为什么 expr1 可以编译,而 expr2 不能编译?
Func<object> func = () => new object();
Expression<Func<object>> expr1 = () => new object();
Expression<Func<object>> expr2 = func; //Cannot implicitly convert type 'System.Func<object>' to 'System.Linq.Expressions.Expression<System.Func<object>>'