This same problem is appearing for me after the .NET upgrade was automatically pushed out for release 5.0.7 (SDK 5.0.301, Runtime 5.0.7)

I believe the below previously worked with 5.0.6 release (SDK 5.0.300, Runtime 5.0.6)

Expected behavior

Worktrees.Add executes without exception

Actual behavior

Differs between Release mode and Debug mode.
When running the app in Release mode the exception is

Throws "LibGit2Sharp.LibGit2SharpException: reference is not a branch".

When running in Debug mode the exception is

System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt

This sounds like a few existing issues logged recently so one of my suspicions was something to do with how calls are being marshalled to unmanaged code in the new runtime with dlls from an older sdk, but pulling down the LibGit2Sharp repo and rebuilding results in the same error.

Version of LibGit2Sharp (release number or SHA1)
0.27.0-preview-0096
0.27.0-preview-0100

Reproduction Steps

Same principal as above, but have included a empty repo to pull down to make reproducing a little bit easier. Included as LinqPad script

<Query Kind="Program">
  <NuGetReference Version="0.27.0-preview-0100" Prerelease="true">LibGit2Sharp</NuGetReference>
  <Namespace>LibGit2Sharp</Namespace>
  <RuntimeVersion>5.0</RuntimeVersion>
</Query>

void Main()
{
	var basePath = @"C:\Temp\Something\"+ Guid.NewGuid().ToString().Substring(0,5);
	Directory.CreateDirectory(basePath);
	//var rootWorktree = Path.Combine(basePath, "master");
	var worktreeName = "random-tree";	
	Repository.Clone("https://github.com/zentron/libg2sharp-bugtest", basePath, new CloneOptions { IsBare = false, RecurseSubmodules = false, BranchName = "main" });


	using (var repo = new Repository(basePath))
	{	
		var workingDirectory = new DirectoryInfo(Path.Combine(basePath, worktreeName));	
		var remoteBranchName = $"origin/second-branch";
		repo.Worktrees.Add(remoteBranchName, worktreeName, workingDirectory.FullName, false);
		Console.Write($"Branch in {workingDirectory.FullName}");
	}
}

Interestingly when I run the above code in LinqPad and toggle between .NET 5 (3.0.7) and .NET 3.1 (3.1.6) it works in the older runtime. If I run the repo.Worktrees.Add in a try catch loop, it also tends to work on the second try.

Located fix and added a PR 👇 @bording Let me know if there is anything else I can help with to get this merged and published.

@zentron Thanks for the PR. I suspect there might more of these lurking around now that .NET seems to be caring about them.

0.27.0-preview-0102 includes the fix.