这是空合并运算符，并很喜欢三元（即时如果）运营商。另请参见？运营商 - MSDN

It's the null coalescing operator, and quite like the ternary (immediate-if) operator. See also ?? Operator - MSDN.

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

扩展为：

FormsAuth = formsAuth != null ? formsAuth : new FormsAuthenticationWrapper();

进一步扩展为：

if(formsAuth != null)
    FormsAuth = formsAuth;
else
    FormsAuth = new FormsAuthenticationWrapper();

在英语中，它的意思是如果无论是左侧是不为空，使用，否则使用什么的权利。

In English, it means "If whatever is to the left is not null, use that, otherwise use what's to the right."

请注意，您可以使用序列中的任何数量的这些。下面的语句将第一个非空回答＃分配给答：

Note that you can use any number of these in sequence. The following statement will assign the first non-null Answer# to Answer:

string Answer = Answer1 ?? Answer2 ?? Answer3 ?? Answer4;