In CodeGenerator, Recommendation scenario Training cause null reference Exception

[LittleLittleCloud]

Just dig in a bit. The null reference exception is because, in the AutoML generated pipeline, one of the fields in the trainer node's property dictionary is null, which crashes CodeGenerator.

[LittleLittleCloud]

The root cause for that is in CodeGenerator, it only handle cases when trainer option's value type is built-in type. see the following code link.

machinelearning/src/Microsoft.ML.CodeGenerator/CodeGenerator/CSharp/TrainerGeneratorBase.cs

Line 56 in f1f8942

foreach (var kv in node.Properties)

While in this case, the null-value field's type is a MatrixFactorizationTrainer.LossFunctionType. So it's been ignored and assigned a null value.

A quick fix can be fairly simple: just pass the string value of MatrixFactorizationTrainer.LossFunctionType in sweepable parameters.
use reflection to get the right type in code gen.