为什么不能将常量字符串的串联分配给常量字符串？ [英] Why can I not assign the concatenation of constant strings to a constant string?

问题描述

有时出于格式原因，我想将常量字符串拆开，通常是SQL。

Occasionally I want to break apart a constant string for formatting reasons, usually SQL.

const string SELECT_SQL = "SELECT Field1, Field2, Field3 FROM TABLE1 WHERE Field4 = ?";

至

const string SELECT_SQL = "SELECT Field1, Field2, Field3 " 
                        + "FROM TABLE1 " 
                        + "WHERE Field4 = ?";

但是C＃编译器不允许第二种形式为常量字符串。为什么？

However the C# compiler will not allow this second form to be a constant string. Why?

推荐答案

嗯，那应该没问题...您确定

Um, that should be fine... are you sure it doesn't compile?

示例代码：

using System;

class Test
{
    const string MyConstant = "Foo" + "Bar" + "Baz";

    static void Main()
    {
        Console.WriteLine(MyConstant);
    }
}

我的猜测是您的真实代码中，您要在串联中包含一些非常数表达式。

My guess is that in your real code you're including some non-constant expression in the concatenation.

例如，这很好：

const string MyField = "Field";
const string Sql = "SELECT " + MyField + " FROM TABLE";

但这不是：

static readonly string MyField = "Field";
const string Sql = "SELECT " + MyField + " FROM TABLE";

这是尝试使用非常量表达式（ MyField ）包含在常量表达式声明中-不允许 。

This is attempting to use a non-constant expression (MyField) within a constant expression declaration - and that's not permitted.

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