Create the correct default values of Q# types

src/Simulation/Core/Default.cs

@@ -0,0 +1,92 @@
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+
+namespace Microsoft.Quantum.Simulation.Core
+{
+    /// <summary>
+    /// Creates default values of Q# types.
+    /// </summary>
+    public static class Default
+    {
+        /// <summary>
+        /// A dictionary from basic types to their default values.
+        /// </summary>
+        private static readonly IReadOnlyDictionary<Type, object> BasicValues = new Dictionary<Type, object>
+        {
+            [typeof(QRange)] = QRange.Empty,
+            [typeof(QVoid)] = QVoid.Instance,
+            [typeof(Result)] = Result.Zero,
+            [typeof(string)] = ""
+        };
+
+        /// <summary>
+        /// A list of all generic tuple types.
+        /// </summary>
+        private static readonly IReadOnlyList<Type> Tuples = new List<Type>
+        {
+            typeof(ValueTuple<>),
+            typeof(ValueTuple<,>),
+            typeof(ValueTuple<,,>),
+            typeof(ValueTuple<,,,>),
+            typeof(ValueTuple<,,,,>),
+            typeof(ValueTuple<,,,,,>),
+            typeof(ValueTuple<,,,,,,>),
+            typeof(ValueTuple<,,,,,,,>)
+        };
+
+        /// <summary>
+        /// Returns the default value of the Q# type. May return null when null is the default value of the type, or if
+        /// the type is not a valid Q# type.
+        /// </summary>
+        [return: MaybeNull]
+        public static T OfType<T>() => OfType(typeof(T)) is T value ? value : default;
+
+        /// <summary>
+        /// Returns the default value of the Q# type. May return null when null is the default value of the type, or if
+        /// the type is not a valid Q# type.
+        /// </summary>
+        private static object? OfType(Type type) => OfAnyType(type).FirstOrDefault(value => !(value is null));
+
+        /// <summary>
+        /// Enumerates the default values of different kinds of types. Yields null if the given type is not the right
+        /// kind, and yields a non-null value if a default value is found.
+        /// </summary>
+        private static IEnumerable<object?> OfAnyType(Type type)
+        {
+            yield return BasicValues.GetValueOrDefault(type);
+            yield return OfArrayType(type);
+            yield return OfTupleType(type);
+            yield return OfUserDefinedType(type);
+        }
+
+        /// <summary>
+        /// If the given type is a Q# array type, returns the default array of that type, or null otherwise.
+        /// </summary>
+        private static object? OfArrayType(Type type) =>
+            type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IQArray<>)
+                ? Activator.CreateInstance(typeof(QArray<>).MakeGenericType(type.GenericTypeArguments))
+                : null;
+
+        /// <summary>
+        /// If the given type is a Q# tuple type, returns the default tuple of that type, or null otherwise.
+        /// </summary>
+        private static object? OfTupleType(Type type) =>
+            type.IsGenericType && Tuples.Contains(type.GetGenericTypeDefinition())
+                ? Activator.CreateInstance(type, type.GenericTypeArguments.Select(OfType).ToArray())
+                : null;
+
+        /// <summary>
+        /// If the given type is a Q# user-defined type, returns the default value of that type, or null otherwise.
+        /// </summary>
+        private static object? OfUserDefinedType(Type type) =>
+            !(type.BaseType is null)
+            && type.BaseType.IsGenericType
+            && type.BaseType.GetGenericTypeDefinition() == typeof(UDTBase<>)
+                ? Activator.CreateInstance(type, type.BaseType.GenericTypeArguments.Select(OfType).ToArray())
+                : null;
+    }
+}

src/Simulation/Core/QArray.cs

@@ -6,7 +6,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Reflection;
 using Newtonsoft.Json;
 
 namespace Microsoft.Quantum.Simulation.Core
@@ -98,17 +97,10 @@ public QArrayInner(params T[] collection)
             }
 
             /// <summary>
-            /// Creates an array of size given by capacity and default-initializes 
-            /// array elements. Uses C# keyword <code>default</code> to initialize array elements. 
+            /// Creates an array of size given by capacity and default-initializes array elements. Uses the default Q#
+            /// value to initialize array elements.
             /// </summary>
-            public QArrayInner(long capacity)
-            {
-                storage = new List<T>((int)capacity);
-                for (var i = 0L; i < capacity; ++i)
-                {
-                    storage.Add(CreateDefault());
-                }
-            }
+            public QArrayInner(long capacity) => Extend(capacity);
 
             public T GetElement(long index) =>
                 storage == null
@@ -143,20 +135,19 @@ public void UnsafeSetElement(long index, T value)
 
             public void Extend(long newLength)
             {
-                if (storage == null)
+                var newLengthInt = Convert.ToInt32(newLength);
+                if (storage is null)
                 {
-                    storage = new List<T>();
+                    storage = new List<T>(newLengthInt);
                 }
-                long oldLength = storage.Count;
-                for (int i = 0; i < (newLength - oldLength); i++)
+                else if (storage.Capacity < newLengthInt)
                 {
-                    T obj = CreateDefault();
-                    storage.Add(obj);
+                    storage.Capacity = newLengthInt;
                 }
+                storage.AddRange(Enumerable.Repeat(Default.OfType<T>(), newLengthInt - storage.Count));
             }
         }
 
