What is the proper way to display the full InnerException?

Question

You can simply print exception.ToString() -- that will also include the full text for all the nested InnerExceptions.

I usually do like this to remove most of the noise:

void LogException(Exception error) {
    Exception realerror = error;
    while (realerror.InnerException != null)
        realerror = realerror.InnerException;

    Console.WriteLine(realerror.ToString())
}    

Edit: I forgot about this answer and is surprised no one pointed out that you can just do

void LogException(Exception error) {
    Console.WriteLine(error.GetBaseException().ToString())
}    

Just use exception.ToString()

https://docs.microsoft.com/en-us/dotnet/api/system.exception.tostring#remarks

The default implementation of ToString obtains the name of the class that threw the current exception, the message, the result of calling ToString on the inner exception, and the result of calling Environment.StackTrace. If any of these members is null, its value is not included in the returned string.

If there is no error message or if it is an empty string (""), then no error message is returned. The name of the inner exception and the stack trace are returned only if they are not null.

exception.ToString() will also call .ToString() on that exception's inner exception, and so on...

@Jon's answer is the best solution when you want full detail (all the messages and the stack trace) and the recommended one.

However, there might be cases when you just want the inner messages, and for these cases I use the following extension method:

public static class ExceptionExtensions
{
    public static string GetFullMessage(this Exception ex)
    {
        return ex.InnerException == null 
             ? ex.Message 
             : ex.Message + " --> " + ex.InnerException.GetFullMessage();
    }
}

I often use this method when I have different listeners for tracing and logging and want to have different views on them. That way I can have one listener which sends the whole error with stack trace by email to the dev team for debugging using the .ToString() method and one that writes a log on file with the history of all the errors that happened each day without the stack trace with the .GetFullMessage() method.

To pretty print just the Messages part of deep exceptions, you could do something like this:

public static string ToFormattedString(this Exception exception)
{
    IEnumerable<string> messages = exception
        .GetAllExceptions()
        .Where(e => !String.IsNullOrWhiteSpace(e.Message))
        .Select(e => e.Message.Trim());
    string flattened = String.Join(Environment.NewLine, messages); // <-- the separator here
    return flattened;
}

public static IEnumerable<Exception> GetAllExceptions(this Exception exception)
{
    yield return exception;

    if (exception is AggregateException aggrEx)
    {
        foreach (Exception innerEx in aggrEx.InnerExceptions.SelectMany(e => e.GetAllExceptions()))
        {
            yield return innerEx;
        }
    }
    else if (exception.InnerException != null)
    {
        foreach (Exception innerEx in exception.InnerException.GetAllExceptions())
        {
            yield return innerEx;
        }
    }
}

This recursively goes through all inner exceptions (including the case of AggregateExceptions) to print all Message property contained in them, delimited by line break.

E.g.

var outerAggrEx = new AggregateException(
    "Outer aggr ex occurred.",
    new AggregateException("Inner aggr ex.", new FormatException("Number isn't in correct format.")),
    new IOException("Unauthorized file access.", new SecurityException("Not administrator.")));
Console.WriteLine(outerAggrEx.ToFormattedString());

Outer aggr ex occurred.
Inner aggr ex.
Number isn't in correct format.
Unauthorized file access.
Not administrator.

---

You will need to listen to other Exception properties for more details. For e.g. Data will have some information. You could do:

foreach (DictionaryEntry kvp in exception.Data)

To get all derived properties (not on base Exception class), you could do:

exception
    .GetType()
    .GetProperties()
    .Where(p => p.CanRead)
    .Where(p => p.GetMethod.GetBaseDefinition().DeclaringType != typeof(Exception));

I do:

namespace System {
  public static class ExtensionMethods {
    public static string FullMessage(this Exception ex) {
      if (ex is AggregateException aex) return aex.InnerExceptions.Aggregate("[ ", (total, next) => $"{total}[{next.FullMessage()}] ") + "]";
      var msg = ex.Message.Replace(", see inner exception.", "").Trim();
      var innerMsg = ex.InnerException?.FullMessage();
      if (innerMsg is object && innerMsg!=msg) msg = $"{msg} [ {innerMsg} ]";
      return msg;
    }
  }
}

This "pretty prints" all inner exceptions and also handles AggregateExceptions and cases where InnerException.Message is the same as Message