What is COMPLUS_ZapDisable and why is it not set by default?

Question

I know that setting COMPLUS_ZapDisable=1 will "disable debugging optimizations in Visual Studio," but I can't seem to find any references that tell me what that actually means.

• What features/functionality are actually disabled by this setting?
• What are the ramifications of always using this setting?
• If the cost is that "[debugging] runs a bit slower," can we try to elaborate a bit? Is the difference significant? Does that difference scale with project size? Is it unbearable on larger projects? Etc.
  • I know this one is a bit more vague, but explanation is really what I'm after
• Is there an actual known reason why this isn't set by default?
  • While opinions are welcomed, I'm really wondering about if there is a factual, known issue that disabling this setting by default addresses

Answer 1

Setting the environment variable COMPlus_ZapDisable=1 disables the use of all NGEN images (*.ni.dll). It can also be set in the registry, but that is not recommended as it would affect all .NET applications. You would normally only use this when debugging the application to get better call stacks. For better performance you can disable the use of NGEN images for only some assemblies with the environment variable COMPlus_DisableNativeImageLoadList (64bit only and needs .NET 4.6+).

For an extensive description see: https://github.com/Microsoft/dotnet/blob/master/Documentation/testing-with-ryujit.md

(In the current CoreCLR it seems DisableNativeImageLoadList has been removed.)