Implicit conversion from dynamic to ref struct is accepted by compiler but throws TypeLoadException

[KalleOlaviNiemitalo]

Inspired by dotnet/csharpstandard#282. The C# compiler allows an implicit conversion from dynamic to a reflike type such as Span<byte>, but the conversion throws TypeLoadException at run time. Should the C# language specification disallow this conversion, or should the C# compiler and the .NET Runtime find some way to make it work?

using System;

namespace ConversionDemo
{
    public class Class1
    {
	public static void Main()
	{
	    dynamic dyn = new byte[20];
	    Span<byte> span2 = dyn;
	}
    }
}

Build with .NET SDK 5.0.202 targeting .NET 5.0, and run:

$ dotnet run
Unhandled exception. System.TypeLoadException: The generic type 'System.Func`3' was used with an invalid instantiation in assembly 'System.Private.CoreLib, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'.
   at ConversionDemo.Class1.Main()

IL disassembly:

.method public hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       77 (0x4d)
  .maxstack  3
  .locals init (object V_0,
           valuetype [System.Runtime]System.Span`1<uint8> V_1)
  IL_0000:  nop
  IL_0001:  ldc.i4.s   20
  IL_0003:  newarr     [System.Runtime]System.Byte
  IL_0008:  stloc.0
  IL_0009:  ldsfld     class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>> ConversionDemo.Class1/'<>o__0'::'<>p__0'
  IL_000e:  brfalse.s  IL_0012
  IL_0010:  br.s       IL_0036
  IL_0012:  ldc.i4.0
  IL_0013:  ldtoken    valuetype [System.Runtime]System.Span`1<uint8>
  IL_0018:  call       class [System.Runtime]System.Type [System.Runtime]System.Type::GetTypeFromHandle(valuetype [System.Runtime]System.RuntimeTypeHandle)
  IL_001d:  ldtoken    ConversionDemo.Class1
  IL_0022:  call       class [System.Runtime]System.Type [System.Runtime]System.Type::GetTypeFromHandle(valuetype [System.Runtime]System.RuntimeTypeHandle)
  IL_0027:  call       class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSiteBinder [Microsoft.CSharp]Microsoft.CSharp.RuntimeBinder.Binder::Convert(valuetype [Microsoft.CSharp]Microsoft.CSharp.RuntimeBinder.CSharpBinderFlags,
                                                                                                                                                                      class [System.Runtime]System.Type,
                                                                                                                                                                      class [System.Runtime]System.Type)
  IL_002c:  call       class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<!0> class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>>::Create(class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSiteBinder)
  IL_0031:  stsfld     class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>> ConversionDemo.Class1/'<>o__0'::'<>p__0'
  IL_0036:  ldsfld     class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>> ConversionDemo.Class1/'<>o__0'::'<>p__0'
  IL_003b:  ldfld      !0 class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>>::Target
  IL_0040:  ldsfld     class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite`1<class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>> ConversionDemo.Class1/'<>o__0'::'<>p__0'
  IL_0045:  ldloc.0
  IL_0046:  callvirt   instance !2 class [System.Runtime]System.Func`3<class [System.Linq.Expressions]System.Runtime.CompilerServices.CallSite,object,valuetype [System.Runtime]System.Span`1<uint8>>::Invoke(!0,
                                                                                                                                                                                                              !1)
  IL_004b:  stloc.1
  IL_004c:  ret
} // end of method Class1::Main

[333fred]

It certainly shouldn't compile. Arguably we should block the conversion in the compiler, but that's a roslyn bug, not a csharplang one.

[KalleOlaviNiemitalo]

It certainly shouldn't compile.

Then, I hope the C# standard committee will include some wording to disallow it before they finish the C# 7.2 work. AFAICT, the standard does not say that the target type of the conversion becomes a type argument for System.Func or any other generic type; it's all implementation-defined.

[333fred]

Yes, but that work is generally tracked by the bug you mentioned.

[KalleOlaviNiemitalo]

I don't see how this aspect of C# 7.2 could be tracked by dotnet/csharpstandard#282, which concerns conversions from dynamic to pointer types and thus applies to C# 4.0 already.

[333fred]

The bug tracks things that can't be converted to dynamic. At least, that's how I read it. Feel free to open a different bug on csharpstandard or comment on the issue if you want to call out ref structs explicitly.

[KalleOlaviNiemitalo]

I commented in dotnet/csharpstandard#333 (comment) now that they're starting to add standardese for ref struct types.