eager loading vs lazy loading c#

When to use eager loading

In "one side" of one-to-many relations that you sure are used every where with main entity. like User property of an Article. Category property of a Product.
Generally When relations are not too much and eager loading will be good practice to reduce further queries on server.
When to use lazy loading

Almost on every "collection side" of one-to-many relations. like Articles of User or Products of a Category
You exactly know that you will not need a property instantly.
Note: like Transcendent said there may be disposal problem with lazy loading.