Catching exceptions with “catch, when”

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Catch blocks already allow you to filter on the type of the exception:

catch (SomeSpecificExceptionType e) {...}

The when clause allows you to extend this filter to generic expressions.

Thus, you use the when clause for cases where the type of the exception is not distinct enough to determine whether the exception should be handled here or not.

A common use case are exception types which are actually a wrapper for multiple, different kinds of errors.

Here’s a case that I’ve actually used (in VB, which already has this feature for quite some time):

try
{
    SomeLegacyComOperation();
}
catch (COMException e) when (e.ErrorCode == 0x1234)
{
    // Handle the *specific* error I was expecting. 
}

Same for SqlException, which also has an ErrorCode property. The alternative would be something like that:

try
{
    SomeLegacyComOperation();
}
catch (COMException e)
{
    if (e.ErrorCode == 0x1234)
    {
        // Handle error
    }
    else
    {
        throw;
    }
}

which is arguably less elegant and slightly breaks the stack trace.

In addition, you can mention the same type of exception twice in the same try-catch-block:

try
{
    SomeLegacyComOperation();
}
catch (COMException e) when (e.ErrorCode == 0x1234)
{
    ...
}
catch (COMException e) when (e.ErrorCode == 0x5678)
{
    ...
}

which would not be possible without the when condition.

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