使用 MEF 去 Compose 指定目錄下的所有 Part，我們可能會像下面這樣透過 DirectoryCatalog 去提供 Export Parts，讓 CompositionContainer 去做 Compose。
private void ComposeCurrentDirectoryParts()
{
using (var catalog = new DirectoryCatalog(Environment.CurrentDirectory))
using (var container = new CompositionContainer(catalog))
{
container.ComposeParts(this);
}
}
多半上面這段程式能夠運行良好。但有的時候會發生不如預期的效果，因為 DirectoryCatalog 會去找目錄下指定的檔案，可能會找到一些組件會相依於其它不存在的組件，導致發生 ReflectionTypeLoadException 錯誤。如果你碰到這樣的問題，可以避開使用 DirectoryCatalog，改成自己去遍巡處理，並用 AssemblyCatalog 載入，載入失敗則將之忽略不予處理。像是下面這樣：
public class SafeDirectoryCatalog : ComposablePartCatalog
{
#region Fields
private AggregateCatalog _catalog;
#endregion Fields
#region Properties
///
/// Gets the catalog.
///
///
/// The catalog.
///
private AggregateCatalog m_Catalog
{
get
{
return _catalog ?? (_catalog = new AggregateCatalog());
}
}
///
/// Gets the part definitions that are contained in the directory catalog.
///
/// The objects that are contained in the .
public override IQueryable Parts
{
get { return m_Catalog.Parts; }
}
#endregion Properties
#region Constructors
///
/// Initializes a new instance of the class.
///
/// Path to the directory to scan for assemblies to add to the catalog.The path must be absolute or relative to .
/// The pattern to search with. The format of the pattern should be the same as specified for .
public SafeDirectoryCatalog(string path, string searchPattern)
{
var files = Directory.EnumerateFiles(path, searchPattern, SearchOption.TopDirectoryOnly);
foreach (var file in files)
{
try
{
var ac = new AssemblyCatalog(file);
if (ac.Parts.Any())
m_Catalog.Catalogs.Add(ac);
}
catch (ReflectionTypeLoadException)
{
}
}
}
///
/// Initializes a new instance of the class.
///
/// Path to the directory to scan for assemblies to add to the catalog.The path must be absolute or relative to .
public SafeDirectoryCatalog(string path)
: this(path, "*.dll")
{
}
#endregion Constructors
}
這邊已參閱Handle ReflectionTypeLoadException during MEF composition - Stack Overflow，將之整理成可重用的類別，使用時只要將本來的 DirectoryCatalog 改成用 SafeDirectoryCatalog 就可以了。
private void ComposeCurrentDirectoryParts()
{
using (var catalog = new SafeDirectoryCatalog(Environment.CurrentDirectory))
using (var container = new CompositionContainer(catalog))
{
container.ComposeParts(this);
}
}