Why Are Optional Constructor Parentheses Allowed in C# Object Initializers?

The reason why C# object initializer allows constructor parentheses to be omitted

C# 3.0 introduced object initializers, whose optional syntax allows the parentheses in the constructor to be omitted in cases where the constructor has a parameterless constructor. As former C# compiler team leader Mads Torgersen explains, this feature has several advantages:

Design considerations:

• Concise and easy to use: Omitting parentheses reduces unnecessary clutter and makes the syntax simpler.
• Common use cases: Object initializers are typically used for "property bags" that don't require custom constructor parameters, so the parentheses are redundant.

Technical implementation:

• Low Development Cost: This feature requires only minimal changes to the compiler's code, so it is relatively cheap to implement.
• Low testing burden: Omitting parentheses does not significantly increase testing requirements.
• Seamless IDE Integration: IntelliSense remains fully functional because optional brackets do not introduce syntactic or semantic ambiguity. This allows for accurate code autocompletion and error detection.

Avoid ambiguity:

While parentheses are allowed to be omitted in object initializers, they are still required in object creation expressions without object initializers. This is because omitting parentheses in this case can create ambiguity, as in the following example:

<code class="language-csharp">class P
{
    class B
    {
        public class M { }
    }
    class C : B
    {
        new public void M(){}
    }

    static void Main()
    {
        // 调用默认构造函数，然后是方法 M
        new C().M();           // 1

        // 使用默认构造函数创建一个新的 B.M 实例
        new C.M();           // 2
    }
}</code>

If you omit the optional brackets on line 1, the compiler cannot determine whether to call C's default constructor or create a new B.M instance. This will lead to semantic ambiguity and potential errors.

Optional constructor parentheses are therefore restricted to object initializers, where omitting them does not pose any risk to the syntax or semantics of the language.

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