{"id":4674,"date":"2017-03-20T22:15:02","date_gmt":"2017-03-20T22:15:02","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=4674"},"modified":"2017-03-20T22:15:02","modified_gmt":"2017-03-20T22:15:02","slug":"how-does-yield-return-work-in-net","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/how-does-yield-return-work-in-net\/","title":{"rendered":"How does yield return work in .NET"},"content":{"rendered":"

A while back I had a chat with somebody who seemed very interested in what the IL code for certain C#\/.NET language features might look like – whilst it’s something I did have an interest in during the early days of .NET, it’s not something I had looked into since then, so it made me interested to take a look again. <\/p>\n

One specific language feature of interest was “what would the IL for yield return<\/em> look like”. <\/p>\n

This is what I found…<\/p>\n

Here’s a stupidly simple piece of C# code that we’re going to use. We’ll generate the binary then decompile it and view the IL etc.<\/p>\n

\r\npublic IEnumerable<string> Get()\r\n{\r\n   yield return "A";\r\n}\r\n<\/pre>\n
Using JetBrains dotPeek (ILDASM, Reflector or ILSpy can ofcourse be used to do generate the IL etc.). So the IL created for the above method looks like this<\/p>\n
\r\n.method public hidebysig instance \r\n   class [mscorlib]System.Collections.Generic.IEnumerable`1<string> \r\n      Get() cil managed \r\n{\r\n   .custom instance void  [mscorlib]System.Runtime.CompilerServices.IteratorStateMachineAttribute::.ctor(class [mscorlib]System.Type) \r\n   = (\r\n      01 00 1b 54 65 73 74 59 69 65 6c 64 2e 50 72 6f \/\/ ...TestYield.Pro\r\n      67 72 61 6d 2b 3c 47 65 74 3e 64 5f 5f 31 00 00 \/\/ gram+<Get>d__1..\r\n      )\r\n   \/\/ MetadataClassType(TestYield.Program+<Get>d__1)\r\n   .maxstack 8\r\n\r\n   IL_0000: ldc.i4.s     -2 \/\/ 0xfe\r\n   IL_0002: newobj       instance void TestYield.Program\/'<Get>d__1'::.ctor(int32)\r\n   IL_0007: dup          \r\n   IL_0008: ldarg.0      \/\/ this\r\n   IL_0009: stfld        class TestYield.Program TestYield.Program\/'<Get>d__1'::'<>4__this'\r\n   IL_000e: ret          \r\n} \/\/ end of method Program::Get\r\n<\/pre>\n
If we ignore the IteratorStateMachineAttribute and jump straight to the CIL code label IL_0002 it’s probably quite obvious (even if you do not know anything about IL) that this is creating a new instance of some type, which appears to be an inner class (within the Program class) named <Get>d__1. The preceeding ldc.i4.s instruction simply pushes an Int32 value onto the stack, in this instance that’s the value -2. <\/p>\n
Note: IteratorStateMachineAttribute<\/a> expects a Type argument which is the state machine type that’s generated by the compiler.<\/em><\/p>\n
Now I could display the IL for this new type and we could walk through that, but it’d be much easier viewing some C# equivalent source to get some more readable representation of this. So I got dotPeek to generate the source for this type (from the IL). <\/p>\n
First let’s see what changes are made by the compiler to the Get() method we wrote<\/p>\n
\r\n[IteratorStateMachine(typeof (Program.<Get>d__1))]\r\npublic IEnumerable<string> Get()\r\n{\r\n   Program.<Get>d__1 getD1 = new Program.<Get>d__1(-2);\r\n   getD1.<>__this = this;\r\n   return (IEnumerable<string>) getD1;\r\n}\r\n<\/pre>\n
You can now see the newobj<\/em> in it’s C# form, creating the <Get>d__1 object with a ctor argument of -2 which is assigned as an initial state.<\/p>\n
Now let’s look at this lt;Get>d__1 generated code <\/p>\n
\r\n[CompilerGenerated]\r\nprivate sealed class <Get>d__1 : \r\n    IEnumerable<string>, IEnumerable, \r\n    IEnumerator<string>, IDisposable, \r\n    IEnumerator\r\n{\r\n   private int <>1__state;\r\n   private string <>2__current;\r\n   private int <>l__initialThreadId;\r\n   public Program <>4__this;\r\n\r\n   string IEnumerator<string>.Current\r\n   {\r\n      [DebuggerHidden] get\r\n      {\r\n         return this.<>2__current;\r\n      }\r\n   }\r\n\r\n   object IEnumerator.Current\r\n   {\r\n      [DebuggerHidden] get\r\n      {\r\n         return (object) this.<>2__current;\r\n      }\r\n   }\r\n\r\n   [DebuggerHidden]\r\n   public <Get>d__1(int <>1__state)\r\n   {\r\n      base..ctor();\r\n      this.<>1__state = param0;\r\n      this.<>l__initialThreadId = Environment.CurrentManagedThreadId;\r\n   }\r\n\r\n   [DebuggerHidden]\r\n   void IDisposable.Dispose()\r\n   {\r\n   }\r\n\r\n   bool IEnumerator.MoveNext()\r\n   {\r\n      switch (this.<>1__state)\r\n      {\r\n         case 0:\r\n            this.<>1__state = -1;\r\n            this.<>2__current = "A";\r\n            this.<>1__state = 1;\r\n            return true;\r\n          case 1:\r\n            this.<>1__state = -1;\r\n            return false;\r\n          default:\r\n            return false;\r\n      }\r\n   }\r\n\r\n   [DebuggerHidden]\r\n   void IEnumerator.Reset()\r\n   {\r\n      throw new NotSupportedException();\r\n   }\r\n\r\n   [DebuggerHidden]\r\n   IEnumerator<string> IEnumerable<string>.GetEnumerator()\r\n   {\r\n      Program.<Get>d__1 getD1;\r\n      if (this.<>1__state == -2 && \r\n          this.<>l__initialThreadId == Environment.CurrentManagedThreadId)\r\n      {\r\n          this.<>1__state = 0;\r\n          getD1 = this;\r\n      }\r\n      else\r\n      {\r\n          getD1 = new Program.<Get>d__1(0);\r\n          getD1.<>4__this = this.<>4__this;\r\n      }\r\n      return (IEnumerator<string>) getD1;\r\n   }\r\n\r\n   [DebuggerHidden]\r\n   IEnumerator IEnumerable.GetEnumerator()\r\n   {\r\n      return (IEnumerator) this.System.Collections.Generic.IEnumerable<System.String>.GetEnumerator();\r\n    }\r\n}\r\n<\/pre>\n
As you can see, yield causes the compiler to create enumerable implementation for just that one of code.<\/p>\n","protected":false},"excerpt":{"rendered":"
A while back I had a chat with somebody who seemed very interested in what the IL code for certain C#\/.NET language features might look like – whilst it’s something I did have an interest in during the early days of .NET, it’s not something I had looked into since then, so it made me […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[49,3],"tags":[],"class_list":["post-4674","post","type-post","status-publish","format-standard","hentry","category-net","category-c"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/4674","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/comments?post=4674"}],"version-history":[{"count":11,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/4674\/revisions"}],"predecessor-version":[{"id":4706,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/posts\/4674\/revisions\/4706"}],"wp:attachment":[{"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/media?parent=4674"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/categories?post=4674"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/putridparrot.com\/blog\/wp-json\/wp\/v2\/tags?post=4674"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}