Develop Reference

Catching base but not sub exceptions

I am writing a HttpHandler and as part of my internal design, I throw different exceptions and catch them at the top level in order to determine what status code to set the request to.
I.e.
ArgumentException triggers Bad Request
AuthenticationException triggers Unauthorised
OracleException triggers InternalServerError
etc
The problem I've found is that I was using InvalidOperationException to trigger NotFound, but a bug in my code caused me to realise that, of course, some system exceptions inherit from these base system exceptions, which causes unexpected responses.
I.e. I found that ObjectDisposedException inherits from InvalidOperationException, which means that the response returns a 404 instead of 500.
Is there a way to catch just the base exception?
I found this thread which suggests I could do a filter and rethrow, but that seems hacky.
Would I be better off just creating my own exception types to save all this hassle?

If you only want to pass the HTTP result code (and maybe error message line) to the client, then I recommend you create a single custom exception:
public class MyCustomException : Exception { // Or maybe InvalidOperationException
public int StatusCode {get;set;}
public string Status {get;set;}
}
etc.

As far as I am aware there's no way to catch an exception but not catch inherited exceptions. The problem is that you're considering the base exception to be more specific than the inherited exception, which is the opposite of how they're intended to work (Inherited exceptions should be more specific subsets of their parent, so conceptually if you can handle the parent you should be able to handle the inherited exception).
Your options basically are:
Create your own exception type & only catch this. If your exception falls under the definition of an InvalidOperationException then you can inherit from this. Then you can catch yours specifically.
Catch, inspect, and re-throw if it's not what you're after. This technique I typically try to avoid but is sometimes required if exception types are too generic. e.g.
catch (InvalidOperationException ex) {
if (ex.GetType() != typeof(InvalidOperationException)) throw;
/* Do stuff */
}

It is not possible to only catch base exceptions.
In this case it is best to create a custom exception, rather than use the system's exception types.
public class HttpErrorException : Exception
{
public HttpStatusCode StatusCode { get; private set; }
public HttpErrorException(HttpStatusCode code, string message)
: base(message)
{
this.StatusCode = code;
}
}
throw new HttpErrorException(400, "You sent a bad request!");
If the original intention was to obfuscate the HTTP status codes, and instead use their text names, factory methods can be created.
public class HttpErrorException : Exception
{
public HttpStatusCode StatusCode { get; private set; }
private HttpErrorException(HttpStatusCode code, string message)
: base(message)
{
this.StatusCode = code;
}
public static HttpErrorException BadRequest(string message)
{
return new HttpErrorException(400, message);
}
public static HttpErrorException InternalServerError(string message)
{
return new HttpErrorException(500, message);
}
// etc
}
throw HttpErrorException.BadRequest("You made a bad request!");

throw new CustomException vs. Marshal.ThrowExceptionForHR

I have a question regarding Exceptions and returning control to a COM caller. Below is a test question with a custom exception class. What is the difference between
Marshal.ThrowExceptionForHR(CustomException.COR_E_ARGUMENT);
and
throw new CustomException("Argument is out of bounds");
I kind of get why 1) and 2 doesn't work as they return an int and an Exception-object. But what's the difference between 3 and 4?
public class CustomException : ApplicationException
{
public static int COR_E_ARGUMENT = unchecked((int)0x80070057);
public CustomException(string msg)
: base(msg)
{
HResult = COR_E_ARGUMENT;
}
}
You need to write a code segment that will use the CustomException class
to immediately return control to the COM caller. You also need to ensure
that the caller has access to the error code. Which code segment should
you use?
return Marshal.GetExceptionForHR(CustomException.COR_E_ARGUMENT);
return CustomException.COR_E_ARGUMENT;
Marshal.ThrowExceptionForHR(CustomException.COR_E_ARGUMENT);
throw new CustomException("Argument is out of bounds"); // correct answer

The COM client code is of course completely agnostic of the .NET Exception type, it only sees the HRESULT error code. So there is no value-add from adding your own exception types.
All standard .NET Exception derived classes already have a precooked HRESULT value baked-in. They assign the Exception.HResult property in their constructor. If you want to generate E_INVALIDARG (0x80070057) then use the standard .NET exception class that already uses that error code. System.ArgumentException.

How to bind exception class and error code of external com exception?

my .NET application calls method of object that located out of assembly boundaries. When this method has thrown exception i've catched COMException.
Exception contains special code in ErrorCode field, for example -123.
try
{
// call outside object
}
catch(COMException e)
{
// e.ErrorCode == -123;
}
Is it posible to define exception class that will be thrown instead of COMException?
eg
[ErrorCode of external error = -123]
public class MyException : Exception;
and rewrite above code
try
{
// call outside object
}
catch(MyException e)
{
//...
}

"Is it posible to define exception class that will be thrown instead of COMException?"
No, but you could catch the COMException, wrap it in a custom exception class, and rethrow it.

