Allow debugging assemblies that are lazily-loaded

Describe the bug

When debugging an assembly loaded using the AppDomain. The assembly is loaded using the Load(byte[] rawAssembly, byte[] rawSymbolStore) method similar to the one on the Assembly class. The debugger refuses to step through to the code found in an external assembly and all attempts to step into will fail.

To Reproduce

This can be easily reproduced using this publicly available repository: PluginTest. To test the issue run the Server Application. You will be presented with the following page:

The Load 'InnerPlugin' Plugin button provides an in assembly variation of a Plugin which can debugged inside the Index.razor - See the LoadInnerPlugin method.

The Load 'StandalonePlugin' Plugin however exposes the debugging issue, where the assembly is loaded from an array of bytes, with the PDB supplied as well. Attempting to step into the plugin.Initialise(); expression will show the issue - Inside the LoadStandalonePlugin method in the same Index.razor.

Exceptions (if any)

No exceptions are raised inside visual studio or the browser.

Further technical details

• ASP.NET Core version: 5.0.0-preview.6.20312.15
• Dotnet Info:

.NET SDK (reflecting any global.json):
 Version:   5.0.100-preview.6.20318.15
 Commit:    4356580024

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.100-preview.6.20318.15\

Host (useful for support):
  Version: 5.0.0-preview.6.20305.6
  Commit:  4ba9ecaabd

.NET SDKs installed:
  2.2.301 [C:\Program Files\dotnet\sdk]
  3.1.100 [C:\Program Files\dotnet\sdk]
  3.1.201 [C:\Program Files\dotnet\sdk]
  3.1.300 [C:\Program Files\dotnet\sdk]
  3.1.400-preview-015178 [C:\Program Files\dotnet\sdk]
  5.0.100-preview.6.20318.15 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-preview.6.20312.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-preview.6.20305.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0-preview.6.20308.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

• Using Visual studio 16.7.0 Preview 3.0, Enterprise

[captainsafia]

@lewing This is the issue that we can use to brainstorm solutions to the problem of adding debugging support for assemblies that are lazily-loaded after the application (and debugging proxy) has been launched.

[captainsafia]

@mkArtakMSFT I think we should track this for rc1 as a tooling follow-up to the lazy-load work.