Deserializing a List with null values - C#

[meboz]

Howdy,

Im struggling to work out how to solve this issue. Im not sure if [BsonDefaultValue] applies here or not.

I have a class as follows...

public class Account : MongoBase

{

  public List<Reminders> Reminders {get; set;}

  public Account(){

    Reminders = new List<Reminder>();

  }

}

For an account that exists in the db without a reminders field, the class gets instatiated and then the Reminders property is set to null, since its null in the db.

How would you overwrite/workaround this behaviour to allow the value from its constructor to not be overwritten in the case where the field doesnt exist?

Cheers,

Byron

[Robert Stam]

See:

https://jira.mongodb.org/browse/CSHARP-467 

So in future versions of the driver, values set in the constructor won't be overwritten unless the database actually has a value for that property.

In the meantime, [BsonDefaultValue] doesn't quite do the trick because an argument to an attribute must be a compile-time constant. You could work around it for now by registering the class map in code like this:

    BsonClassMap.RegisterClassMap<Account>(cm =>

    {

        cm.AutoMap();

        cm.GetMemberMap(c => c. Reminders).SetDefaultValue(new List<Reminders>());

    });

Or you could get the latest source for the driver from github because CSHARP-467 has already been implemented.

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[meboz]

Thanks Robert.

Ill grab latest source. Thanks for the quick reply.

Cheers,

Byron

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[meboz]

Hi Robert,

I grabbed the latest source but the same problem existed.

I rolled back to 1.4.2, and tried the SetDefaultValue approach as you mentioned, but received this error which I thought was very odd...

Unable to cast object of type 'NameSpace.Reminder[]' to type 'NameSpace.MongoAccount'.

note: there are some sematic differences to my original post (class names, types etc).

Any ideas?

Cheers,

Byron

[Robert Stam]

I'm surprised to hear about either problem. I did a quick test on my machine before posting my reply and did not get any errors.

If you are willing to post a complete program I can probably help you further. Even a stack trace for the error message you received would be helpful.

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[Robert Stam]

Here's the test program I used:

http://www.pastie.org/3893335

When I run the test with the 1.4.2 C# driver I get this output:

List is: null

Press Enter to continue

which shows the issue you described, that the value you initialized in the constructor is being set back to null during deserialization because the document is missing a List element.

When I run the test with the latest driver from the master branch in github I get this output:

List is: System.Collections.Generic.List`1[[System.Int32, mscorlib, Version=4.0.

0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]

Press Enter to continue

which shows the issue is fixed in the master branch on github because the value set in the constructor is no longer being overwritten.

If I uncomment the code that initializes the class map I get the same output whether I run the test with the 1.4.2 C# driver or the master branch from github.

Let me know if the test program I am using doesn't reflect your use case.

[meboz]

Hi Robert,

I can see where I was making a mistake here.

Rather than my Reminders field being 'unset' in the db, it was infact, null. So the expected behaviour of mongo is now no surprise.

This will help me in the situation im in...

BsonClassMap.RegisterClassMap<Account>(x =>

                                      {

                                      x.AutoMap();

                                      x.GetMemberMap(r => r.Reminders).SetIgnoreIfNull(true);

                                      });

Cheers,

Byron