C# re-throwing exception outside of scope

I am fully aware that what I am about to ask is not good practice... but:
Let's say I have a class containing a function that I want to always return a value, but store any exceptions that might occur for later processing. Something Like:
public Exception _error { get; set; }
public bool IsValid()
{
try
{
//do something here to cause exception
return true;
}
catch (Exception ex)
{
_error = ex;
return false;
}
}
Now that I have stored the exception, is it at all possible to throw the exception from an outside method while maintaining both the original stack trace and exception type?
throw _error; //lose stack trace
throw new Exception("", _error) //lose type
Thanks for looking or answering.
EDIT:
Thanks to some additional points, I realize that the below idea only takes away information and doesn't really add or simplify the situation. Thanks again to everyone.
After pondering Pieter's answer and comments, I'm now wondering if making a wrapper Exception class like the below could be a partial solution. This overrides as much of the exception as possible to make the New exception look like its innerexception, including the stacktrace.. dirty I know, but interesting:
public class ExceptionWrapper : Exception
{
private Exception _innerException;
public ExceptionWrapper(Exception ex) : base("", ex)
{
_innerException = ex;
this.Source = ex.Source;
this.HelpLink = ex.HelpLink;
}
public override string StackTrace
{
get
{
return _innerException.StackTrace;
}
}
public override System.Collections.IDictionary Data
{
get
{
return _innerException.Data;
}
}
public override string Message
{
get
{
return _innerException.Message;
}
}
public new Exception InnerException
{
get
{
return _innerException.InnerException;
}
}
}

No, this is not possible.
However, you normally solve this is by wrapping the exception in a new exception:
throw new MyException("Wrapper", _error);
This does maintain the stack trace of _error, but you do get a new exception. Your solution in your second example is the correct way of handling these cases.

Consider using reflection to create a wrapper exception of the correct type (Activator.CreateInstance) and calling the constructor that will accept the inner exception you have stored.
For example:
[Test]
public void test()
{
Exception ex = new ArgumentNullException();
Exception wrapped = (Exception)Activator.
CreateInstance(ex.GetType(), "wrapped", ex);
Type expectedType = typeof(ArgumentNullException);
Assert.IsInstanceOf(expectedType, wrapped, "Is ArgumentNullException.");
Assert.AreEqual(ex, wrapped.InnerException, "Exception is wrapped.");
}
Update
In order to mitigate the constructor issue, you could consider using the default constructor (should be there for an exception that follows design guidelines, but not mandatory) and then patching up the new instance by setting its fields via reflection.
I agree the approach is highly "meh" it's more an exploration of an idea. I wouldn't recommend it.
The exception design guidelines require a default constructor, so this sort of behaviour may go on in the framework somewhere anyway. Perhaps for some sort of icky serialization\deserialization of exceptions across some sort of communications boundary?

It seems that .net-4.5 added a new API for capturing stack/info about exceptions and rethrowing them in different contexts. This is called ExceptionDispatchInfo. It is useful if you find yourself needing more control over running tasks indirectly, like if you do manual thread management for jobs or Task does not exactly fit your needs. In your example, it should look like this:
public ExceptionDispatchInfo _error { get; private set; }
public bool IsValid()
{
try
{
//do something here to cause exception
return true;
}
catch (Exception ex)
{
_error = ExceptionDispatchInfo.Capture(ex);
return false;
}
}
/// <summary>Throw underlying exception if invalid.</summary>
public void AssertWasValid() => _error?.Throw();
Now, it doesn’t preserve the original caller. The displayed stack trace shows the calls from the original try block into the code in there, a statement breaking the original and new parts of the stack, and then the calls into ExceptionDispatchInfo.Throw() itself as the new part of the shown stack. This seems similar to how traces with async code look. If you care about the original caller, seems this won’t work. But if you care about getting the line/method that threw the exception, this should be sufficient.

How to throw custom exception when exception type was passed by constructor

I've created class that takes Exception type in constructor
private readonly Exception _exception;
public StringToObject(Exception exception)
{
_exception = exception;
}
i wanted to throw exception
throw new _exception("");
but i got error:
'._exception' is a 'field' but is used like a 'type'
is any possible ways to throw it?

This is not a good practice at all. Doing so will cause you to lose your stack trace related information. Please consider reading this section of Eric Lippert's blog:
Too Much Reuse
When you write
throw new Exception();
you instantiate this new exception. But then, since your private member _exception is already instantiated, you don't need to re-instantiate it, that is instantiating an instance, which doesn't make sense. Instead, use the following:
throw _exception;
This will do it.

To rethrow an existing exception like that use
throw _exception;
However, that will modify the call stack in the exception instance, so you will lose the original source of the exception. If you want to avoid that, you can throw a new exception with the instance as an inner exception.

I'm actually very confused about why you want to do this? Are you trying to create a custom exception to provide more information? If so, then you want to use this pattern.
First define a custom exception class that derives from Exception:
public class MyCustomException : Exception // Or you could derive from ApplicationException
{
public MyCustomException(string msg, Exception innerException)
: base(msg, innerException)
{
}
}
You could also define additional parameters in your custom exception constructor to contain even more information if you wish. Then, in your application code...
public void SomeMethod()
{
try
{
// Some code that might throw an exception
}
catch (Exception ex)
{
throw new MyCustomException("Additional error information", ex);
}
}
You'll want to be sure to keep track of the inner exception, because that will have the most useful call stack information about what caused the exception in the first place.

throw _exception;

This example should work. I´ve included all the classes involved in the example.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication1
{
public class MyException : Exception
{
public MyException(string message) : base(message)
{}
//...
}
public class MyClass
{
private Exception exception;
public MyClass(Exception e)
{
this.exception = e;
}
public void ThrowMyException()
{
throw exception;
}
}
class Program
{
static void Main(string[] args)
{
MyException myExceptionInstance = new MyException("A custom message");
MyClass myClassInstance = new MyClass(myExceptionInstance);
myClassInstance.ThrowMyException();
}
}
}

I suspect that what you're really looking for is to throw a new exception of your suggested type, in which case passing in a "Type" parameter (or even using a generic) would be the way forward.
However, I can't imagine a situation where this is a sensible design choice, so I'd have to urge you to reconsider (and perhaps post more of your requirements so that someone can suggest a better alternative!).