AssemblyBuilder still lacks SetEntryPoint

[masonwheeler]

Description

In the recent work on Reflection.Emit saving in .NET 9 preview, the SetEntryPoint API was removed entirely, with the developer left to implement it manually using PersistableAssemblyBuilder and various System.Reflection.Metadata APIs. This works well enough if you're trying to save to a file, but one important use case is still failing hard: generating executable assemblies in-memory. This has real-world applications for a REPL, or for a compiler test suite.

Reproduction Steps

1. Try to generate an executable assembly in-memory with System.Reflection.Emit
2. You can't do it because there's no SetEntryPoint API available.

Expected behavior

It should be possible to generate an executable assembly in-memory, as it was in .NET Framework.

Actual behavior

It is not possible to generate an executable assembly in-memory.

Regression?

This worked in .NET Framework.

Known Workarounds

It's possible to use PersistableAssemblyBuilder and save the whole thing to a MemoryStream, and then call Assembly.Load on the bytes in the stream, but this feels like a gross hack.

Configuration

.NET Core 9, Preview 3, Windows 10, x64

Other information

No response

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[jkotas]

It's possible to use PersistableAssemblyBuilder and save the whole thing to a MemoryStream, and then call Assembly.Load on the bytes in the stream, but this feels like a gross hack.

I expect that this it is what we are going to do under the covers eventually.

The fundamental feature currently missing in PersistableAssemblyBuilder compared to in-memory RefEmit is ability to append new types to already executing assembly. If you do not need to append new types to already executing assembly, you may want to switch to PersistableAssemblyBuilder + Assembly.Load to get the features that are unavailable with in-memory RefEmit.

[buyaa-n]

It is not possible to generate an executable assembly in-memory.

PersistableAssemblyBuilder is only for saving, in order to execute need to load back the saved stream, closing the issue as it is by design

[masonwheeler]

@buyaa-n This was not meant to be about PersistableAssemblyBuilder, but rather AssemblyBuilder, because building and executing an executable assembly in-memory, without ever saving it, has always been a valid use case.

[buyaa-n]

Based on @jkotas comment:

I expect that this it is what we are going to do under the covers eventually.

I assumed we wouldn't fix in runnable AssemblyBuilder either, feel free to reopen if that was not the case