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Many of the other C# 6 features I’ve covered are implemented using syntactic sugar, exception filters isn’t. This has been in the deep parts of .NET for some time and in C# 6 it is getting surfaced finally. This is very much the introduction to Exception Filtering, and I have another post with advanced topics I would love for you to read.

Catching Exceptions

Catching exceptions today uses the exceptions type as the condition for the the catch block.

try
{
    // code which could throw exception
}
catch (HttpException ex)
{
   // catching only HttpExceptions
}

In the above code, any HttpException will be caught but any exception of any other type won’t.

try
{
    // code which could throw exception
}
catch (HttpException ex)
{
   // catching only HttpExceptions
}
catch (Exception ex)
{
    // catching everything except HttpExceptions
}

We can catch everything by catching the base Exception class. Order here matters, but the compiler will help prevent you from hurting yourself:

Today, if you want more fine grain control of what a catch block responds to you need to do that in the catch block itself.  For example, handling an exception differently based on values in the exception.

try
{
    // some code
}
catch (HttpException ex)
{
    if (ex.HttpCode == 401)
    {
        // need to do an authenticate
    }
    else
    {
        // some other HTTP error we need to handle
    }
}

Another good example is logging, based on a setting you may want to log or not log exceptions.

try
{
    // some code
}
catch (HttpException ex)
{
    if (loggingTurnedOn)
    {
        // log it
    }

    throw;
}

Exception Filters

Exception Filters adds a way to have additional requirements for a catch block in the form of a boolean expression which we append with the when keyword. We can now rewrite the two examples as follows:

//example 1
try
{
    // some code
}
catch (HttpException ex)
    when (ex.HttpCode == 401)
{
    // need to do an authenticate
}
catch (HttpException ex)
{
    // some other HTTP error we need to handle
}

//example 2
var loggingTurnedOn = false;
try
{
    // some code
}
catch (HttpException ex)
    when (loggingTurnedOn)
{
    // log it               
    throw;
}

The when clause can take in ANYTHING which evaluates to a boolean expression. You can put ands or ors and chain things together:

when (loggingTurnedOn && ex.HttpCode != 401)

You can pass methods in to the when condition too:

public static bool ThrowException()
{
    //elided
}

public static void OldPain2()
{  
    try
    {
        // some code
    }
    catch (HttpException ex)
        when (ThrowException())
    {
       // do something
    }
}

When isn’t if

Lastly, the language chosen for this feature is important, it isn’t IF, because it isn’t an IF statement – it is a WHEN condition. One of the important distinctions around that is that an IF statement supports ELSE and WHEN does not.