It can be surprising to discover that building a Release version of a .NET application in Visual Studio includes .pdb files in the output folder alongside the binaries. However, this is expected behavior.

The Release configuration utilizes the compiler with specific flags: /optimize+ and /debug:pdbonly.

The /optimize+ flag prompts the compiler to generate optimized MSIL code. While the C# compiler implements some optimizations during MSIL generation, it’s important to note that the JIT compiler performs most optimizations when converting MSIL to native code. The primary impact of unoptimized MSIL seems to be the inclusion of NOPs to facilitate debugging. It remains unclear how unoptimized MSIL affects JIT compilation. While the JIT generally aims for optimization, exceptions occur when methods are marked with MethodImplAttribute set to NoOptimization or during debugging with specific options (Suppress JIT optimization on module load). The influence of the /optimize flag on JIT behavior, particularly its potential to embed metadata instructing JIT optimization, is also uncertain. Interestingly, JIT behavior can also be manipulated through an .ini file (this article).

The /debug flag instructs the compiler to generate .pdb files and specifies the level of detail for debugging information (full or pdbonly). A comprehensive analysis (This excellent post) sheds light on this process. Notably, it highlights the [DebuggableAttribute](http://msdn.microsoft.com/en-us/library/system.diagnostics.debuggableattribute.aspx) attribute, which influences the extent of JIT optimization. The inclusion of MSIL NOPs appears to be more closely tied to the /debug flag than /optimize.

PDBs are crucial for debugging. A detailed explanation of PDBs and their significance can be found here: This article. It is highly recommended to generate PDBs for release builds and store them securely alongside the source code for potential debugging needs.

Beyond debugging, PDBs serve additional purposes:

• The .NET runtime utilizes PDB information to generate detailed stack traces, including filenames and line numbers, by constructing StackFrame objects. While stack traces are generated regardless of build configuration, their informativeness is enhanced by the debug symbols present in Debug builds.

• Decompilers like ILSpy rely on PDBs to retrieve local variable names, which are absent from Assembly Metadata. Assembly Metadata contains method names and parameter names but omits local variable names. Consequently, ILSpy consults the associated PDBs to display meaningful variable names when decompiling an assembly into C# code. This difference is evident in the following decompiled outputs from ILSpy:

  • Debug build with PDBs:
  • Release build with PDBs:
  • Build without PDBs:

Interestingly, Java adopts a different approach, embedding debugging information directly within .class files rather than using separate files (here).