Why Are Parentheses Optional in C# 3.0 Object Initializers?

Optional brackets in C# 3.0 object initializer constructor

In C# 3.0, when initializing an object using object initializer syntax, you can omit the parentheses around the constructor if the constructor does not have any parameters. This is because the curly brace after the type name indicates the beginning of the object's initializer.

Design Considerations

The decision to make brackets optional was driven by several factors, including:

• Minimize complexity: Adding support for optional brackets is relatively simple and does not significantly increase the complexity of the language.
• Reduced redundancy: In most cases, constructor calls are redundant because properties are set directly in the object initializer.
• Disambiguation: Removing the brackets does not introduce any grammatical ambiguity.
• Common usage patterns: Many objects without parameters are simply used as "property bags".

Consideration of ambiguity

While optional parentheses pose no problem in the context of an object initializer, they can introduce ambiguity if applied to a default constructor call without an object initializer. Consider the following code:

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
    }
}

Line 1 creates a new C object and calls instance method M. Line 2 creates a new B.M object and calls its constructor. If the parentheses on line 2 were optional, the program would be ambiguous because it is unclear whether C.M refers to the constructor of C or to the class M nested in B .

Heuristic-based disambiguation

To resolve potential ambiguities, the compiler uses heuristics to determine the intent of the code. For example, if there is a type, field, or method with the same name as a dot identifier, the dot identifier is considered to refer to that member. However, the heuristics used do not always lead to the expected interpretation, especially in complex code.

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