[P/Invoke] 使用 SetDllImportResolver[1] 改写 DllImport 的库解析规则
- [P/Invoke] 使用
SetDllImportResolver[1] 改写DllImport的库解析规则- 问题导入
- 尝试解决
- 总结
问题导入
我们都知道，DllImport 在加载本机库时，是在程序文件夹或者环境变量指定的路径里按照特定的规则来寻找库。那么，如果我们想要稍微改变一下加载规则，要怎么做呢？
这个问题我是在 F# 交互环境中引用 PaddleOCRSharp 遇到的。当时，我像在项目中引用一样，直接在交互环境中引用 PaddleOCRSharp，配好模型参数，然后构建 PaddleOCREngine，结果报错了：
- let engine = PaddleOCREngine(config);;
System.DllNotFoundException: Unable to load DLL 'PaddleOCR' or one of its dependencies: 找不到指定的模块。 (0x8007007E)at PaddleOCRSharp.PaddleOCREngine.Initialize(String det_infer, String cls_infer, String rec_infer, String keys, OCRParameter parameter)at ...
已因出错而停止
尝试解决
看起来是缺 DLL 文件。由于交互环境是在 dotnet 安装目录里面的，我只能将所需的 DLL 复制出来放到另外的文件夹，然后试试库自带的更改加载路径方法：
- let dllPath = "..."
- PaddleOCREngine.PaddleOCRdllPath <- dllPath
- let engine = PaddleOCREngine(config);;
System.DllNotFoundException: Unable to load DLL 'PaddleOCR' or one of its dependencies: 找不到指定的模块。 (0x8007007E)at PaddleOCRSharp.PaddleOCREngine.Initialize(String det_infer, String cls_infer, String rec_infer, String keys, OCRParameter parameter)at ...
已因出错而停止
还是报错。此时我还没有意识到报错的真正原因，只是怀疑还是找不到 DLL 。于是我又换了个方法——使用 SetDllImportResolver 来试试。
SetDllImportResolver 的说明文档主要如下：
Sets a callback for resolving native library imports from an assembly.
static member SetDllImportResolver : System.Reflection.Assembly * System.Runtime.InteropServices.DllImportResolver -> unitThis per-assembly resolver is the first attempt to resolve native library loads initiated by this assembly.
可以看到，SetDllImportResolver 是用于改写某个程序集内解析本机库导入规则的。
而 DllImportResolver 是一个接受三个参数而返回一个本机库句柄的委托：
type DllImportResolver = delegate oflibraryName: string *assembly : Assembly *searchPath : Nullable<DllImportSearchPath> -> nativeint
说明文档有示例，那就简单了，很快就可以写出下面的代码：
- NativeLibrary.SetDllImportResolver(
- // PaddleOCREngine 所在的程序集
- typeof<PaddleOCREngine>.Assembly,
- fun libraryName assembly searchPath ->
- if libraryName = "PaddleOCR" then
- let path = Path.Combine(dllPath, libraryName + ".dll")
- // 加载本机库，返回句柄
- NativeLibrary.Load(path, assembly, searchPath)
- // 如果不是 PaddleOCR，则使用默认的加载规则
- else 0n)
- let engine = PaddleOCREngine(config);;
System.DllNotFoundException: Unable to load DLL 'D:\l\Desktop\sp\dll\ocr\PaddleOCR.dll' or one of its dependencies: 找不到指定的模块。 (0x8007007E)at System.Runtime.InteropServices.NativeLibrary.LoadLibraryByName(String libraryName, Assembly assembly, Nullable`1 searchPath, Boolean throwOnError)at ...
已因出错而停止
依然报错，但是输出改变了。于是我终于发现了问题所在：依赖库没有被复制过来。将依赖库复制过来后，程序终于运行起来了。
总结
SetDllImportResolver 可以用来改写某个程序集内解析本机库导入的规则。
对于 PaddleOCREngine 来说，修改 DLL 加载路径方法最好是修改 PaddleOCREngine.PaddleOCRdllPath，因为它是通过修改环境变量实现的，可以让本机库也找得到依赖。如果是用 SetDllImportResolver 让程序在托管部分运行起来的话，在非托管那边也会报错：
System.Exception: Initialize err: --------------------------------------
C++ Traceback (most recent call last):
--------------------------------------
Not support stack backtrace yet. ----------------------
Error Message Summary:
----------------------
PreconditionNotMetError: The third-party dynamic library (mklml.dll) that Paddle depends on is not configured correctly. (error code is 126) Suggestions: 1. Check if the third-party dynamic library (e.g. CUDA, CUDNN) is installed correctly and its version is matched with paddlepaddle you installed.2. Configure third-party dynamic library environment variables as follows: - Linux: set LD_LIBRARY_PATH by `export LD_LIBRARY_PATH=...` - Windows: set PATH by `set PATH=XXX; (at D:\MyWorks\Paddle\PaddleBuild\Paddle-v2.6\paddle\phi\backends\dynload\dynamic_loader.cc:312) at PaddleOCRSharp.PaddleOCREngine..ctor(OCRModelConfig config, OCRParameter parameter)at PaddleOCRSharp.PaddleOCREngine..ctor(OCRModelConfig config) at FSI_0014.staticInitialization@() in d:\l\Desktop\sp\240929 ocr.fsx:line 27 at <StartupCode$FSI_0014>.$FSI_0014.main@() at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor) at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
已因出错而停止
SetDllImportResolver仅在 .NET Core 3.1 和 .NET 5+ 中可用。 ↩︎
