How to serialize an Exception object in C#?

Create a custom Exception class with the [Serializable()] attribute. Here's an example taken from the MSDN:

[Serializable()]
public class InvalidDepartmentException : System.Exception
{
    public InvalidDepartmentException() { }
    public InvalidDepartmentException(string message) : base(message) { }
    public InvalidDepartmentException(string message, System.Exception inner) : base(message, inner) { }

    // Constructor needed for serialization 
    // when exception propagates from a remoting server to the client.
    protected InvalidDepartmentException(System.Runtime.Serialization.SerializationInfo info,
        System.Runtime.Serialization.StreamingContext context) : base(info, context) { }
}

What I've done before is create a custom Error class. This encapsulates all the relevant information about an Exception and is XML serializable.

[Serializable]
public class Error
{
    public DateTime TimeStamp { get; set; }
    public string Message { get; set; }
    public string StackTrace { get; set; }

    public Error()
    {
        this.TimeStamp = DateTime.Now;
    }

    public Error(string Message) : this()
    {
        this.Message = Message;
    }

    public Error(System.Exception ex) : this(ex.Message)
    {
        this.StackTrace = ex.StackTrace;
    }

    public override string ToString()
    {
        return this.Message + this.StackTrace;
    }
}

The Exception class is marked as Serializable and implements ISerializable. See MSDN: http://msdn.microsoft.com/en-us/library/system.exception.aspx

If you are attempting to serialize to XML using the XmlSerializer, you will hit an error on any members that implement IDictionary. That is a limitation of the XmlSerializer, but the class is certainly serializable.