数组协方差

string[] strings = new[] {"foo", "bar"};
object[] objects = strings; // implicit conversion from string[] to object[]

此转换不是类型安全的。以下代码将引发运行时异常：

string[] strings = new[] {"Foo"};
object[] objects = strings;

objects[0] = new object(); // runtime exception, object is not string
string str = strings[0];   // would have been bad if above assignment had succeeded