Why is the binary output not equal when compiling again?

by

ANOTHER UPDATE:

Since 2015 the compiler team has been making an effort to get sources of non-determinism out of the compiler toolchain, so that identical inputs really do produce identical outputs. See the “Concept-determinism” tag on the Roslyn github for more details.

UPDATE: This question was the subject of my blog in May 2012. Thanks for the great question!

How is this possible?

Very easily.

Isn’t the binary result supposed to be exactly equal for the same input?

Absolutely not. The opposite is true. Every time you run the compiler you should get a different output. Otherwise how could you know that you’d recompiled?

The C# compiler embeds a freshly generated GUID in an assembly on every compilation, thereby guaranteeing that no two compilations produce exactly the same result.

Moreover — even without the GUID, the compiler makes no guarantees whatsoever that two “identical” compilations will produce the same results.

In particular, the order in which the metadata tables are populated is highly dependent on details of the file system; the C# compiler starts generating metadata in the order in which the files are given to it, and that can be subtly changed by a variety of factors.

due to the way our build server works the checked in changes trigger a rebuild, causing the once again modified binary files to be checked in in a circle.

I’d fix that if I were you.

More Related Contents:

  1. Why does my .NET 4 application know .NET 4 is not installed
  2. What are the true benefits of ExpandoObject?
  3. No generic implementation of OrderedDictionary?
  4. Row_number over (Partition by xxx) in Linq?
  5. When should TaskCompletionSource be used?
  6. Does using Tasks (TPL) library make an application multithreaded?
  7. Breaking changes in .NET 4.0
  8. String.Replace() vs. StringBuilder.Replace()
  9. Is the C# compiler smart enough to optimize this code?
  10. What are C# lambda’s compiled into? A stackframe, an instance of an anonymous type, or?
  11. Read binary file into a struct
  12. Adding scripting functionality to .NET applications
  13. How to suppress a dialog box displayed by code that I can’t change?
  14. lowering priority of Task.Factory.StartNew thread
  15. C# compiler number literals
  16. Why is the C# compiler emitting a callvirt instruction for a GetType() method call?
  17. Can you use optional parameters in code targeting .NET 3.5?
  18. Duck typing in the C# compiler
  19. If Int32 is just an alias for int, how can the Int32 class use an int?
  20. C# Empty Statement
  21. Json.NET adding backslash while returning json serialized string
  22. Why only literal strings saved in the intern pool by default?
  23. How Can I add properties to a class on runtime in C#?
  24. Reverse a string with accent chars?
  25. variable scope in statement blocks
  26. Mono Compiler as a Service (MCS)
  27. GetSystemMetrics() returns different results for .NET 4.5 & .NET 4.0
  28. C# Compiler optimization – Unused methods
  29. How to construct WMI query
  30. Why and how does C# allow accessing private variables outside the class itself when it’s within the same containing class?

Leave a Comment