dotnet core 应用是如何跑起来的 通过AppHost理解运行过程

    /**
 * Detect if the apphost executable is allowed to load and execute a managed assembly.
 *
 *    - The exe is built with a known hash string at some offset in the image
 *    - The exe is useless as is with the built-in hash value, and will fail with an error message
 *    - The hash value should be replaced with the managed DLL filename with optional relative path
 *    - The optional path is relative to the location of the apphost executable
 *    - The relative path plus filename are verified to reference a valid file
 *    - The filename should be "NUL terminated UTF-8" by "dotnet build"
 *    - The managed DLL filename does not have to be the same name as the apphost executable name
 *    - The exe may be signed at this point by the app publisher
 *    - Note: the maximum size of the filename and relative path is 1024 bytes in UTF-8 (not including NUL)
 *        o https://en.wikipedia.org/wiki/Comparison_of_file_systems
 *          has more details on maximum file name sizes.
 */

                const pal::char_t* dotnet_root_cstr = fxr.dotnet_root().empty() ? nullptr : fxr.dotnet_root().c_str();
            rc = hostfxr_main_bundle_startupinfo(argc, argv, host_path_cstr, dotnet_root_cstr, app_path_cstr, bundle_header_offset);

        hostfxr_resolver_t fxr{app_root};

    #define EMBED_HASH_HI_PART_UTF8 "c3ab8ff13720e8ad9047dd39466b3c89" // SHA-256 of "foobar" in UTF-8
#define EMBED_HASH_LO_PART_UTF8 "74e592c2fa383d4a3960714caef0c4f2" // 这两句代码就是 foobar 的 UTF-8 二进制的 SHA-256 字符串
#define EMBED_HASH_FULL_UTF8    (EMBED_HASH_HI_PART_UTF8 EMBED_HASH_LO_PART_UTF8) // NUL terminated

bool is_exe_enabled_for_execution(pal::string_t* app_dll)
{
    constexpr int EMBED_SZ = sizeof(EMBED_HASH_FULL_UTF8) / sizeof(EMBED_HASH_FULL_UTF8[0]);
    // 这里给的是就是最长 1024 个 byte 的 dll 名，加上一个 \0 一共是 1025 个字符
    constexpr int EMBED_MAX = (EMBED_SZ > 1025 ? EMBED_SZ : 1025); // 1024 DLL name length, 1 NUL

    // 这就是定义在 AppHost.exe 二进制文件里面的一段空间了，长度就是 EMBED_MAX 长度，内容就是 c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2 这段字符串
    static char embed[EMBED_MAX] = EMBED_HASH_FULL_UTF8;     // series of NULs followed by embed hash string

    static const char hi_part[] = EMBED_HASH_HI_PART_UTF8;
    static const char lo_part[] = EMBED_HASH_LO_PART_UTF8;

    // 将 embed 的内容复制到 app_dll 变量里面
    pal::clr_palstring(embed, app_dll);
}


int exe_start(const int argc, const pal::char_t* argv[])
{
	// 读取嵌入到二进制文件的 App 名，也就是 dotnet 的入口 dll 路径，可以是相对也可以是绝对路径
    pal::string_t embedded_app_name;
    if (!is_exe_enabled_for_execution(&embedded_app_name))
    {
        trace::error(_X("A fatal error was encountered. This executable was not bound to load a managed DLL."));
        return StatusCode::AppHostExeNotBoundFailure;
    }

    // 将 embedded_app_name 的内容赋值给 app_path 变量，这个变量的定义代码我没有写
    append_path(&app_path, embedded_app_name.c_str());

    const pal::char_t* app_path_cstr = app_path.empty() ? nullptr : app_path.c_str();
    // 跑起来 dotnet 应用
    rc = hostfxr_main_bundle_startupinfo(argc, argv, host_path_cstr, dotnet_root_cstr, app_path_cstr, bundle_header_offset);
}

        /// <summary>
    /// Embeds the App Name into the AppHost.exe
    /// If an apphost is a single-file bundle, updates the location of the bundle headers.
    /// </summary>
    public static class HostWriter
    {
        /// <summary>
        /// hash value embedded in default apphost executable in a place where the path to the app binary should be stored.
        /// </summary>
        private const string AppBinaryPathPlaceholder = "c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2";
        private static readonly byte[] AppBinaryPathPlaceholderSearchValue = Encoding.UTF8.GetBytes(AppBinaryPathPlaceholder);

        /// <summary>
        /// Create an AppHost with embedded configuration of app binary location
        /// </summary>
        /// <param name="appHostSourceFilePath">The path of Apphost template, which has the place holder</param>
        /// <param name="appHostDestinationFilePath">The destination path for desired location to place, including the file name</param>
        /// <param name="appBinaryFilePath">Full path to app binary or relative path to the result apphost file</param>
        /// <param name="windowsGraphicalUserInterface">Specify whether to set the subsystem to GUI. Only valid for PE apphosts.</param>
        /// <param name="assemblyToCopyResorcesFrom">Path to the intermediate assembly, used for copying resources to PE apphosts.</param>
        public static void CreateAppHost(
            string appHostSourceFilePath,
            string appHostDestinationFilePath,
            string appBinaryFilePath,
            bool windowsGraphicalUserInterface = false,
            string assemblyToCopyResorcesFrom = null)
        {
            var bytesToWrite = Encoding.UTF8.GetBytes(appBinaryFilePath);
            if (bytesToWrite.Length > 1024)
            {
                throw new AppNameTooLongException(appBinaryFilePath);
            }

            void RewriteAppHost()
            {
                // Re-write the destination apphost with the proper contents.
                using (var memoryMappedFile = MemoryMappedFile.CreateFromFile(appHostDestinationFilePath))
                {
                    using (MemoryMappedViewAccessor accessor = memoryMappedFile.CreateViewAccessor())
                    {
                        BinaryUtils.SearchAndReplace(accessor, AppBinaryPathPlaceholderSearchValue, bytesToWrite);

                        appHostIsPEImage = PEUtils.IsPEImage(accessor);

                        if (windowsGraphicalUserInterface)
                        {
                            if (!appHostIsPEImage)
                            {
                                throw new AppHostNotPEFileException();
                            }

                            PEUtils.SetWindowsGraphicalUserInterfaceBit(accessor);
                        }
                    }
                }
            }

            // 忽略代码
        }
    }

    AppHostPatcher.exe Foo.exe .\Application\Foo.dll