A different view on creation
The following code shows several increasingly complex way to create a shared instance of an object:
What are the differences? The _requestExecuters value is a concurrent dictionary. And the major difference is the kind of allocations that happen in each call.
In the first scenario, we’ll create a new RequestExecuter each time that we call this line. We’ll still use only a single instance, of course, but we still create (and discard) an instance per call.
In the second scenario, we are passing a delegate, so we’ll only create the RequestExecuter once. Or so it seems. The problem is that under concurrent load, it is possible that we’ll have two RequestExecuters created, only one of which will be used. If we have any unmanaged resources in the RequestExecuter, that can cause a leak. Another issue is that we are using the method parameter, which forces the compiler to capture it, and allocate a new delegate instance per call.
The third scenario is the same as the second one, but we aren’t capturing the parameter, so the compiler will not need to create a new delegate instance per call.
The forth one is using a lazy value. This way, we avoid the race in creating the RequestExecuter, but we still create a new lazy instance per call.
And the fifth one is using a lazy instance and a cached delegate version, so there are no extra allocations there. There is still the race to create the lazy instance, but that should happen rarely, and it isn’t holding any expensive resources.