-
         private QArrayInner storage;
         private long start = 0;
         private long step = 1;
@@ -174,25 +165,6 @@ private void CopyAndCompress()
             step = 1;
         }
 
-        // Returns the default value of an object of this type of array. Normally null or 0, but for things like
-        // ValueTuples, it returns an empty instance of that value tuple.
-        private static T CreateDefault()
-        {
-            if (typeof(T).IsValueType || typeof(T).IsAbstract || typeof(T) == typeof(String) || typeof(T) == typeof(QVoid))
-            {
-                return default(T);
-            }
-            else
-            {
-                // First look for an empty constructor
-                ConstructorInfo defaultConstructor = typeof(T).GetConstructor(Type.EmptyTypes);
-                return defaultConstructor != null
-                    ? (T)(defaultConstructor.Invoke(new object[] { }))
-                    : Activator.CreateInstance<T>();
-            }
-        }
-
-
         /// <summary>
         /// Create an array of length 0.
         /// </summary>
@@ -508,7 +480,7 @@ public QArrayEnumerator(QArray<T> qArray)
                 currentIndex = -1;
             }
 
-            public T Current => currentIndex >= 0 ? array[currentIndex] : CreateDefault();
+            public T Current => currentIndex >= 0 ? array[currentIndex] : Default.OfType<T>();
 
             object IEnumerator.Current => this.Current;
 

src/Simulation/Core/QRange.cs

@@ -14,7 +14,7 @@ namespace Microsoft.Quantum.Simulation.Core
     /// </summary>
     public class QRange : IEnumerable<long>
     {
-        public QRange() : this(0, 1, 0)
+        public QRange() : this(1, 1, 0)
         {
         }
 
@@ -61,8 +61,7 @@ public QRange(long start, long end) : this(start, 1, end)
         /// <summary>
         /// Returns an empty range.
         /// </summary>
-        public static QRange Empty =>
-            new QRange(0L, -1L);
+        public static QRange Empty => new QRange();
 
         /// <summary>
         /// Returns true if the range is empty.

src/Simulation/CsharpGeneration.Tests/SimulationCodeTests.fs

@@ -2924,7 +2924,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class U : UDTBase<IUnitary>, IApplyData
     {
-        public U() : base(default(IUnitary))
+        public U() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<IUnitary>())
         {
         }
 
@@ -2950,7 +2950,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class AA : UDTBase<A>, IApplyData
     {
-        public AA() : base(default(A))
+        public AA() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<A>())
         {
         }
 
@@ -2976,7 +2976,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class Q : UDTBase<Qubit>, IApplyData
     {
-        public Q() : base(default(Qubit))
+        public Q() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<Qubit>())
         {
         }
 
@@ -3002,7 +3002,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class QQ : UDTBase<Q>, IApplyData
     {
-        public QQ() : base(default(Q))
+        public QQ() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<Q>())
         {
         }
 
@@ -3028,7 +3028,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class Qubits : UDTBase<IQArray<Qubit>>, IApplyData
     {
-        public Qubits() : base(new QArray<Qubit>())
+        public Qubits() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<IQArray<Qubit>>())
         {
         }
 
@@ -3054,7 +3054,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class udt_args1 : UDTBase<(Int64,IQArray<Qubit>)>, IApplyData
     {
-        public udt_args1() : base(default((Int64,IQArray<Qubit>)))
+        public udt_args1() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(Int64,IQArray<Qubit>)>())
         {
         }
 
@@ -3084,7 +3084,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class udt_Real : UDTBase<Double>, IApplyData
     {
-        public udt_Real() : base(default(Double))
+        public udt_Real() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<Double>())
         {
         }
 
@@ -3104,7 +3104,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class udt_Complex : UDTBase<(udt_Real,udt_Real)>, IApplyData
     {
-        public udt_Complex() : base(default((udt_Real,udt_Real)))
+        public udt_Complex() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(udt_Real,udt_Real)>())
         {
         }
 
