使用C#中的Marshal.StructureToPtr将结构传递给C++ API

Question

我在我的代码中使用C++编写的API(用C#编写).API需要一个参数作为指向结构的指针.结构由"Int"和Char Arrays组成:例如

 unsafe public struct ToBePassed 
    { 
        Int32 Num1;
        Int32 Num2; 
        Char[] Data; // or fixed Char Data[255];
    }

我不能直接将结构指针传递给API,因为在这种情况下,我收到错误,因为"指针不能引用Marshaled结构".代码编译成功但是当我执行(调试)代码时会出现此错误.

现在我有两个选择:第一: - 通过参考传递结构:我想问一下当我通过ref传递结构时,需要结构指针的API是否可以接收地址.请注意,API将在"Char [] Data"中返回Data.

第二: - 使用Marshal.StructureToPtr:这会将结构指针转换为IntPtr.怀疑是相同的,API会正确接收吗？

谢谢你的时间!

此致,Swanand

Answer 1

如果它只需要指针,您可以分配一些非托管内存,将结构编组到内存中,并将该指针传递给您的函数.然后你可以编组回到结构(如果你愿意)并释放内存.在编组任何内容之前,您需要正确定义结构.像这样的东西:

[StructLayout(
    LayoutKind.Sequential,      //must specify a layout
    CharSet = CharSet.Ansi)]    //if you intend to use char
public struct ToBePassed
{
    public Int32 Num1;
    public Int32 Num2;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 255)]
    public Char[] Data;    //specify the size using MarshalAs
}

[DllImport("...")]
public static extern void APICall(IntPtr argPtr);


public static void CallFunction(ToBePassed managedObj)
{
    IntPtr unmanagedAddr = Marshal.AllocHGlobal(Marshal.SizeOf(managedObj));

    Marshal.StructureToPtr(managedObj, unmanagedAddr, true);

    APICall(unmanagedAddr);

    Marshal.PtrToStructure(unmanagedAddr, managedObj);

    Marshal.FreeHGlobal(unmanagedAddr);
    unmanagedAddr = IntPtr.Zero;
}

[edit]
要模拟可变长度数组,请在结构中分配非托管内存并像往常一样进行初始化.

[StructLayout(LayoutKind.Sequential)]
public struct SomeStruct
{
    public Int32 X;
    public Int32 Y;
}

[StructLayout(LayoutKind.Sequential)]
public struct VLA
{
    public Int32 intArrayLength;
    public Int32 SomeStructArrayLength;
    public IntPtr intArray;
    public IntPtr SomeStructArray;
}

public static VLA CreateVLA(int[] intArray, SomeStruct[] SomeStructArray)
{
    var vla = new VLA()
    {
        intArrayLength = intArray.Length,
        SomeStructArrayLength = SomeStructArray.Length,
        intArray = Marshal.AllocHGlobal(intArray.Length * Marshal.SizeOf(typeof(int))),
        SomeStructArray = Marshal.AllocHGlobal(SomeStructArray.Length * Marshal.SizeOf(typeof(SomeStruct))),
    };
    Marshal.Copy(intArray, 0, vla.intArray, intArray.Length);
    //there's no overload to copy arbitrary arrays, do it manually
    for (int i = 0; i < SomeStructArray.Length; i++)
    {
        Marshal.StructureToPtr(
            SomeStructArray[i],
            vla.SomeStructArray + i * Marshal.SizeOf(typeof(SomeStruct)),
            true);
    }
    return vla;
}