter uses a C# struct (SStruct1) and the getter a C# class (CStruct1):

[StructLayout(LayoutKind.Sequential)]
public struct SStruct1
{
    public bool b1;
    public uint ui1;
    public uint ui2;
    public uint ui3;
    public uint ui4;
    public uint ui5;
}

[StructLayout(LayoutKind.Sequential)]
public class CStruct1 { /* same as struct */ }

I have not been able to get P/Invoke working using just one, either a class or a struct.

Do you have any tips regarding a more convenient solution for this problem?

Changing the struct to class and vice versa does give a PInvokeStackImbalance exception.

I am guessing this has to do with the fact that the setter is expecting a struct by value and the getter by reference, given it's out-parameter-equivalent. I've tried any Marshal attribute and parameter definitions, I could think of, but am hitting a dead end. Google does not seem to help.

Any help is greatly apprechiated.

Edit

I've been mistaken: The API is ANSI C, not C++.

Update

I've tried the solution suggested (SStruct for setter, out SStruct for getter) and although working fine for a function with a struct consisting entirely of uints, it produces complete garbage for the struct in question:

| Name | correct value   | returned value |
| ---- | --------------- | -------------- |
| b1   | False           | True           |
| ui1  | 0               | 3355443223     |
| ui2  | 0               | 1677721600     |
| ui3  | 0               | 0              |
| ui4  | 0               | 0              |
| ui5  | 0               | 0              |

Remember: When using using aforementioned DllImports, this works just fine.

Answer 1

1. Check your build settings x86 or x64 for C# projects.
2. Verify calling convention. I think that in VC++ default is cdecl , try to specify it in DllImport.

Answer 2

Building on the comment from @HansPassant, you want to use out SStruct for the getter method. Also, you need to make sure that the size of a bool is the same both in the dotnet struct and the compiled api.dll struct (the struct packing might be 1 byte but it is very unlikely see here :).

Also, you use CallingConvention=StdCall . The symptom of using the wrong calling convention is a stack imbalance exception. You want to try using CallingConvention.Cdecl if you don't know it.

Most often, when opening the dll in depends, if symbols don't end-up with @<some int> , the convention is Cdecl with no guarantee though.