C＃：如何传递null期待裁判的函数吗？ [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);

，我想这样的呼吁：

Which I would like to call like this:

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

不幸的是，我无法通过空在领域，需要键入 REF映射并没有投我试过修复了这个。

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？

推荐答案

我假设映射是一个结构？如果是这样你可以有 FILES_GetMemoryMapping（）原型不同签名的两个版本。对于您要传递的第二个重载空，使参数的的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);

通话例如：

Call example:

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

如果映射实际上是一类，而不是一个结构，只需将它传递下来之前为空值

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

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