System.AccessViolationException on methods with generic arguments with type constraints

[ociaw]

When run with NullGuard, the code below throws a System.AccessViolationException. This is caused by invalid IL weaved by Fody, as it does not box the argument (which can be a struct) before checking it for null. The argument is only not boxed when a generic type constraint is present.

class Program
{
    static void Main()
    {
        Class.Generic(new Implementation());
    }
}
public static class Class
{
    public static void Generic<T>(T implementation) where T : IInterface { }
}

public interface IInterface { }

struct Implementation : IInterface { }

Install Fody and NullGuard.Fody, then run dotnet run --framework netcoreapp3.1.

Full solution

Configuration

I've tested this on:
OS: Windows 10 1903 (10.0.18362)
.NET Core 3.1 and .NET Framework 4.7.2
x86 and x64
Fody 6.2.0
NullGuard.Fody 2.2.0 and 3.0.0-beta1

Other information

Exception thrown:

Fatal error. System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
   at Class.Generic[[Implementation, Repro, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]](Implementation)
   at Program.Main()

IL after weaving by Fody (note the lack of boxing before btrue.s):

.method public hidebysig static 
	void Generic<(IInterface) T> (
		!!T implementation
	) cil managed 
{
	// Method begins at RVA 0x2074
	// Code size 20 (0x14)
	.maxstack 2

	IL_0000: ldarg.0
	IL_0001: brtrue.s IL_0013

	IL_0003: ldstr "implementation"
	IL_0008: ldstr "[NullGuard] implementation is null."
	IL_000d: newobj instance void [System.Runtime]System.ArgumentNullException::.ctor(string, string)
	IL_0012: throw

	IL_0013: ret
} // end of method Class::Generic

If the type constraint is removed, the argument is boxed correctly:

.method public hidebysig static 
	void Generic<T> (
		!!T implementation
	) cil managed 
{
	// Method begins at RVA 0x2074
	// Code size 25 (0x19)
	.maxstack 2

	IL_0000: ldarg.0
	IL_0001: box !!T
	IL_0006: brtrue.s IL_0018

	IL_0008: ldstr "implementation"
	IL_000d: ldstr "[NullGuard] implementation is null."
	IL_0012: newobj instance void [System.Runtime]System.ArgumentNullException::.ctor(string, string)
	IL_0017: throw

	IL_0018: ret
} // end of method Class::Generic

[SimonCropp]

@ociaw have you made any progress on a PR that fixes this?

[ociaw]

I've been having trouble writing a test to reproduce the exception, but I was planning on doing the same as #343.

[tom-englert]

The fix is now deployed.
Thanks to @ltrzesniewski for the quick response.