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On exception - convert object to string?

I've tried to gather as much info as possible, when my VB.Net app crashes.
I have some objects that I need to convert to string, so I can send the objects value to me in an email.
How can I convert an object to string - even if they are not serializable?
In my test, if I try to serialize my test object, I get an error, since it inherits from IDictionary, it can't be serialized. But I can add it to the "Watch" window and still see all its properties. So there must be a way to convert an non serializable object to a string?

I had such question before - Logging instance data
So you can use reflection and go through properties by your own or you can take something, that is already written, for example ServiceStack has Dump method:
http://www.servicestack.net/mythz_blog/?p=202
BTW, not sure what you mean by "not serializable". If object can't be serialized to xml using default .net classes, that doesn't mean it can't be serialized using custom classes or different serialization format, like JSON or binary.

You can do it by using reflection
Reflection (C# Programming Guide)

So, are you basically asking for a way to serialize an object? Why don't you try to serialize it to Json, for example. I'm sure it can deal with IDictionary types. Give it a try: json.codeplex.com

Why do you have to mark a class with the attribute [serializable]?

Seeing as you can convert any document to a byte array and save it to disk, and then rebuild the file to its original form (as long as you have meta data for its filename etc.).
Why do you have to mark a class with [Serializable] etc? Is that just the same idea, "meta data" type information so when you cast the object to its class things are mapped properly?

Binary serialization is pretty powerful, it can create an instance of a class without running the constructor and can set fields in your class that you declared private. Regular code can of course not do this. By applying the [Serializable] attribute, you explicitly give it the go-ahead to mess with your private parts. And you implicitly give that permission to only the BinaryFormatter class.
XML serialization doesn't need this kind of okay, it only serializes members that are public.
DataContractSerializer can serialize private members as well. It therefore needs an explicit okay again, now with the [DataContract] attribute.

First off, you don't have to.
It is simply a marker interface that tells the serializer that the class is composed of items that it can serialize (which may or may not be true) and that is can use the default serialization.
The XMLSerializer has the additional requirement to have a zero parameter constructor to the class.
There are other serializers that use contracts for serialization (such as the DataContractSerializer) - they give you more control over serialization than simply marking a class as Serializable. You can also get more control by implementing the ISerializable interface.

It's basically metadata that indicates that a class can be serialized, nothing more.
It is required by a lot of framework serializers, which refuse to deal with types not having this attribute applied to them.

Serialization can create security holes and may be plagued by versioning problems. On top of that, for some classes, the very idea of serialization is outright nonsense.
For details, see the excellent answers to Why Java needs Serializable interface?, especially this one, this one, and this one. They make the case that serialization should be a feature you have to explicitly opt into.
For a counterpoint, the accepted answer to that question makes the case that classes should be serializable by default.

It indicates to the serializer that you want that class to be serialized as you may not want all properties or classes to be serialized.

I see it as a reminder that I will allow the class to be serialized. So you don't implicitly serialize something you shouldn't.
Don't know it that is designers' intention.
BTW, I just love BinaryFormatter and use it as much as I can. It handles pretty much of the stuff automatically (like rebuilding complex object graphs with recurring references spread throughout the graph).

Serializing an object but none of its references c#

I have a situation where I need to serialize an object but don't want to serialize any of its references. This is because I don't know in advance which dlls the object might be referencing and therefore can't ensure that they are serializable objects. This has arisen from needing to serialise plugins to preserve their state.
Am I right in thinking that this is the case with XML serialization (shallow)? But that this will ignore anything private in the object - which isn't what I want?
Is this somehow possible?

Xml Serialization will only work on things that are publicly accessible. Also, unless you mark a public property / field with the [XmlIgnore] attribute, it will be serialized.
If you're just looking at some method of serialization, then use binary serialization. It will serialize the internal state of the object (all fields, private or otherwise). You can use the [NonSerialized] attribute to ignore specific references if you want.

If you know at the type declaration time which references should not be serialized you can use binary serialization and filter out members with the [NonSerialized] attribute.

Put NonSerialized attribute in case of binary and XmlIgnore attribute in case of xml serialization to reference properties or fields

You do know which properties you can serialize, though, correct? Are these plugins implementing a common interface? If that is the case, you should be able to write a generic serializer that will only serialize the specific properties that you choose.
Here is a basic example that will give you the idea of what you need to do:
Object Serialization using C#
If you are just looking to serialize native types within your class instances, you should just be able to implement ISerializable, though, and decorate the properties that you do not want to be serialized.

