Most pages will not really make much difference in practice.
The first example instantiates when Page is created. oSystem will be available for the entire resource of the page.
The second example will create an instance in the Page_Load event, which will not happen until about the middle of the page life cycle.
For more information on the page life cycle, see ASP.NET Page Life Cycle Overview .
If you want to use the instance earlier, for example, in the Page_Init event, then the previous example will not select the object sooner.
If your application needs to be high-performance, requiring very efficient memory management, you will probably prefer the last example. In this example, the memory will be closer to when it will be used, so it will not bind resources longer than necessary. However, if you want efficient memory management, you can do many optimizations.
So, on most pages there is no practical difference.
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