当前位置：网站首页>[MEF] Chapter 04 MEF's Multi-Component Import (ImportMany) and Directory Services

[MEF] Chapter 04 MEF's Multi-Component Import (ImportMany) and Directory Services

2022-08-08 21:01:00 【thinking barefoot】


I. Demo overview
This demo shows how MEF uses the ImportMany feature to import multiple export parts that match the same constraints at the same time, and introduces the Catalog service, which tells the MEF framework where to search for components that match the specified constraintsThe export part, that is, where the export part is located.
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2. ImportMany of multiple parts
The export settings of the component do not change, and the import place is modified like the following:
[ImportMany(typeof(ILogger))]public IEnumerable Loggers { get; set; }
There are two differences between the above code and the import of a single component:
1. Use the ImportMany feature instead of the Import feature.
2. The IEnumerable type is used to encapsulate the imported components.Because, there may be multiple components imported, and a collection method is required to carry them.

After the import element is modified as above, it only needs to traverse to access each export component when calling.As shown in the following code:
static void Main(string[] args){Program program = new Program();program.Compose();foreach (ILogger logger in program.Loggers){logger.WriteLog("Hello World!");}Console.ReadLine();}

III. Catalog
In MEF, the so-called directory (Catalog) is different from the directory (Directory) in the file system, and its role is to tell the MEF framework where to find the exported components, such as:
AssemblyCatalog: Finds the exported assembly in the specified assembly.
DirectoryCatalog: Find the export component in the specified file directory, the file directory can be passed in the constructor, which can be an absolute path or a relative path.
The code is as follows:
/// /// Assembles the host and widget together via the container object./// public void Compose(){AggregateCatalog aggregateCatalog = new AggregateCatalog();AssemblyCatalog assemblyCatalog = new AssemblyCatalog(typeof(Program).Assembly);DirectoryCatalog directoryCatalog = new DirectoryCatalog("imps");aggregateCatalog.Catalogs.Add(assemblyCatalog);aggregateCatalog.Catalogs.Add(directoryCatalog);var container = new CompositionContainer(aggregateCatalog);container.ComposeParts(this);}
There is a catalog class of AggregateCatalog in the above code, which is an aggregate catalog, which can organize multiple catalogs together, such as AssemblyCatalog and DirectoryCatalog in the above column.In this way, MEF will search for a matching exported part in multiple places.

IV. Relevant resources
1. MSDN official information:http://msdn.microsoft.com/en-us/library/dd460648(v=vs.110).aspx



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