MEF - Handle ReflectionTypeLoadException during MEF composition

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。

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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:

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  public class SafeDirectoryCatalog : ComposablePartCatalog
{
#region Fields 
private AggregateCatalog _catalog;
#endregion Fields 

#region Properties 
/// <summary>
/// Gets the catalog.
/// </summary>
/// <value>
/// The catalog.
/// </value>
private AggregateCatalog m_Catalog
{
get
{
return _catalog ?? (_catalog = new AggregateCatalog());
}
}
/// <summary>
/// Gets the part definitions that are contained in the directory catalog.
/// </summary>
/// <returns>The <see cref="T:System.ComponentModel.Composition.Primitives.ComposablePartDefinition" /> objects that are contained in the <see cref="T:System.ComponentModel.Composition.Hosting.DirectoryCatalog" />.</returns>
public override IQueryable<ComposablePartDefinition> Parts
{
get { return m_Catalog.Parts; }
}
#endregion Properties 

#region Constructors 
/// <summary>
/// Initializes a new instance of the <see cref="SafeDirectoryCatalog" /> class.
/// </summary>
/// <param name="path">Path to the directory to scan for assemblies to add to the catalog.The path must be absolute or relative to <see cref="P:System.AppDomain.BaseDirectory" />.</param>
/// <param name="searchPattern">The pattern to search with. The format of the pattern should be the same as specified for <see cref="M:System.IO.Directory.GetFiles(System.String,System.String)" />.</param>
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)
{
}
}
}

/// <summary>
/// Initializes a new instance of the <see cref="SafeDirectoryCatalog" /> class.
/// </summary>
/// <param name="path">Path to the directory to scan for assemblies to add to the catalog.The path must be absolute or relative to <see cref="P:System.AppDomain.BaseDirectory" />.</param>
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。

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private void ComposeCurrentDirectoryParts()
{
using (var catalog = new SafeDirectoryCatalog(Environment.CurrentDirectory))
using (var container = new CompositionContainer(catalog))
{
container.ComposeParts(this);
}
}