XML cannot be serialized - parameterless constructor error, even with constructor

Question

I'm trying to serialize an object to xml using XmlSerializer but keep getting this error:

System.InvalidOperationException: [MySql.Data.MySqlClient.MySqlParameter] cannot be serialized because it does not have a parameterless constructor.

I followed the solution to why xml serializable class need a parameterless constructor which seems to have solved the problem for everyone else, but I still get an error. When I add public MyClass() it gives me 'MyClass.MyClass()' must declare a body because it is not marked abstract, extern, or partial, so I add a body and the first error comes back.

Note that originally there aren't any constructors, parameterless or otherwise, so I don't understand why it isn't just automatically creating its own.

What am I doing wrong?

Code added:

MyFile.cs

public async Task Save(MyClass xmlObj)
{
    XmlSerializer xmlSer = new XmlSerializer(this.GetType()); //I think the problem is likely to be here
    StringWriter strWriter = new StringWriter();

    xmlSer.Serialize(strWriter, xmlObj);
    objToUpdate.ColName = (strWriter.ToString());
    await dbContext.SaveChangesAsync();
}

File.cs

public async Task<IActionResult> SaveData(params...)
{
    await new MyFile<MySqlParameter>(accessParam, dbContext).Save(xmlObj);
}

The issue is surely that `MySqlParameter` can't be serialised, not `MyClass` - so why are you adding a constructor to `MyClass`? And what version of `MySql` are you using? Because [`MySqlParameter` *does* have a parameterless constructor](https://dev.mysql.com/doc/dev/connector-net/8.0/html/M_MySql_Data_MySqlClient_MySqlParameter__ctor.htm). Can you post a compilable reproduction of your problem? — Matthew Watson, Feb 20 '20 at 08:45
Why would it be trying to serialise `MySqlParameter`? That's only the data being sent to the db when saving, it's in the parent method, not with the `XmlSerializer` — half of a glazier, Feb 20 '20 at 09:13
are you tried to change your constructor to ``private`` or ``internal``? — Mohammed Sajid, Feb 20 '20 at 09:26
`Why would it be trying to serialise` - I don't know why, but the error message clearly states that it *is* being serialised. You will need to post some code. — Matthew Watson, Feb 20 '20 at 09:29

score 1 · Answer 1 · answered Feb 20 '20 at 09:08

1

You should have added

public MyClass(){}

to the class. There is a difference between no body and an empty body which is what you want on this constructor.

answered Feb 20 '20 at 09:08

David Hinchliffe

480
2
6

From my question: *so I add a body and the first error comes back* - I already tried that – half of a glazier Feb 20 '20 at 09:09
as @Matthew asked, please post the code for the class you're trying to serialize, it does look like you trying to serialize a none searilizable object – David Hinchliffe Feb 20 '20 at 09:14

score 1 · Accepted Answer · answered Feb 20 '20 at 09:45

I figured it out...

I had been creating a new XmlSerializer and passing in a parameter as posted in a SO answer:

XmlSerializer xmlSer = new XmlSerializer(this.GetType());

When really it works perfectly fine to use the same XmlSerializer as the deserialization:

XmlSerializer objSer = new XmlSerializer(typeof(MyClass));

XML cannot be serialized - parameterless constructor error, even with constructor

2 Answers2