@@ -3127,7 +3127,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
     public class udt_TwoDimArray : UDTBase<IQArray<IQArray<Result>>>, IApplyData
     {
-        public udt_TwoDimArray() : base(new QArray<IQArray<Result>>())
+        public udt_TwoDimArray() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<IQArray<IQArray<Result>>>())
         {
         }
 
@@ -3149,7 +3149,7 @@ internal partial class EmptyInternalOperation : Operation<QVoid, QVoid>, ICallab
         """
 internal class InternalType : UDTBase<QVoid>, IApplyData
 {
-    public InternalType() : base(default(QVoid))
+    public InternalType() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<QVoid>())
     {
     }
 
@@ -3171,7 +3171,7 @@ internal class InternalType : UDTBase<QVoid>, IApplyData
         """
 public class NamedTuple : UDTBase<((Int64,Double),Int64)>, IApplyData
 {
-    public NamedTuple() : base(default(((Int64,Double),Int64)))
+    public NamedTuple() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<((Int64,Double),Int64)>())
     {
     }
 
@@ -3231,7 +3231,7 @@ namespace Microsoft.Quantum
 {
     public class Pair : UDTBase<(Int64,Int64)>, IApplyData
     {
-        public Pair() : base(default((Int64,Int64)))
+        public Pair() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(Int64,Int64)>())
         {
         }
 
@@ -3251,7 +3251,7 @@ namespace Microsoft.Quantum
 
     public class Unused : UDTBase<(Int64,Int64)>, IApplyData
     {
-        public Unused() : base(default((Int64,Int64)))
+        public Unused() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(Int64,Int64)>())
         {
         }
 
@@ -3337,7 +3337,7 @@ namespace Microsoft.Quantum
 {
     public class Pair : UDTBase<(Int64,Int64)>, IApplyData
     {
-        public Pair() : base(default((Int64,Int64)))
+        public Pair() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(Int64,Int64)>())
         {
         }
 
@@ -3361,7 +3361,7 @@ namespace Microsoft.Quantum
 
     public class NestedPair : UDTBase<(Double,((Boolean,String),Int64))>, IApplyData
     {
-        public NestedPair() : base(default((Double,((Boolean,String),Int64))))
+        public NestedPair() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<(Double,((Boolean,String),Int64))>())
         {
         }
 
@@ -3616,7 +3616,7 @@ namespace Microsoft.Quantum.Core
 {
     public class Attribute : UDTBase<QVoid>, IApplyData
     {
-        public Attribute() : base(default(QVoid))
+        public Attribute() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<QVoid>())
         {
         }
 
@@ -3636,7 +3636,7 @@ namespace Microsoft.Quantum.Diagnostics
 {
     public class Test : UDTBase<String>, IApplyData
     {
-        public Test() : base(default(String))
+        public Test() : base(global::Microsoft.Quantum.Simulation.Core.Default.OfType<String>())
         {
         }
 

src/Simulation/CsharpGeneration/SimulationCode.fs

@@ -1461,15 +1461,12 @@ module SimulationCode =
         let context = globalContext.setUdt udt
         let name = userDefinedName None udt.FullName.Name.Value
         let qsharpType = udt.Type
-        let buildEmtpyConstructor =
-            let baseTupleType =
-                match qsharpType.Resolution with
-                | ArrayType b -> roslynTypeName context b |> sprintf "QArray<%s>"
-                | _ -> (roslynTypeName context qsharpType)
-            let defaultValue = match qsharpType.Resolution with | ArrayType _ -> [ sprintf "new %s()" baseTupleType] | _ -> [ sprintf "default(%s)" baseTupleType ]
-            let args = []
-            ``constructor`` name ``(`` args ``)``
-                ``:`` defaultValue
+        let buildEmptyConstructor =
+            let defaultValue =
+                roslynTypeName context qsharpType
+                |> sprintf "global::Microsoft.Quantum.Simulation.Core.Default.OfType<%s>()"
+            ``constructor`` name ``(`` [] ``)``
+                ``:`` [ defaultValue ]
                 [ ``public`` ]
                 ``{``
                     []
@@ -1540,7 +1537,7 @@ module SimulationCode =
         let baseClass     = ``simpleBase`` baseClassName
         let modifiers     = [ classAccessModifier udt.Modifiers.Access ]
         let interfaces    = [ ``simpleBase`` "IApplyData" ]
-        let constructors  = [ buildEmtpyConstructor; buildBaseTupleConstructor ]
+        let constructors  = [ buildEmptyConstructor; buildBaseTupleConstructor ]
         let qubitsField   = buildQubitsField context qsharpType
         let itemFields    = buildNamedItemFields @ buildItemFields
         let allFields     = itemFields @ qubitsField