在属性中使用 int 常量

Use int constant in attribute(在属性中使用 int 常量)

问题描述

谁能解释为什么我不能在 C# 属性中使用 const Int32?

Can anybody explain why I can't use a const Int32 in an C# attribute?

例子:

private const Int32 testValue = 123;  
[Description("Test: " + testValue)]  
public string Test { get; set; }

让编译器说:

"属性参数必须是常量表达式，..."

"An attribute argument must be a constant expression, ..."

为什么?

推荐答案

由于错误状态，属性参数必须是 constant 表达式.

As the error states, an attribute argument must be a constant expression.

连接字符串和整数不是常量表达式.

Concatenating a string and an integer is not a constant expression.

因此，如果你通过 "Test:"+ 123 直接，它会给出同样的错误.另一方面，如果您将 testValue 更改为字符串，它将编译.

Thus, if you pass "Test: " + 123 directly, it will give the same error. On the other hand, if you change testValue to a string, it will compile.

常量表达式的规则规定常量表达式可以包含算术运算符，前提是两个操作数本身都是常量表达式.

The rules for constant expressions state that a constant expression can contain arithmetic operators, provided that both operands are themselves constant expressions.

因此，A";+ B" 仍然是不变的.

Therefore, "A" + "B" is still constant.

但是，A";+ 1 使用字符串运算符+(string x, object y);，其中整数操作数被装箱到一个对象中.
常量表达式规则明确指出

However, "A" + 1 uses the string operator +(string x, object y);, in which the integer operand is boxed to an object.
The constant-expression rules explicitly state that

常量表达式中不允许进行其他转换，包括装箱、拆箱和非空值的隐式引用转换.

Other conversions including boxing, unboxing and implicit reference conversions of non-null values are not permitted in constant expressions.

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