"二进制运算符添加未对类型'System.String'和'System.String'&QUOT定义; - 真的吗？ [英] "The binary operator Add is not defined for the types 'System.String' and 'System.String'." -- Really?

问题描述

当试图运行下面的代码：

 表达式来; Func键<字符串，字符串>> stringExpression = Expression.Lambda< Func键<字符串，字符串>>（
 Expression.Add（
 StringParam中，
 Expression.Constant（A）
）
新的List< ParameterExpression>（）{} StringParam中
）; 
 
串AB = stringExpression.Compile（）（B）; 
  

我得到在标题中引用的错误：二元运算符添加不为类型定义 System.String'和'System.String'。事实是否真的如此呢？显然，在C＃中它的作品。做字符串s =A+B在C＃中的特殊语法糖，表达式编译器不会访问？

< DIV CLASS =h2_lin>解决方案

这是绝对正确的，是的。有没有这样的运营商 - C＃编译器转换字符串+字符串成调用 string.Concat 。 （这一点很重要，因为这意味着 X + Y + Z 可转换成 string.Concat（X，Y，Z）避免了无谓创建中间字符串。

有一个看的文档的字符串运算符 - 仅 == 和 = 的定义！由框架

When trying to run the following code:

    Expression<Func<string, string>> stringExpression = Expression.Lambda<Func<string, string>>(
        Expression.Add(
            stringParam,
            Expression.Constant("A")
        ),
        new List<ParameterExpression>() { stringParam }
    );

    string AB = stringExpression.Compile()("B");

I get the error referenced in the title: "The binary operator Add is not defined for the types 'System.String' and 'System.String'." Is that really the case? Obviously in C# it works. Is doing string s = "A" + "B" in C# special syntactic sugar that the expression compiler doesn't have access to?

解决方案

It's absolutely right, yes. There is no such operator - the C# compiler converts string + string into a call to string.Concat. (This is important, because it means that x + y + z can be converted into string.Concat(x, y, z) which avoids creating intermediate strings pointlessly.

Have a look at the docs for string operators - only == and != are defined by the framework.

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