Making an image easier to debug in .NET

When compiling unmanaged code, you can configure an executable image for debugging by setting IDE switches or command-line options. For example, you can use the /Zi command-line option in Visual C++ to ask it to emit debug symbol files (file extension .pdb). Similarly, the /Od command-line option tells the compiler to disable optimization. The resulting code runs more slowly, but it's easier to debug, should this be necessary.

When compiling .NET Framework managed code, compilers such as Visual C++, Visual Basic, and C# compile their source program into common intermediate language (CIL). CIL is then JIT-compiled, just before execution, into native machine code. As with unmanaged code, you can configure an executable image for debugging by setting IDE switches or command-line options. You can also configure the JIT compilation for debugging in much the same way.

This JIT configuration has two aspects:

  • You can request the JIT compiler to generate tracking information. This makes it possible for the debugger to match up a chain of CIL with its machine code counterpart, and to track where local variables and function arguments are stored. In .NET Framework version 2.0 and later, the JIT compiler always generates tracking information, so there's no need to request it.

  • You can request the JIT compiler to not optimize the resulting machine code.

Normally, the compiler that generates the CIL sets these JIT compiler options appropriately, based upon the IDE switches or command-line options you specify, for example, /Od.

In some cases, you might want to change the behavior of the JIT compiler so that the machine code it generates is easier to debug. For example, you might want to generate JIT tracking information for a retail build or control optimization. You can do so with an initialization (.ini) file.

For example, if the assembly you want to debug is called MyApp.exe, then you can create a text file named MyApp.ini, in the same folder as MyApp.exe, which contains these three lines:

[.NET Framework Debugging Control]

You can set the value of each option to 0 or 1, and any absent option defaults to 0. Setting GenerateTrackingInfo to 1 and AllowOptimize to 0 provides the easiest debugging.

Starting with .NET Framework 2.0, the JIT compiler always generates tracking information regardless of the value for GenerateTrackingInfo; however, the AllowOptimize value still has an effect. When using the Ngen.exe (Native Image Generator) to precompile the native image without optimization, the .ini file must be present in the target folder with AllowOptimize=0 when Ngen.exe executes. If you've precompiled an assembly without optimization, you must remove the precompiled code using NGen.exe /uninstall option before rerunning Ngen.exe to precompile the code as optimized. If the .ini file isn't present in the folder, by default Ngen.exe precompiles the code as optimized.

The System.Diagnostics.DebuggableAttribute controls the settings for an assembly. DebuggableAttribute includes two fields that control whether the JIT compiler should optimize and/or generate tracking information. In .NET Framework 2.0 and later versions, the JIT compiler always generates tracking information.

For a retail build, compilers don't set any DebuggableAttribute. By default, the JIT compiler generates the highest performance, hardest to debug machine code. Enabling JIT tracking lowers performance a little, and disabling optimization lowers performance a lot.

The DebuggableAttribute applies to a whole assembly at a time, not to individual modules within the assembly. Development tools must therefore attach custom attributes to the assembly metadata token, if an assembly has already been created, or to the class called System.Runtime.CompilerServices.AssemblyAttributesGoHere. The ALink tool then promotes these DebuggableAttribute attributes from each module to the assembly they become a part of. If there's a conflict, the ALink operation fails.


In version 1.0 of the .NET Framework, the Microsoft Visual C++ compiler adds the DebuggableAttribute when the /clr and /Zi compiler options are specified. In version 1.1 of the .NET Framework, you must either add the DebuggableAttribute manually in your code or use the /ASSEMBLYDEBUG linker option.

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