Simulators with missing gates throw NullReferenceException

[cgranade]

Describe the bug

If a simulator in the C# runtime does not implement the interface for a given intrinsic operation / gate, then when the corresponding operation is called from Q#, the simulator throws a NullReferenceException.

To Reproduce

The experimental open systems simulator at

qsharp-runtime/src/Simulation/Simulators/OpenSystemsSimulator/OpenSystemsSimulator.cs

Line 17 in f659395

public partial class OpenSystemsSimulator : SimulatorBase, IType1Core, IDisposable

implements the IType1Core interface, but not the IQSharpCore interface, such that the IGate_Z interface is not implemented by Microsoft.Quantum.Experimental.OpenSystemsSimulator. If Z is called from a Q# program, a NullReferenceException is thrown.

Expected behavior

The IType1Core interface should allow for the Z operation to be implemented by the ApplyUncontrolledZ__Body C# method.

Actual behavior

Unhandled exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Quantum.Intrinsic.Z.<get___Body__>b__9_0(Qubit __in__)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable<I,O>.Apply(I args)      
   at Microsoft.Quantum.Synthesis.ApplyDirectly1C.<get___Body__>b__31_0(ValueTuple`2 __in__)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable<I,O>.Apply(I args)      
   at Microsoft.Quantum.Synthesis.Apply1C.<get___Body__>b__15_0(ValueTuple`2 __in__)
   at Microsoft.Quantum.Simulation.Core.OperationPartial`3.<get___Body__>b__22_0(P a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable.Apply(Object args)      
   at Microsoft.Quantum.Canon.ApplyToEach`1.<get___Body__>b__12_0(ValueTuple`2 __in__)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply[GenO](Object args)
   at Microsoft.Quantum.Simulation.Core.GenericCallable.Apply[O](Object args)
   at Microsoft.Quantum.Simulation.Core.GenericCallable.Microsoft.Quantum.Simulation.Core.ICallable.Apply(Object args)
   at Microsoft.Quantum.Characterization.[omitted].<get___Body__>b__30_0(Int64 __in__)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable<I,O>.Apply(I args)      
   at Microsoft.Quantum.Characterization.[omitted].<get___Body__>b__11_0(ValueTuple`2 __in__)  
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable<I,O>.Apply(I args)      
   at Microsoft.Quantum.Experimental.[omitted].<get___Body__>b__23_0(ValueTuple`2 __in__) in [omitted]
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Microsoft.Quantum.Simulation.Core.ICallable<I,O>.Apply(I args)      
   at Microsoft.Quantum.Experimental.[omitted].<get___Body__>b__12_0(QVoid __in__) in [omitted]
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply(I a)
   at Microsoft.Quantum.Simulation.Core.Operation`2.Apply[GenO](Object args)
   at Microsoft.Quantum.Simulation.Common.SimulatorBase.Execute[T,I,O](I args)
   at Microsoft.Quantum.Simulation.Common.SimulatorBase.<>c__DisplayClass64_0`3.<Run>b__0()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.<>c.<.cctor>b__274_0(Object obj)
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.Quantum.EntryPointDriver.Simulation`3.RunSimulator(IEntryPoint`2 entryPoint, ParseResult parseResult, Func`1 createSimulator)
   at Microsoft.Quantum.EntryPointDriver.Simulation`3.Simulate(DriverSettings settings, IEntryPoint`2 entryPoint, ParseResult parseResult, String simulator)
   at Microsoft.Quantum.EntryPointDriver.Driver`3.Simulate(ParseResult parseResult, String simulator)
   at System.CommandLine.Invocation.CommandHandler.GetResultCodeAsync(Object value, InvocationContext context)
   at System.CommandLine.Invocation.ModelBindingCommandHandler.InvokeAsync(InvocationContext context)
   at System.CommandLine.Invocation.InvocationPipeline.<>c__DisplayClass4_0.<<BuildInvocationChain>b__0>d.MoveNext()    
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<<UseParseErrorReporting>b__20_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass15_0.<<UseHelp>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass24_0.<<UseVersionOption>b__0>d.MoveNext()--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass22_0.<<UseTypoCorrections>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<<UseSuggestDirective>b__21_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<<UseParseDirective>b__19_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<<UseDebugDirective>b__11_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c.<<RegisterWithDotnetSuggest>b__10_0>d.MoveNext()      
--- End of stack trace from previous location where exception was thrown ---
   at System.CommandLine.Builder.CommandLineBuilderExtensions.<>c__DisplayClass13_0.<<UseExceptionHandler>b__0>d.MoveNext()

Additional context

Looking at Z.g.cs, the issue seems to be the use of as to cast the operation factory to an IGate_Z instance:

// Z.g.cs
public Z(IOperationFactory m) : base(m)
{
    this.Gate = m as IGate_Z; // this cast fails, but doesn't throw immediately
}
private protected IGate_Z Gate
{
    get;
}
public override Func<Qubit, QVoid> __Body__ => (__in__) =>
{
    var qubit = __in__;
    Gate.Z__Body(qubit); // 💣
    return QVoid.Instance;
}

As @swernli pointed out in offline discussion, the issue can be mitigated by modifying *.csproj files:

<Project Sdk="Microsoft.Quantum.Sdk/0.15.210223170-alpha">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>netcoreapp3.1</TargetFramework>
        <DefaultSimulator>Microsoft.Quantum.Experimental.OpenSystemsSimulator</DefaultSimulator>
        <IncludeQSharpCorePackages>false</IncludeQSharpCorePackages>
    </PropertyGroup>
    <ItemGroup>
        <PackageReference Include="Microsoft.Quantum.EntryPointDriver" Version="0.15.210223170-alpha" />
        <PackageReference Include="Microsoft.Quantum.Type1.Core" Version="0.15.210223170-alpha" />
    </ItemGroup>
</Project>

[bettinaheim]

@swernli Is the reason that this resulted in a runtime failure for Chris that the project originally included the Q# core package by default? Even so, I am a bit confused why this was a runtime failure?

[swernli]

Yes, that's correct. The new simulator was written against an alternate target package, but without IncludeQSharpCorePackages being set to false the wrong target package is loaded and unsupported intrinsics are invoked. Once we have automatic target package selection, this problem will be further mitigated, but for dev/test scenarios and bring up of new simulators, it would be better to fail earlier instead of later.

[swernli]

The proposed fix here would be updating this line https://github.com/microsoft/qsharp-runtime/blob/main/src/Simulation/CSharpGeneration/SimulationCode.fs#L940 so that instead of generating lines like this:

this.Gate = m as IGate_Exp;

it generates lines like this:

this.Gate = (IGate_Exp)m;

So that the cast fails immediately instead of producing a null that then fails later.