C# StructLayout/FieldOffset 和数组索引 [英] C# StructLayout/FieldOffset and indexing in arrays

问题描述

我在数组中正确使用 FieldOffset 时遇到了一些问题.下面的代码是一个例子，它对我来说不能正常工作:

I'm having a bit of a problem using FieldOffset correctly with arrays. The code below is an example where it doesn't work correctly for me:

[StructLayout(LayoutKind.Explicit)]
public struct IndexStruct {
    [FieldOffset(0)]
    public byte[] data;

    [FieldOffset(0)]
    public short[] idx16;

    [FieldOffset(0)]
    public int[] idx32;
}

例如，如果我将名为data"的数组设置为序列化的字节数组，然后尝试使用idx16"字段检索作为 short 的数据，则索引仍与字节 [] 对齐.这意味着 idx161 获取数据中的第二个字节，而不是第二个 16 位字(字节 2 和 3).如果我做反向 I 索引 shorts 而不是字节，这意味着偏移对齐是从源数据继承的.我的问题，有没有办法解决这个问题?我知道我可以通过乘以元素的大小来补偿索引值，但还有其他方法吗?

If I for example sets the array named "data" to a serialized byte array and then try to retrieve data as shorts using the "idx16" field the indexing is still aligned as a byte[]. Meaning that idx161 fetches the second byte in data, not the second 16bit word (byte 2 and 3). If I do the inverse I index shorts instead of bytes, meaning that the offset alignment is inherited from the source data. My question, is there a way to work around this? I know that I can compensate the index value by multiplying with the size of the element, but is there another way?

这里 是我在 StackOverflow 上找到的一个答案，但是在尝试该代码时发现它无法正常工作.使用以下代码在 VS 中使用单元测试进行了尝试，但没有成功:

Here is an answer I found here on StackOverflow, but when trying that code it turned out that it wasn't working properly. Tried it out using a Unit test in VS with the following code without any success:

[TestMethod()]
public void SumTest() {
    float[] fArr = {2.0f, 0.5f, 0.0f, 1.0f};
    MemoryStream ms = new MemoryStream();
    for (int i = 0; i < fArr.Length; i++) {
        ms.Write(BitConverter.GetBytes(fArr[i]), 0, sizeof(float));
    }
    byte[] buff = ms.ToArray();
    double expected = 3.5f;
    double actual = Sum(buff);
    Assert.AreEqual(expected, actual);
}

非常感谢！

推荐答案

问题是(从我所见)你已经联合了数组的引用 - 所以无论哪个数组被设置最后会赢.一旦有一个数组，它就会使用索引器(而不是字节偏移量)——所以大小无关紧要.

The problem is (from what I can see) that you've unioned the references of the arrays - so whichever array gets set last will win. Once there is an array, it is using the indexer (not byte offset) - so the size doesn't matter.

正确"(或不正确，视情况而定)执行此操作的方法可能是使用不安全的代码 - 将指针指向数组 - 类似于:

The way to do this "properly" (or improperly, as the case may be) would probably be with unsafe code - taking the pointer to the array - something like:

    IndexStruct s = new IndexStruct();
    s.data = new byte[] { 1, 0, 0, 0, 1, 1 };

    unsafe
    {
        fixed (short* data = s.idx16)
        {
            Console.WriteLine(data[0]); // should be 1 (little-endian)
            Console.WriteLine(data[1]); // should be 0
            Console.WriteLine(data[2]); // should be 257
        }
    }

当然，我不确定我是否推荐它 - 但这似乎达到了您想要的效果?

Of course, I'm not sure I recommend it - but that seems to achieve what you want?

我也想知道您是否可以完全删除 struct 并直接使用对 byte[] 的不安全访问:

I also wonder whether you can drop the struct completely and just use unsafe access to a byte[] directly:

    byte[] raw = new byte[] { 1, 0, 0, 0, 1, 1 };
    unsafe
    {
        fixed (byte* addr = raw)
        {
            short* s = (short*)addr;
            Console.WriteLine(s[0]); // should be 1
            Console.WriteLine(s[1]); // should be 0
            Console.WriteLine(s[2]); // should be 257
        }
    }

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