{"id":4523,"date":"2008-03-24T15:47:00","date_gmt":"2008-03-24T15:47:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vbteam\/2008\/03\/24\/stathread-vs-mtathread-whorst\/"},"modified":"2024-07-05T14:26:28","modified_gmt":"2024-07-05T21:26:28","slug":"stathread-vs-mtathread-whorst","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/vbteam\/stathread-vs-mtathread-whorst\/","title":{"rendered":"STAThread vs. MTAThread (WHorst)"},"content":{"rendered":"

I recently ran into an interesting threading problem that I wanted to share, so that perhaps I can save some people from the same confusion I had.<\/font><\/p>\n

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I started with a C# application someone else had written.  <\/span>I’ll call it “DeltaEngine” for the purpose of this post.  <\/span>DeltaEngine calls into a native assembly and handles certain events from it.<\/font><\/p>\n

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I created a solution where I referenced DeltaEngine as a library project, and called into it from a VB project.  <\/span>I’ll call this project “VBApp”.  <\/span>So, the reference structure looks like this:<\/font><\/p>\n

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VBApp (VB) -> DeltaEngine (C#) -> NativeCode<\/font><\/p>\n

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When I ran my solution with VBApp as the startup project, it would wait indefinitely for an event to be caught in DeltaEngine from the Native Code.  <\/span>However, I noticed if I made the exact same calls with DeltaEngine as the startup project, it handled the event as expected.  <\/span>I spent a lot of time trying to figure this out and was completely stumped.<\/font><\/p>\n

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Eventually I talked to someone who pointed out that VB projects use Single Threaded Apartment (STA) by default, while C# projects use Multithreaded Apartment (MTA) by default.  <\/span>Because DeltaEngine was originally developed with a C# project as the startup application, it assumed MTA threading.  <\/span>Since I started calling the code from a VB startup project, I was running it with STA threading, without realizing it.  <\/span>Because of this, the DeltaEngine code started waiting for the event after it had already been raised, so it waited indefinitely.  <\/span>If I set DeltaEngine as the startup project and made the same calls, it would use MTA threading and work as expected.<\/font><\/p>\n

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The fix was to add the MTAThreadAttribute to my Sub Main in VBApp.  <\/span>After this, it started working just fine.<\/p>\n

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<MTAThread> Sub Main()<\/p>\n

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Similarly, if you wanted to use the STAThread attribute on a C# Main method, it would look like the code below.  <\/span><\/p>\n

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[STAThread]<\/p>\n

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static void Main()<\/p>\n

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Windows Forms require STA threading, so if you were to create a C# Winapp, you’d see code like this in Program.cs.<\/p>\n

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I don’t want to duplicate the existing documentation about STAThread and MTAThread, so I’ve included some links below that you can use to learn more.<\/font><\/p>\n

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STAThreadAttribute<\/p>\n

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http:\/\/msdn2.microsoft.com\/en-us\/library\/system.stathreadattribute(VS.71).aspx<\/font><\/a><\/p>\n

http:\/\/blogs.msdn.com\/jfoscoding\/archive\/2005\/04\/07\/406341.aspx<\/font><\/a><\/p>\n

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MTAThreadAttribute<\/p>\n

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http:\/\/msdn2.microsoft.com\/en-us\/library\/system.mtathreadattribute(VS.71).aspx<\/font><\/a><\/p>\n

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– Bill Horst, VB IDE Test<\/font><\/p><\/p>\n","protected":false},"excerpt":{"rendered":"

I recently ran into an interesting threading problem that I wanted to share, so that perhaps I can save some people from the same confusion I had.   I started with a C# application someone else had written.  I’ll call it “DeltaEngine” for the purpose of this post.  DeltaEngine calls into a native assembly and […]<\/p>\n","protected":false},"author":260,"featured_media":8818,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[192,195],"tags":[41],"class_list":["post-4523","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-featured","category-visual-basic","tag-bill-horst"],"acf":[],"blog_post_summary":"

I recently ran into an interesting threading problem that I wanted to share, so that perhaps I can save some people from the same confusion I had.   I started with a C# application someone else had written.  I’ll call it “DeltaEngine” for the purpose of this post.  DeltaEngine calls into a native assembly and […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/4523","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/users\/260"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/comments?post=4523"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/posts\/4523\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media\/8818"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/media?parent=4523"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/categories?post=4523"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/vbteam\/wp-json\/wp\/v2\/tags?post=4523"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}