How to map a nullable enum to string in Mongo C# client

Question

I've a document with a nullable enum like this:

public enum Gender
{
    Male,
    Female
}

public class Person {
    public Gender? Gender { get; set;}
}

I'm using MongoDB C# Driver and has mapped the enum to be serialize as a string:

BsonClassMap.RegisterClassMap<Person>(map =>
{
    map.AutoMap();
    map.SetIgnoreExtraElements(true);
    map.MapMember(x => x.Gender).SetSerializer(new EnumSerializer<Gender>(BsonType.String);
});

This works fine for non-nullable types, but failed for this nullable enum:

Value type of serializer is Gender and does not match member type System.Nullable`1[[Gender]]. (Parameter 'serializer')

How can I map a nullable enum to a string ?

Or this would help : github.com/mongodb/mongo-csharp-driver/blob/master/src/… — Gabriel Scavassa
– Gabriel Scavassa, Commented Dec 1, 2021 at 14:32
You can fix that by setting a DefaultValue attribute on the property, e.g. [DefaultValue(MyEnumType.Default)] — Sohan
– Sohan, Commented Dec 1, 2021 at 19:29

Yonatan · Accepted Answer · 2021-12-01 19:02:16Z

3

There is a special wrapper just for your case - NullableSerializer<T>. Use it like this:

BsonClassMap.RegisterClassMap<Person>(map =>
{
    map.AutoMap();
    map.SetIgnoreExtraElements(true);
    map.MapMember(x => x.Gender).SetSerializer(new NullableSerializer<Gender>(new EnumSerializer<Gender>(BsonType.String)));
});

answered Dec 1, 2021 at 19:02

Yonatan

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1 Comment

dhrm Over a year ago

Thanks for your reply. What are the pros and cons using this SetSerializer method vs. using ConventionRegistry and the EnumRepresentationConvention posted above? Except that the first is for each map member whereas the other is on a global level.

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