对象初始化器语法困惑：new Cat{...}与new Cat(){...}的差异及使用

Great question! This is one of those C# syntax sugar details that seems trivial but can confuse folks when they first encounter both styles. Let's break it down simply:

Core Equivalence for Parameterless Constructors #

In most cases you'll encounter (when using a parameterless constructor), new Cat { ... } and new Cat() { ... } are 100% functionally identical. The C# compiler treats them exactly the same way—they generate identical IL code under the hood. Omitting the parentheses is just a shorthand syntax the language provides to make your code cleaner.

For example, these two lines do exactly the same thing:

// Shorthand (no parentheses)
Cat fluffy = new Cat { Age = 10, Name = "Fluffy" };

// Explicit parentheses
Cat fluffy2 = new Cat() { Age = 10, Name = "Fluffy" };

This even applies in collection initializers, like the example you shared. You can safely omit the parentheses for each new Cat instance:

// Both versions work and behave identically
List<Cat> cats = new List<Cat> { 
    new Cat(){ Name = "Sylvester", Age=8 }, 
    new Cat(){ Name = "Whiskers", Age=2 } 
};

List<Cat> catsShorthand = new List<Cat> { 
    new Cat{ Name = "Sylvester", Age=8 }, 
    new Cat{ Name = "Whiskers", Age=2 } 
};

When You Must Use Parentheses #

There are two scenarios where you can't skip the parentheses:

1. Calling a constructor with parameters: If your Cat class has a constructor that takes arguments (e.g., public Cat(string name) { Name = name; }), you need to include parentheses to pass those values. You can't write new Cat "Fluffy" { Age = 10 }—that's invalid syntax. Instead:

   // Correct: Use parentheses to pass constructor arguments
   Cat fluffy = new Cat("Fluffy") { Age = 10 };
   

2. Resolving syntax ambiguity: Rarely, the compiler might misinterpret your code if you omit parentheses in complex expressions. Adding them removes any ambiguity, though this scenario is uncommon in everyday code.

Anonymous Type Exception #

One important edge case: anonymous types can only use the new { ... } syntax (no parentheses). Since anonymous types don't expose explicit constructors, the language forbids adding parentheses here. For example:

// Valid anonymous type initialization
var anonymousPet = new { Species = "Cat", Age = 3, Name = "Mittens" };

// Invalid! Anonymous types don't support parentheses
// var anonymousPet = new () { Species = "Cat", Age = 3, Name = "Mittens" };

Final Takeaway #

• For parameterless constructors (like your Cat class), parentheses are optional—use whichever style you or your team prefer for readability.
• For constructors with parameters, parentheses are mandatory.
• Anonymous types require no parentheses.

内容的提问来源于stack exchange，提问作者knee pain