In .NET, by default, a normal reference to any object is a strong reference. So, when you declare a variable of a reference type, you are creating a strong reference by default. The garbage collector cannot collect an object in use by an application while the application’s code can reach that object. The application is said to have a strong reference to the object.
A weak reference permits the garbage collector to collect the object while still allowing the application to access the object. A weak reference is valid only during the indeterminate amount of time until the object is collected when no strong references exist. When you use a weak reference, the application can still obtain a strong reference to the object, which prevents it from being collected. However, there is always the risk that the garbage collector will get to the object first before a strong reference is reestablished.
Weak references are useful for objects that use a lot of memory, but can be recreated easily if they are reclaimed by garbage collection.