c# - 没有 SafeHandle 的 DllImport 导致 MissingMethodException

Question

我在 C# 中为 QuickUsb 编写了一个包装类来访问非托管库。有关完整的实现，请参阅此 gist .

本题的主要兴趣点有以下几个部分:

public class QuickUsbPort
{
    private class SafeQuickUsbHandle : SafeHandleZeroOrMinusOneIsInvalid
    {
        [DllImport("QuickUsb.dll", CharSet = CharSet.Ansi)] static extern
            int QuickUsbClose(IntPtr handle);

        public SafeQuickUsbHandle(IntPtr handle) : base(true)
        {
            SetHandle(handle);
        }

        [ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
        protected override bool ReleaseHandle()
        {
            return QuickUsbClose(handle) != 0;
        }
    }

    private static class NativeLib
    {
        [DllImport("QuickUsb.dll", CharSet = CharSet.Ansi)] static extern
            int QuickUsbOpen(out SafeQuickUsbHandle handle, string deviceName);

        public static SafeQuickUsbHandle Open(string deviceName)
        {
            if (QuickUsbOpen(out SafeQuickUsbHandle handle, deviceName) == 0)
            {
                throw new QuickUsbException("Open", new List<Tuple<string, string>>
                {
                    new Tuple<string, string>("deviceName", deviceName),
                });
            }
            return handle;
        }
    }
}

将 handle 编码为 SafeQuickUsbHandle 似乎存在问题，因为在调用 Open() 时此代码会抛出 MissingMethodException。然而，以下修改不会引发此类异常:

        [DllImport("QuickUsb.dll", CharSet = CharSet.Ansi)] static extern
            int QuickUsbOpen(out IntPtr handle, string deviceName);

        public static SafeQuickUsbHandle Open(string deviceName)
        {
            if (QuickUsbOpen(out IntPtr handle, deviceName) == 0)
            {
                throw new QuickUsbException("Open", new List<Tuple<string, string>>
                {
                    new Tuple<string, string>("deviceName", deviceName),
                });
            }
            return new SafeQuickUsbHandle(handle);
        }

所以我想知道我是否遗漏了我的 SafeQuickUsbHandle 实现的某些方面，以允许 c# 正确编码和处理句柄。

注意在dll中，handle是指向句柄的指针:

/// <param name="handle">
/// A PQHANDLE that points to a QHANDLE in which to place the new device ID.
/// If successful, hDevice will contain the new QHANDLE</param>

Answer 1

您必须为派生自 SafeHandle 的类提供公共(public)无参数构造函数，尤其是当您使用 p/invoke 时，定义如下:SafeHandle

Your subclass of SafeHandle is only required to provide three methods

.ctor() – A default constructor that initializes the SafeHandle. This method is used by P/Invoke when it returns a SafeHandle to your process

bool IsInvalid { get; } – a property to determine if the current value of the handle is valid or not

bool ReleaseHandle() – clean up the contained resource

p/invoke 无论如何都会神奇地设置值。它也在 official documentation 中:

When you inherit from SafeHandle, you must override the following members: IsInvalid and ReleaseHandle. You should also provide a default constructor that calls the base constructor with a value that represent an invalid handle value, and a Boolean value indicating whether the native handle is owned by the SafeHandle and consequently should be freed when that SafeHandle has been disposed.

由于 SafeHandleZeroOrMinusOneIsInvalid 没有定义公共(public)无参数构造函数，您必须这样做。