Explain Primary Constructor in C# 12

A Primary Constructor is a new way to write constructors in C# 12 that makes your code shorter and easier to read.

How Does It Work?

1. Where to Write It: You write the primary constructor right after the class or struct name inside parentheses.
2. Using Parameters: The parameters you define in the primary constructor can be used throughout the class or struct.

For example, you can declare a class with a primary constructor like this.

public class Student(string name, int age)
{
    // primary constructor parameters are in scope here
}

The primary constructor parameters name and age are not public properties, but they can be used to initialize public properties like this.

public class Student(string name, int age)
{
    public string Name { get; } = name; // initialize a readonly property
    public int Age { get; set; } = age; // initialize a read-write property
}

You can also use primary constructor parameters to call a base constructor like this.

public class Alumni(string name, int age, string school) : Student(name, age)
{
    public string School { get; } = school;
}

In this example,

• Student is a class with a primary constructor that takes name and age as parameters.
• These parameters are used to set the values of the properties Name and Age.

Why Use It?

• Less Code: You don’t need to write a separate constructor method.
• Clearer Code: It’s easier to see what the class needs to be created.

Important Note

• If you use a primary constructor, the class won’t have a default (parameterless) constructor unless you add one yourself.

Some Limitations

• Primary constructor parameters are not stored as fields unless they are needed. This means they do not consume memory or affect the size of the type.
• Primary constructor parameters are not members of the type. This means you cannot access them using this or base or use them in attribute arguments or default parameter values.
• Primary constructor parameters can be assigned to them, but they are not properties. This means they do not have getters or setters, and they do not support attributes or modifiers.
• Primary constructor parameters are only available in the type where they are declared. This means derived types do not inherit them and are not visible to external callers.
• Every other constructor for a class must call the primary constructor, directly or indirectly, through this() constructor invocation. This ensures that primary constructor parameters are always assigned and available in the type.