{"id":55937,"date":"2010-03-02T13:46:00","date_gmt":"2010-03-02T13:46:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/pfxteam\/2010\/03\/02\/faq-parallel-foreach-and-non-generic-collections\/"},"modified":"2010-03-02T13:46:00","modified_gmt":"2010-03-02T13:46:00","slug":"faq-parallel-foreach-and-non-generic-collections","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/dotnet\/faq-parallel-foreach-and-non-generic-collections\/","title":{"rendered":"FAQ :: Parallel.ForEach and non-generic collections?"},"content":{"rendered":"

.NET 2.0 introduced Generics to allow enhanced code reusability and type safety.  Since then, generic collections (IEnumerable<T>, List<T>, Dictionary<T>, etc.) have become standard and are recommended over their non-generic counterparts (IEnumerable, ArrayList, HashTable, etc.).  As a result, Parallel.ForEach only supports generic collections, so code like the following will fail to compile.<\/p>\n

<\/font><\/font><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

IEnumerable<\/span> nonGenericCollection = …;<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

Parallel<\/span>.ForEach(nonGenericCollection, currentElement =><\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

});<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

This issue applies to all non-generic collections (pretty much anything that was added before .NET 2.0), but here are some usual suspects that we’ve seen folks run into: XmlNodeList, DataRowCollection, DataTableCollection.  The error message is typically something like the following:<\/p>\n

<\/font><\/font><\/p>\n

·<\/font>         <\/span><\/span><\/span>“The type arguments for method ‘System.Threading.Tasks.Parallel.ForEach<TSource>) cannot be inferred from the usage. Try specifying the type arguments explicitly.”<\/p>\n

<\/font><\/font><\/p>\n

·<\/font>         <\/span><\/span><\/span>“The best overloaded method match for ‘System.Threading.Tasks.Parallel.ForEach<object>(System.Collections.Generic.IEnumerable<object>, System.Action<object>)’ has some invalid arguments.”<\/p>\n

<\/font><\/font><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

Fortunately, the workaround is simple.  Since non-generic collections produce objects (IEnumerator.Current returns Object), it is always possible to produce an IEnumerable<Object> from an IEnumerable.  For example:<\/p>\n

<\/font><\/font><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

static<\/span> IEnumerable<\/span><object<\/span>> Cast(IEnumerable<\/span> source)<\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

    foreach<\/span> (object<\/span> o in<\/span> source)<\/p>\n

<\/span><\/p>\n

        yield<\/span> return<\/span> o;<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

Even more fortunately, LINQ already provides this functionality:<\/p>\n

<\/font><\/font><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

public<\/span> static<\/span> IEnumerable<\/span><TResult> <\/span><\/p>\n

    <\/span>Cast<TResult>(this<\/span> IEnumerable<\/span> source);<\/p>\n

<\/span><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

So we can easily fix the initial example using this Cast extension method.<\/p>\n

<\/font><\/font><\/p>\n

\n

 <\/font><\/p>\n<\/p>\n

using<\/span><\/b> System.Linq;<\/p>\n

<\/span><\/b><\/p>\n

…<\/p>\n

<\/font><\/font><\/p>\n

Parallel<\/span>.ForEach(nonGenericCollection.Cast<object<\/span>>()<\/b>, <\/span><\/p>\n

    c<\/span>urrentElement =><\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

});<\/p>\n

<\/span><\/p><\/p>\n","protected":false},"excerpt":{"rendered":"

.NET 2.0 introduced Generics to allow enhanced code reusability and type safety.  Since then, generic collections (IEnumerable<T>, List<T>, Dictionary<T>, etc.) have become standard and are recommended over their non-generic counterparts (IEnumerable, ArrayList, HashTable, etc.).  As a result, Parallel.ForEach only supports generic collections, so code like the following will fail to compile.   IEnumerable nonGenericCollection = […]<\/p>\n","protected":false},"author":485,"featured_media":58792,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[7908],"tags":[7907,7923,7909,7912],"class_list":["post-55937","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pfxteam","tag-net-4","tag-faq","tag-parallel-extensions","tag-task-parallel-library"],"acf":[],"blog_post_summary":"

.NET 2.0 introduced Generics to allow enhanced code reusability and type safety.  Since then, generic collections (IEnumerable<T>, List<T>, Dictionary<T>, etc.) have become standard and are recommended over their non-generic counterparts (IEnumerable, ArrayList, HashTable, etc.).  As a result, Parallel.ForEach only supports generic collections, so code like the following will fail to compile.   IEnumerable nonGenericCollection = […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts\/55937","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/users\/485"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/comments?post=55937"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/posts\/55937\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/media\/58792"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/media?parent=55937"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/categories?post=55937"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/dotnet\/wp-json\/wp\/v2\/tags?post=55937"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}