where T:class generic type constraints

Type parameter constraints,. NET supports type parameter constraints in the following five ways:

where T:struct | T must be a struct type
Where T:class T must be a class type
where T:new () | T must have an argument-free constructor
where T:nameofbaseclass | T must inherit a class named Nameofbaseclass
where T:nameofinterface | T must implement an interface named Nameofinterface

Microsoft MSDN Detailed Description: http://msdn.microsoft.com/zh-cn/library/bb384067.aspx

where T:class generic type constraints