C#:如何将 null 传递给需要 ref 的函数? [英] C#: How to pass null to a function expecting a ref?

问题描述

我有以下功能:

public static extern uint FILES_GetMemoryMapping(
    [MarshalAs(UnmanagedType.LPStr)] string pPathFile,
    out ushort Size,
    [MarshalAs(UnmanagedType.LPStr)] string MapName,
    out ushort PacketSize,
    ref Mapping oMapping,
    out byte PagesPerSector);

我想这样称呼:

FILES_GetMemoryMapping(MapFile, out size, MapName,
    out PacketSize, null, out PagePerSector);

不幸的是，我无法在需要类型 ref Mapping 的字段中传递 null 并且我尝试过的任何强制转换都无法解决此问题.

Unfortunately, I cannot pass null in a field that requires type ref Mapping and no cast I've tried fixes this.

有什么建议吗?

推荐答案

我假设 Mapping 是一种结构?如果是这样，您可以拥有具有不同签名的两个版本的 FILES_GetMemoryMapping() 原型.对于要传递 null 的第二个重载，将参数设为 IntPtr 并使用 IntPtr.Zero

I'm assuming that Mapping is a structure? If so you can have two versions of the FILES_GetMemoryMapping() prototype with different signatures. For the second overload where you want to pass null, make the parameter an IntPtr and use IntPtr.Zero

public static extern uint FILES_GetMemoryMapping(
    [MarshalAs(UnmanagedType.LPStr)] string pPathFile,
    out ushort Size,
    [MarshalAs(UnmanagedType.LPStr)] string MapName,
    out ushort PacketSize,
    IntPtr oMapping,
    out byte PagesPerSector);

调用示例:

FILES_GetMemoryMapping(MapFile, out size, MapName,
   out PacketSize, IntPtr.Zero, out PagePerSector);

如果 Mapping 实际上是一个类而不是一个结构，只需在传递之前将值设置为 null.

If Mapping is actually a class instead of a structure, just set the value to null before passing it down.

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