You can try something like this:
Type myType = currentObject.GetType();
Then check to see if the object is serializable by using:
myType.IsSerializable; //returns a bool
That should tell you whether or not the object is serializable. If you really need to know whether every single object inside of a class is serializable, such as other nested classes or custom types, then you could probably use reflection to read each object, use the code above, and verify whether or not it is serializable. This, however, might be a more complicated approach, and may not be plausible, especially if you have overhead issues to deal with.

It may be useful for you to separate what you want to persist from how you persist it.
It seems like you want control over how you want to persist data, but obviously, cannot know what it is, because of your plugin model.
One scheme that may make sense to you is to give your plugins some sort of object or interface they can write to and read from when its time to save / load. Its fine to document these constraints.
For example, when persisting, allow your plugins to pass to you:
some arbitrary byte array which they are responsible for serializing / deserializing. Then it is a plugins responsibility to make sure they use objects that are appropriately serializable.
a dictionary of strings
an xml file
others...
Store this information per plugin (in whichever form you want), and loading up again, pass back the same information.
This is simply an approach around the fact that in the end, the plugin knows what it needs to save, and needs to own that piece of information.

Why does XmlSerializer requires the classes of the serialized object declared as public?

It's totally well known that in order to be able to serialize your objects using XmlSerializer you have to declare their classes as public -otherwise you get an InvalidOperationException. The question here is why? I Googled and I found out that XmlSerializer actually generates and compiles a brand new assembly and then uses this assembly to serialize your objects. The question is, still, why does it require the class to be public, while it's easy to get access to internal types in my assembly using reflection?

Quite simply because it doesn't use reflection in order to serialise/deserialise your class - it access the public properties (and classes) directly.
Using refleciton to access members would be extremely expensive so instead, as you mention in your question, it generates a serializer class once using reflection, caches it*, and from this point onwards uses direct member access.
I should qualify this: it only generates a serializer once and caches it for certain constructor overloads on the XmlSerializer. For others, it re-generates the serializer class every time you create an instance of the serializer.
As long as you use the vanilla constructor you are alright:
XmlSerializer ser = new XmlSerializer(typeof(MyType));

The simple reason is because it's been that way since Day 1.
Also, Reflection is expensive. Why do it if you don't have to?
Also, the XML Serializer isn't intended to serialize every class in the world. It's meant to serialize classes designed to be serialized. As such, it's no great burden to make sure the data you want is in public fields and properties of a public class with a public parameterless constructor.
It's only when you try to serialize a type that was not designed to be serialized that you run into trouble.

Having DataContractSerializer serialize the same class in two different ways?

I'm using the DataContractSerializer to serialize an objects properties and fields marked with DataMember attributes to xml.
Now a have another use case for the same class, where I need to serialize other properties and other fields.
Are there a way to add "another DataMemberAttribute" that can be used for my other serialization scenario?

No, basically.
If you want to use the existing DataContractSerializer, you'll have to maintain a second version of the DTO class and convert the data between them.
Options if you are writing your own serialization code:
declare your own [DataMember]-style attribute(s) and interpret them at runtime in your own serialization code
use a "buddy class"
use external metadata (such as a file)
use code-based configuration (i.e. via a DSL)
In reality, I expect the first will be the simplest choice.

In a similar scenario in the past, we've taken an Object Oriented approach, and created a new class that extends from the main class.
To help you achieve inhertience with the DataContractSerializer, check out KnownTypeAttribute
In one of your comments to your question,
If the same class is implementing multiple interfaces, certain data elements may be relevant to only one of the interfaces.
If that is the case in your scenario, then perhaps your Data Service Contracts should be exposing just the Interfaces, and not the Class?
For example, if you have a class like:
[DataContract]
public class DataObject : IRed, IBlue
then rather than have your operation contract expose DataObject, you have two operation contracts one for IRed and one for IBlue.
This eliminates the need for custom serialization code.

There is a way to do it, but it's an ugly hack.
The DataContractSerializer can serialize objects that implement the IXmlSerializable interface. You could implement the interface and create your own ReadXml(XmlReader reader) and WriteXml(XmlWriter writer) methods that could serialize the object in different ways.
Note that you'd have to have a flag embedded within the class itself to determine which way to serialize the object. (There's no way to tell the DataContractSerializer which mode to use, so the flag has to be contained in the object itself.)
A second version of the DTO class, as #Marc suggests, would be much cleaner.