c# - C# 中的联合 - 不正确对齐或与非对象字段重叠

Question

我正在通过 PInvoke 编码到需要以下调用的 native C dll。

private static extern int externalMethod(IntPtr Data, [MarshalAs(UnmanagedType.U4)] ref int dataLength);

dataLength 参数是通过 IntPtr Data 参数传递的结构的长度。如果两者不匹配，它会抛出异常。外部方法使用 C 联合将四种类型连接在一起。

我已经设法通过使用 FieldOffsetAttribute 在 C# 中重新创建联合。然后我计算 C# 联合的长度并使用以下方法调用该方法:

int len = Marshal.SizeOf(data);
IntPtr ptr = Marshal.AllocCoTaskMem(len);
externalMethod(ptr, len);

但是，我得到错误 System.TypeLoadException : ... Could not load type because it contains an object field at offset 0 that is importantly aligned or overlapped by a non-object field. 与以下代码。我相信它可能是字符串之一或整数数组(变量 B7)？我将如何改变它以使其工作 - 我是否必须将整数数组分解为多个变量？

[StructLayoutAttribute(LayoutKind.Explicit)]
public struct Union{
    [FieldOffset(0)]
    public A a;

    [FieldOffset(0)]
    public B b;

    [FieldOffset(0)]
    public C c;

    [FieldOffset(0)]
    public D d;
}

[StructLayout(LayoutKind.Sequential)]
public struct A
{
    public int A1;
    public int A2;
    public int A3;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 17)]
    public string A4;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 4)]
    public string A5;
}

[StructLayout(LayoutKind.Sequential)]
public struct B
{
    public int B1;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 2)]
    public string B2;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 4)]
    public string B3;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 6)]
    public string B4;
    public int B5;
    public int B6;
    [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U4, SizeConst = 255)]
    public int[] B7;
}

[StructLayout(LayoutKind.Sequential)]
public struct C
{
    public int C1;
    public int C2;
    public int C3;
    public int C4;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 32)]
    public string C5;
    public float C6;
    public float C7;
    public float C8;
    public float C9;
    public float C10;
    public float C11;
    public float C12;
    public float C13;
    public float C14;
}

[StructLayout(LayoutKind.Sequential)]
public struct D
{
    public int D1;
    [MarshalAs(UnmanagedType.LPTStr, SizeConst = 36)]
    public string D2;
}

Answer 1

只需直接使用 A/B/C/D 结构并跳过联合即可。在您的外部调用中，只需在方法声明中替换正确的结构。

extern void UnionMethodExpectingA( A a );

如果非托管方法实际上接受联合并且根据传递的类型表现不同，那么您可以声明不同的外部方法，这些方法最终都会调用相同的非托管入口点。

[DllImport( "unmanaged.dll", EntryPoint="ScaryMethod" )]
extern void ScaryMethodExpectingA( A a );

[DllImport( "unmanaged.dll", EntryPoint="ScaryMethod" )]
extern void ScaryMethodExpectingB( B b );

针对“长度”参数进行了更新。逻辑仍然适用。只需创建一个“包装器”方法并执行相同的操作即可。

void CallScaryMethodExpectingA( A a )
{
  ScaryMethodExpectingA( a, Marshal.SizeOf( a ) );
}