A variable of a reference type does not contain its data directly; it contains a reference to its data. When you pass a reference-type parameter by value, it is possible to change the data pointed to
by the reference, such as the value of a class member. However, you cannot change the value of the reference itself; that is, you cannot use the same reference to allocate memory for a new class and have it persist outside the block. To do that, pass
the parameter using the ref or
out keyword.
C# - Passing Reference-Type Parameters
原文：http://www.cnblogs.com/liguangsunls/p/6756862.html
