Another use for Guard

Posted by Nicholas Blumhardt | November 11, 2007 08:30

I wrote earlier about a handy utility class based on IDisposable.

Well, I think I'm going to end up using this one more than I expected. I did some refactoring of ContainerBuilder last night and changed one of the APIs I've never been happy with. The WithScope() syntax is fine but when registering a bunch of factories the typing can get repetitive. So, previously, to change the default instance scope you could do this:

var oldDefault = builder.DefaultInstanceScope;
builder.DefaultInstanceScope = InstanceScope.Factory;
// Register a bunch of factories
// then revert to the saved default
builder.DefaultInstanceScope = oldDefault;

This is a bit repetitive in itself, and to be truly correct requires a try/finally block.

The new syntax is:

using (builder.SetDefaultScope(InstanceScope.Factory))
{
  // Register factories
}
// At the end of the using block, the old default is reinstated

The C++ warriors of old will recognise this pattern. What's nifty is the implementation of SetDefaultScope():

public IDisposable SetDefaultScope(InstanceScope scope)
{
    var oldValue = _defaultInstanceScope;
    _defaultInstanceScope = scope;
    return new Guard(() => _defaultInstanceScope = oldValue);
}

Pretty minimal, I think.

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