很好 MSDN 明确指出

常量表达式是可以在
编译时完全评估的表达式。因此，
引用类型的常量的唯一可能值为字符串和null。

A constant expression is an expression that can be fully evaluated at compile time. Therefore, the only possible values for constants of reference types are string and null.

来自C＃语言规范的10.4节。可以使用这些类型。

From section 10.4 of C# language specification. These are the types that can be used.

在常量声明中指定的类型必须是sbyte，byte，
short， ushort，int，uint，long，ulong，char，float，double，decimal，
bool，字符串，枚举类型或引用类型。每个
常量表达式都必须产生目标类型或
类型的值，可以通过隐式转换将其转换为目标类型

The type specified in a constant declaration must be sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, an enum-type, or a reference-type. Each constant-expression must yield a value of the target type or of a type that can be converted to the target type by an implicit conversion