Why Are Parentheses Optional in Some C# Object Initializers but Required in Others?

Optional brackets in C# object initializer constructor: reasons and disambiguation

In C# 3.0, object initializer constructors allow parentheses to be omitted if a parameterless constructor is present. This feature was added based on the following factors:

• Simplify the design, specification and development process and reduce workload.
• Eliminate redundancy because when using object initializers for property assignment, the parameter list is usually not present.
• Low maintenance burden as this feature has not had any known bugs since its implementation.

Missing brackets in default constructor call

However, in object creation expressions without object initializers, parentheses in default constructor calls are still required. This restriction is to avoid ambiguity. For example, in the following code:

<code class="language-csharp">class P
{
    class B
    {
        public class M { }
    }
    class C : B
    {
        new public void M(){}
    }
    static void Main()
    {
        new C().M(); // 1
        new C.M();   // 2
    }
}</code>

Line 1 creates a new C object, calls its default constructor, and calls the M instance method on the new object. Line 2 creates an instance of B.M and triggers its default constructor. If the brackets in line 1 were optional, line 2 would be ambiguous because it could also refer to C.M.

Ambiguous detection method

Determining ambiguity in C# functions involves several methods:

• Manually check and analyze syntax and semantics.
• Formal proof, usually by an external research team.
• Use the parser generator for machine analysis to search for potential ambiguities.

Examples of potential ambiguity

Consider adding a new prefix operator "frob":

<code class="language-csharp">x = frob 123 + 456;</code>

This can be interpreted as performing a frob operation on the result of 123 456, or assigning 10 to a variable of type frob named x.

In another example, the following expression in C# 2.0 is ambiguous:

<code class="language-csharp">yield(x);</code>

It can mean producing x in an iterator, or it can mean calling the yield method with x as a parameter. The ambiguity is removed by changing it to "yield return(x)".

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