C# - Failed to serialize a class with nullable generic field inherited from base class

[hhuberman]

Having the following schema:

struct TestBase<T>
{
    0: nullable<T> Value;
}
struct Test : TestBase<double> { }

I'm getting the following exception when trying to serialize an instance of a Test object:

 System.InvalidOperationException: The binary operator NotEqual is not defined for the types 'System.Double' and 'System.Object'.
   at System.Linq.Expressions.Expression.GetEqualityComparisonOperator(ExpressionType binaryType, String opName, Expression left, Expression right, Boolean liftToNull)
   at System.Linq.Expressions.Expression.NotEqual(Expression left, Expression right, Boolean liftToNull, MethodInfo method)
   at System.Linq.Expressions.Expression.NotEqual(Expression left, Expression right)
   at Bond.Expressions.ObjectParser.Nullable(ContainerHandler handler)
   at Bond.Expressions.ObjectParser.Container(Nullable`1 expectedType, ContainerHandler handler)
   at Bond.Expressions.SerializerTransform`2.Container(IParser parser, RuntimeSchema schema)
   at Bond.Expressions.SerializerTransform`2.<>c__DisplayClass12_0.<GenerateSerialize>b__0(IParser p)
   at Bond.Expressions.SerializerGenerator`2.GenerateSerialize(Serialize serialize, IParser parser, ParameterExpression writer, Boolean inline)
   at Bond.Expressions.SerializerTransform`2.GenerateSerialize(SerializeWithSchema serializeWithSchema, IParser parser, RuntimeSchema schema)
   at Bond.Expressions.SerializerTransform`2.Value(IParser parser, Expression valueType, RuntimeSchema schema)
   at Bond.Expressions.SerializerTransform`2.<>c__DisplayClass15_1.<Struct>b__6(IParser fieldParser, Expression fieldType)
   at Bond.Expressions.Field.Bond.Expressions.IField.Value(IParser parser, Expression valueType)
   at Bond.Expressions.ObjectParser.Field(ITransform transform, Expression structVar, UInt16 id, ISchemaField schemaField, IField field)
   at Bond.Expressions.ObjectParser.<>c__DisplayClass21_0.<Apply>b__4(<>f__AnonymousType0`2 <>h__TransparentIdentifier0, IField knownField)
   at System.Linq.Enumerable.SelectManyIterator[TSource,TCollection,TResult](IEnumerable`1 source, Func`2 collectionSelector, Func`3 resultSelector)+MoveNext()
   at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
   at System.Collections.Generic.List`1.AddRange(IEnumerable`1 collection)
   at Bond.Expressions.ObjectParser.Apply(ITransform transform)
   at Bond.Expressions.SerializerTransform`2.Struct(IParser parser, RuntimeSchema schema, Boolean isBase)
   at Bond.Expressions.SerializerTransform`2.<>c__DisplayClass15_0.<Struct>b__4(IParser baseParser)
   at Bond.Expressions.Transform.Bond.Expressions.ITransform.Base(IParser parser)
   at Bond.Expressions.ObjectParser.Apply(ITransform transform)
   at Bond.Expressions.SerializerTransform`2.Struct(IParser parser, RuntimeSchema schema, Boolean isBase)
   at Bond.Expressions.SerializerTransform`2.Struct(IParser parser, RuntimeSchema schema)
   at Bond.Expressions.SerializerTransform`2.<>c__DisplayClass12_0.<GenerateSerialize>b__0(IParser p)
   at Bond.Expressions.SerializerGenerator`2.GenerateSerialize(Serialize serialize, IParser parser, ParameterExpression writer, Boolean inline)
   at Bond.Expressions.SerializerTransform`2.GenerateSerialize(SerializeWithSchema serializeWithSchema, IParser parser, RuntimeSchema schema)
   at Bond.Expressions.SerializerTransform`2.Generate(IParser parser)
   at Bond.Serializer`1.SerializerHelper..ctor(ObjectParser parser, Type type, Boolean inlineNested)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at System.Activator.CreateInstance(Type type, Object[] args)
   at Bond.Serializer`1..ctor(Type type, IParser parser, Boolean inlineNested)
   at Bond.Serializer`1..ctor(Type type)

However, it works if I change the schema to:

struct TestBase<T>
{
    0: T Value;
}

struct Test : TestBase<nullable<double>> { }

[chwarr]

Interesting. It's getting confused somewhere by the generic, the nullable, and the nullable inner type.

(Somewhat related: Generation of C# serializer for a = nothing field with a default value fails with an obscure exception)

[katruno]

I faced a similar problem. In my case I'm not using a generic value, instead I've a nullable field of an string alias:

using StrAlias = string;
struct Test
{
    0: nullable<StrAlias> Value;
}

StrAlias is defined as a struct (value type):

public struct StrAlias 
{
    public string Value { get; }
}

Serialization for Test object is failing with similar error:

Error: System.InvalidOperationException: The binary operator NotEqual is not defined for the types 'MyNamespace.StrAlias' and 'System.Object'.
at System.Linq.Expressions.Expression.GetEqualityComparisonOperator(ExpressionType binaryType, String opName, Expression left, Expression right, Boolean liftToNull)
at System.Linq.Expressions.Expression.NotEqual(Expression left, Expression right, Boolean liftToNull, MethodInfo method)
at Bond.Expressions.ObjectParser.Nullable(ContainerHandler handler) in D:\a\1\s\cs\src\core\expressions\ObjectParser.cs:line 376
at Bond.Expressions.DeserializerTransform`1.FieldValue(IParser parser, Expression var, Expression valueType, Type schemaType, Boolean initialize) in D:\a\1\s\cs\src\core\expressions\DeserializerTransform.cs:line 502
at Bond.Expressions.DeserializerTransform`1.<>c__DisplayClass18_2.b__3(IParser fieldParser, Expression fieldType) in D:\a\1\s\cs\src\core\expressions\DeserializerTransform.cs:line 222
at Bond.Expressions.ObjectParser.Field(ITransform transform, Expression structVar, UInt16 id, ISchemaField schemaField, IField field) in D:\a\1\s\cs\src\core\expressions\ObjectParser.cs:line 125
at Bond.Expressions.ObjectParser.<>c__DisplayClass21_0.b__4(<>f__AnonymousType0`2 <>h__TransparentIdentifier0, IField knownField) in D:\a\1\s\cs\src\core\expressions\ObjectParser.cs:line 88

[chwarr]

These very well could be related. I wonder if it's the struct that's causing the expression generation to get confused. Nullable<Double> is itself a struct...

[katruno]

By looking a little bit more at the code: https://github.com/microsoft/bond/blob/master/cs/src/core/expressions/ObjectParser.cs
and the expression that is throwing the exception:
ln 376: var notNull = Expression.NotEqual(nullableValue, Expression.Constant(null));

Could be possible that it is missing the specific type while creating the constant?
By default it used object but you can pass the specific type in an overload:
https://docs.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression.constant?view=netcore-3.1

[chwarr]

Could very well be the case. I don't have time for the foreseeable future to add a regression test and attempt a fix. If you're interested, feel free to play around here and submit a PR if you come up with a fix.