空合并运算符和 lambda 表达式
看一下我尝试在构造函数内编写的以下代码：
private Predicate<string> _isValid;
//...
Predicate<string> isValid = //...;
this._isValid = isValid ?? s => true;
该代码无法编译 - 只是“无效的表达式术语”等等。
相比之下，它确实可以编译，我可以使用它：
this._isValid = isValid ?? new Predicate<string>(s => true);
但是，我仍然想知道为什么不允许这种语法。
有什么想法吗？
take a look at the following code I attempted to write inside a constructor:
private Predicate<string> _isValid;
//...
Predicate<string> isValid = //...;
this._isValid = isValid ?? s => true;
The code doesn't compile - just "invalid expression term"s and so one.
In contrast that does compile and I could just use it:
this._isValid = isValid ?? new Predicate<string>(s => true);
However, I still wonder why this syntax is not allowed.
Any ideas?
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会起作用的:)
它是这样解析的：
这没有任何意义。
Will work :)
It parsed it this way:
which does not make any sense.
查看 C# 语法的这一部分：
parenthesized-expression: ( expression ) ..... simple-name: identifier type-argument-listopt ..... conditional-or-expression: conditional-and-expression conditional-or-expression || conditional-and-expression null-coalescing-expression: conditional-or-expression conditional-or-expression ?? null-coalescing-expression conditional-expression: null-coalescing-expression null-coalescing-expression ? expression : expression lambda-expression: anonymous-function-signature => anonymous-function-body由于
null-coalescing-expression以conditional-or-expression终止，因此示例中的s将解析作为简单名称。通过将其括在括号中，可以将其解析为括号表达式。Check out this portion of the C# grammar:
parenthesized-expression: ( expression ) ..... simple-name: identifier type-argument-listopt ..... conditional-or-expression: conditional-and-expression conditional-or-expression || conditional-and-expression null-coalescing-expression: conditional-or-expression conditional-or-expression ?? null-coalescing-expression conditional-expression: null-coalescing-expression null-coalescing-expression ? expression : expression lambda-expression: anonymous-function-signature => anonymous-function-bodySince
null-coalescing-expressionterminates withconditional-or-expressionthesin your example will parse as asimple-name. By wrapping it in parentheses it can then be parsed as aparenthesized-expression.