C# Academy · Lesson

Capturing Parameters in Members

Use primary constructor parameters throughout the type.

Lesson 2 of 413 steps

Capturing Parameters in Members is a free C# Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Using Parameters in Members

The power of primary constructors comes from using the parameters inside members: methods, properties, and field initializers. The compiler arranges storage as needed.

using System;

class Multiplier(int factor) {
    public int Apply(int n) => n * factor;
}

Console.WriteLine(new Multiplier(3).Apply(5)); // 15

Capture vs Initializer-Only

If a parameter is used only in a field/property initializer, no extra storage is needed. If it is used in a method, the compiler captures it into a hidden field.

using System;

class A(int x) {
    public int Stored = x;     // initializer only
}
class B(int x) {
    public int Get() => x;     // captured into a field
}

Console.WriteLine(new A(7).Stored); // 7
Console.WriteLine(new B(9).Get());  // 9

Parameters Are Mutable

Primary constructor parameters behave like ordinary parameters: they are mutable within the instance unless you treat them as read-only. Mutating one changes the captured field.

using System;

class Toggle(bool on) {
    public void Flip() => on = !on;
    public bool State => on;
}

var t = new Toggle(false);
t.Flip();
Console.WriteLine(t.State); // True

Exposing as Read-Only Properties

A common pattern is to expose a parameter through a get-only property, giving a clean public surface while keeping the parameter internal.

using System;

class User(string email) {
    public string Email { get; } = email;
}

Console.WriteLine(new User("a@b.com").Email); // a@b.com

Computed Properties from Parameters

Parameters feed naturally into expression-bodied computed properties.

using System;

class Money(decimal amount, string currency) {
    public string Display => amount.ToString("0.00") + " " + currency;
}

Console.WriteLine(new Money(9.5m, "USD").Display); // 9.50 USD

Parameters in Multiple Methods

The same captured parameter can be shared by many methods, acting like a private field initialized at construction.

using System;

class Range2(int lo, int hi) {
    public bool Contains(int n) => n >= lo && n <= hi;
    public int Clamp(int n) => Math.Max(lo, Math.Min(hi, n));
}

var r = new Range2(0, 10);
Console.WriteLine(r.Contains(5)); // True
Console.WriteLine(r.Clamp(15));   // 10

Capturing Reference Types

Captured reference-type parameters let you store injected dependencies and call them from any method.

using System;
using System.Collections.Generic;

class Repo(List<string> store) {
    public void Add(string item) => store.Add(item);
    public int Count => store.Count;
}

var list = new List<string>();
var repo = new Repo(list);
repo.Add("x");
Console.WriteLine(repo.Count); // 1

Avoiding Accidental Double Storage

If you both assign a parameter to a field and use the parameter in methods, you may end up storing the value twice. Prefer one approach for clarity.

using System;

class Clean(int x) {
    // expose once via property, reference property in methods
    public int Value { get; } = x;
    public int Doubled() => Value * 2;
}

Console.WriteLine(new Clean(4).Doubled()); // 8

Parameters in Lambdas

Parameters can be captured by lambdas defined inside the class, just like fields.

using System;
using System.Collections.Generic;
using System.Linq;

class Filter(int threshold) {
    public IEnumerable<int> KeepAbove(IEnumerable<int> xs) =>
        xs.Where(n => n > threshold);
}

var f = new Filter(3);
Console.WriteLine(string.Join(",", f.KeepAbove(new[]{1,2,3,4,5}))); // 4,5

Scope Lifetime

A captured parameter lives as long as the object. It is effectively instance state, so do not assume it is reset between method calls.

using System;

class Accumulator(int seed) {
    private int total = seed;
    public int Add(int n) { total += n; return total; }
}

var a = new Accumulator(100);
Console.WriteLine(a.Add(1)); // 101
Console.WriteLine(a.Add(1)); // 102

Putting It Together

This service captures two dependencies and uses them across methods, showing the typical real-world shape.

using System;

class PriceService(decimal taxRate, string currency) {
    public decimal WithTax(decimal net) => net * (1 + taxRate);
    public string Format(decimal v) => v.ToString("0.00") + " " + currency;
}

var s = new PriceService(0.2m, "EUR");
Console.WriteLine(s.Format(s.WithTax(100m))); // 120.00 EUR

Quick Check

Confirm how primary constructor parameters are stored.

Answer it in the interactive lesson

Recap

You learned how primary constructor parameters are used in members.

Used in methods, parameters are captured into hidden fields.
Used only in initializers, no extra storage is needed.
Expose them via get-only properties for a clean surface.
They behave as instance state for the object lifetime.

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Frequently Asked Questions

Is the “Capturing Parameters in Members” lesson free?

Yes — the full text of “Capturing Parameters in Members” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Capturing Parameters in Members”?

Use primary constructor parameters throughout the type. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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