简单把这个是制约泛型参数的一类（或更具体地说这可能是一个类，接口，委托或数组类型的引用类型）。

请参阅此 MSDN文章进一步的细节。

In C# what does where T : class mean?

Ie.

public IList<T> DoThis<T>() where T : class

Simply put this is constraining the generic parameter to a class (or more specifically a reference type which could be a class, interface, delegate, or array type).

See this MSDN article for further details.