如何从结构中使用结构类型的参数 [英] How can I use a parameter of a struct type from a struct

问题描述

您好，

我在结构中声明了结构类型的参数，当我尝试从结构中使用该参数时，我得到错误NullReferencePointer。



我尝试了什么：



另外我知道你不能初始化结构中的结构。

有没有办法使用该参数而不会出现错误？

Hello,
I have declared a parameter of a struct type in a struct and when I try to use that parameter from the struct I get the error NullReferencePointer.

What I have tried:

Also I know that you can't initialize a struct in a struct.
Is there a way to use that parameter without getting that error?

[StructLayout(LayoutKind.Sequential)]
public struct TCommandParam
{
    public int iValue;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4096)]
    public byte[] sValue;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 64)]
    public byte[] FrameFD;

}

[StructLayout(LayoutKind.Sequential)]
public struct TCommandBuffer
{
    public int Command;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
    public TCommandParam[] Param;
    public int ReturnValue;
}

推荐答案

好的 - 它不是导致问题的结构，它是struct。

.NET中任何东西的数组都是引用类型，即使它是struct（或任何值类型，如int或byte）的数组 - 如果你没有初始化引用类型，当你尝试使用它时会得到一个空引用错误。

在你的结构中，你有三个数组：sValue，FrameFD和Param，你没有显示它们被初始化。请记住，当您开始将信息传递给非托管代码时，MarshalAs属性只会产生影响：如果您没有将数组实例分配给这些阵列中的任何一个，那么每当您尝试使用它们时，都会得到一个空引用错误。

首先看看你实例化结构的位置，看看你是否为它们分配了数组空间。

只实例化TCommandBuffer结构，也是我没有为它们分配数组空间。我尝试在struct中创建一个构造函数并分配一个数组大小，但它不起作用。我如何为所有这些都分配数组空间，以便我可以使用这些struct参数？



我强烈建议您使用类而不是结构，部分原因是因为您可以提供初始值设定项（禁止使用哪些结构）以及无参数构造函数：

OK - it's not the struct that is causing the problem, it's the contents of the struct.
An array of anything in .NET is a reference type, even if it's an array of struct (or any value type such as int, or byte) - and if you don't initialize a reference type, you get a null reference error when you try to use it.
In your structs, you have three arrays: sValue, FrameFD, and Param none of which you show being initialized. Bear in mind that the MarshalAs attribute only has an effect when you start transferring information to unmanaged code: if you don't assign an array instance to any one of those arrays, you will get a null reference error whenever you try to use them.
So start by looking at where you instantiate the structs, and see if you do allocate array space for them all.
" only instantiate the TCommandBuffer struct, also I don't allocate array space for them. I tried creating a constructor inside the struct and allocate an array size but it didn't work. How can I allocate array space for all of them that so I can use those struct parameters?"

I'd strongly suggest you use a classes instead of structs, partly because you can provide initializers (which structs are forbidden to do) and also parameterless constructors:

[StructLayout(LayoutKind.Sequential)]
public sealed class TCommandParam
    {
    public int iValue;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4096)]
    public byte[] sValue = new Byte[4096];
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 64)]
    public byte[] FrameFD = new byte[64];

    }

[StructLayout(LayoutKind.Sequential)]
public sealed class TCommandBuffer
    {
    public int Command;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
    public TCommandParam[] Param = new TCommandParam[10];
    public int ReturnValue;
    public TCommandBuffer()
        {
        for (int i = 0; i < Param.Length; i++)
            {
            Param[i] = new TCommandParam();
            }
        }
    }

无论如何，拥有大型结构是一个坏主意;它会严重影响性能：使用结构和类 - 这是什么所有关于？ [ ^ ] - MS建议结构不要超过16个字节，如果编译为64位环境，则TCommandParam会执行，因为引用是8个字节。

Having large structs is a bad idea anyway; it can really badly affect performance: Using struct and class - what's that all about?[^] - MS recommendations are that structs shoudln't exceed 16 bytes, and your TCommandParam does if you compile for a 64 bit environment, since a reference is 8 bytes.

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