Setting the message of a custom Exception without passing it to the base constructor

I want to make a custom Exception in C#, but in theory I do need to do a little parsing first before I can make a human readable ExceptionMessage.

The problem is that the orginal Message can only be set by calling the base constructor of Messsage, so I can't do any parsing in advance.

I tried overring the Message property like this:

public class CustomException : Exception
{
    string _Message;

    public CustomException(dynamic json) : base("Plep")
    {
        // Some parsing to create a human readable message (simplified)
        _Message    = json.message;
    }

    public override string Message
    {
        get { return _Message; }
    }
}

The problem is that the Visual Studio debugger still shows the message that I've passed into the constructor, Plep in this case.

throw new CustomException( new { message="Show this message" } )

results in:

If I leave the base constructor empty it will show a very generic message:

An unhandled exception of type 'App.CustomException' occurred in App.exe

Question

It looks like the Exception Dialog reads some field/property that I don't have any access too. Is there any other way to set a human readable error message outside the base constructor on Exception.

Note that I'm using Visual Studio 2012.

Just put the formatting code into a static method?

public CustomException(dynamic json) : base(HumanReadable(json)) {}
private static string HumanReadable(dynamic json) {
    return whatever you need to;
}

Consider the Microsoft Guidelines for creating new exceptions:

  using System;
  using System.Runtime.Serialization;

  [Serializable]
  public class CustomException : Exception
  {
    //
    // For guidelines regarding the creation of new exception types, see
    //    https://msdn.microsoft.com/en-us/library/ms229064(v=vs.100).aspx
    //

    public CustomException()
    {
    }

    public CustomException(string message) : base(message)
    {
    }

    public CustomException(string message, Exception inner) : base(message, inner)
    {
    }

    protected CustomException(SerializationInfo info, StreamingContext context) : base(info, context)
    {
    }

    public static CustomException FromJson(dynamic json)
    {
      string text = ""; // parse from json here

      return new CustomException(text);
    }
  }

Note the static factory method (not part of the pattern), that you can use in your program like this:

throw CustomException.FromJson(variable);

That way you followed best practice and can parse your json inside the exception class.