How I replaced if statements with a Dictionary delegate in C#

Let's say you need to implement a feature that returns a different package based on the user-provided coupon code.

So you start with a model:

public record Package
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
}

And you write a function that returns a different package based on the coupon code:

private static Package GetPackageFromCoupon(string coupon)
{
    if (coupon == "ABC")
    {
        return new Package { Id = 1, Name = "PS5 Controller", Price = 50.00 };
    }
    if (coupon == "EBC")
    {
        return new Package { Id = 2, Name = "Iphone X", Price = 200.00 };
    }
    if (coupon == "DDD")
    {
        return new Package { Id = 3, Name = "X7 Mouse", Price = 20.00 };
    }
    return new Package
    {
        Id = 1000,
        Name = "Soda",
        Price = 1.00
    };
}

And invoke it from your main method:

internal class Program
{
    static void Main(string[] args)
    {
        var package = GetPackageFromCoupon("ABC");
        Console.WriteLine(package);
        Console.ReadLine();

    }
}

Quick test

Provide expected parameters and inspect the results.

"ABC" => Package { Id = 1, Name = PS5 Controller, Price = 50 }
"EBC" => Package { Id = 2, Name = Iphone X, Price = 200 }
"DDD" => Package { Id = 3, Name = X7 Mouse, Price = 20 }

Works as expected. Also, if you enter something that doesn't exist:

"a" => Package { Id = 1000, Name = Soda, Price = 1 }

The Problem

What if you need to add more coupon codes and return different variations of the Package object? Well, it's gonna get pretty messy very soon.

Quick solution - Dictionary

Rather than writing every possible variation in the if block, create a dictionary where the key is the coupon code and the value is the Package:

private static readonly Dictionary<string, Package> _packages = new()
{
    ["ABC"] = new Package { Id = 1, Name = "PS5 Controller", Price = 50.00 },
    ["EBC"] = new Package { Id = 2, Name = "Iphone X", Price = 200.00 },
    ["DDD"] = new Package { Id = 3, Name = "X7 Mouse", Price = 20.00 },
};

The next step is to enter the coupon code and either read the value from the dictionary or return a default value.

private static Package GetPackageFromCoupon(string coupon)
{
    var isExistingPackage = _packages.TryGetValue(coupon, out var package);

    if (isExistingPackage)
    {
        return package;
    }

    return new Package
    {
        Id = 1000,
        Name = "Soda",
        Price = 1.00
    };
}

Run the program, and you'll get back the same result!

But why a dictionary? Why not an array or a list? The answer is Big O notation. Reading an item by key is immensely faster than querying a collection. Dictionary lookup is O(1), while list search is O(n).

Let's spice things up a bit.

What if the output varies?

Change request comes in. You need to introduce seasons. If it's the holiday season, the price should be lower.

You need to create a function that takes the season as a parameter and adjusts the price accordingly. To keep it simple, add static methods in the Package class:

public record Package
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }

    public static Package PS5Controller(bool isHolidaySeason)
    {
        return new Package { 
            Id = 1, 
            Name = "PS5 Controller", 
            Price = isHolidaySeason ? 20.00 : 50.00
        };
    }

    public static Package IphoneX(bool isHolidaySeason)
    {
        return new Package
        {
            Id = 2,
            Name = "Iphone X",
            Price = isHolidaySeason ? 50.00 : 200.00
        };
    }

    public static Package X7Mouse(bool isHolidaySeason)
    {
        return new Package
        {
            Id = 3,
            Name = "X7 Mouse",
            Price = isHolidaySeason ? 10.00 : 20.00
        };
    }

    public static Package DefaultPackage(bool isHolidaySeason)
    {
        return new Package
        {
            Id = 1000,
            Name = "Soda",
            Price = isHolidaySeason ? 0.00 : 1.00
        };
    }

}

But how do you call this function the dictionary? How do you pass that parameter?

private static readonly Dictionary<string, Package> _packages = new()
{
    ["ABC"] = Package.PS5Controller(), // ?
};

The Func Delegate

The Func<T, TResult> is a built-in generic delegate that encapsulates a method (that takes between 0 and 16 input parameters) and returns a value.

So instead of Dictionary<string, Package>, create a
Dictionary<string, Func<bool, Package>> where the bool is the input parameter (holiday season) and the Package is the output (method that returns a particular package).

private static readonly Dictionary<string, Func<bool, Package>> _packages = new()
{
    ["ABC"] = (isHoliday) => Package.PS5Controller(isHoliday),
    ["EBC"] = (isHoliday) => Package.IphoneX(isHoliday),
    ["DDD"] = (isHoliday) => Package.X7Mouse(isHoliday),
};

This can be further simplified:

private static readonly Dictionary<string, Func<bool, Package>> _packages = new()
{
    ["ABC"] = Package.PS5Controller,
    ["EBC"] = Package.IphoneX,
    ["DDD"] = Package.X7Mouse,
};

This works because C# allows method groups to be assigned to delegates when the signatures match.

The last piece of the puzzle is to adapt the GetPackageFromCoupon() method to also accept the holiday season:

private static Package GetPackageFromCoupon(string coupon, bool isHoliday)
{
    var isExistingPackage = _packages.TryGetValue(coupon, out var packageFactory);

    if (isExistingPackage)
    {
        return packageFactory(isHoliday);
    }

    return Package.DefaultPackage(isHoliday);
}

Clarifications

This time around, the Dictionary returns a function (using a coupon code):

var isExistingPackage = _packages.TryGetValue(coupon, out var packageFactory);

The function is invoked by passing the isHoliday parameter:

return packageFactory(isHoliday);

The packageFactory() is a pointer to the function returned from the Dictionary. It can be any of the three specified functions.

Lastly, if there is no match, return a default value:

return Package.DefaultPackage(isHoliday);

Final test

Invoke the function with different variants:

static void Main(string[] args)
{

    var isHolidaySeason = true;
    var package = GetPackageFromCoupon("ABC", isHolidaySeason);

    Console.WriteLine(package);
    Console.ReadLine();

}

• During the holiday season:

Package { Id = 1, Name = PS5 Controller, Price = 20 }

• When not:

Package { Id = 1, Name = PS5 Controller, Price = 50 }

This also works for the default scenario:

var isHolidaySeason = true;
var package = GetPackageFromCoupon("aaaa", isHolidaySeason);

// Package { Id = 1000, Name = Soda, Price = 0 }

Full Example (Program.cs)

using CSharpPractice.Coupons;

namespace CSharpPractice
{
    internal class Program
    {
        static void Main(string[] args)
        {

            var isHolidaySeason = true;
            var package = GetPackageFromCoupon("ABC", isHolidaySeason);

            Console.WriteLine(package);
            Console.ReadLine();
        }

        private static Package GetPackageFromCoupon(string coupon, bool isHoliday)
        {
            var isExistingPackage = _packages.TryGetValue(coupon, out var packageFactory);

            if (isExistingPackage)
            {
                return packageFactory(isHoliday);
            }

            return Package.DefaultPackage(isHoliday);
        }

        private static readonly Dictionary<string, Func<bool, Package>> _packages = new()
        {
            ["ABC"] = Package.PS5Controller,
            ["EBC"] = Package.IphoneX,
            ["DDD"] = Package.X7Mouse,
        };


    }

}

Summary

This approach also aligns with the Open–Closed Principle: the code is open for extension (add new coupon codes or pricing rules) but closed for modification (no need to touch existing logic).
Adding a new coupon doesn't require rewriting method conditions. Just one dictionary entry.

Until next time 👋