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Strategy

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A route planner can optimize the same trip for time, distance, or toll cost. The navigation workflow stays put while the routing algorithm changes.

Strategy puts replaceable algorithms behind one contract. A context delegates the variable step to that contract, and a client or selection policy supplies the implementation. The context knows how to use an IShippingCostStrategy. It does not need the pricing rules for flat-rate or zone-based shipping. This boundary is useful when algorithms change independently of the workflow around them.

classDiagram
    class ShippingContext {
        -strategy IShippingCostStrategy
        +CalculateCost(order) decimal
    }
    class IShippingCostStrategy {

        +Calculate(order) decimal
    }
    class FlatRateStrategy {
        +Calculate(order) decimal
    }
    class WeightBasedStrategy {
        +Calculate(order) decimal
    }
    class ZoneBasedStrategy {
        +Calculate(order) decimal
    }
    ShippingContext o--> IShippingCostStrategy
    IShippingCostStrategy <|.. FlatRateStrategy
    IShippingCostStrategy <|.. WeightBasedStrategy
    IShippingCostStrategy <|.. ZoneBasedStrategy

Strategy vs State vs Command

Strategy varies an algorithm chosen for a context. Software Architecture/Patterns/Design Patterns/Behavioral/State models behavior that follows an internal mode and transition rules. Software Architecture/Patterns/Design Patterns/Behavioral/Command packages a request so it can be queued, logged, or invoked later.

Problem

ShippingService.CalculateCost() has an if/else chain — adding a new strategy means editing the method:

public class ShippingService
{
    // ⚠️ Adding "same-day delivery" requires editing this method
    public decimal CalculateCost(Order order, string strategy)
    {
        if (strategy == "flat_rate")
        {
            return 9.99m;
        }
        else if (strategy == "weight_based")
        {
            var weightKg = order.Items.Sum(i => i.Product.WeightKg * i.Quantity);
            return weightKg * 2.50m;
        }
        else if (strategy == "zone_based")
        {
            var zone = GetShippingZone(order.ShippingAddress);
            return zone switch { 1 => 5.99m, 2 => 9.99m, 3 => 14.99m, _ => 19.99m };
        }
        else if (strategy == "free" && order.Customer.Tier == CustomerTier.Gold)
        {
            return 0m;
        }
        // ⚠️ String comparison — typos cause silent failures
        throw new ArgumentException($"Unknown strategy: {strategy}");
    }
}

Adding same-day delivery changes the same branch that already carries every other pricing rule. Each new variant raises the chance of disturbing an existing one.

Solution

Each algorithm becomes a strategy class. The context selects the strategy via DI or a registry:

// Strategy interface
public interface IShippingCostStrategy
{
    int Priority { get; }
    decimal Calculate(Order order);
    bool AppliesTo(Order order); // ✅ strategy knows when it's applicable
}
 
// Concrete strategies
public class FlatRateStrategy : IShippingCostStrategy
{
    public int Priority => 0;
    public decimal Calculate(Order order) => 9.99m;
    public bool AppliesTo(Order order) => true; // always applicable as fallback
}
 
public class WeightBasedStrategy : IShippingCostStrategy
{
    private const decimal RatePerKg = 2.50m;
    public int Priority => 100;
 
    public decimal Calculate(Order order)
    {
        var totalWeightKg = order.Items.Sum(i => i.Product.WeightKg * i.Quantity);
        return Math.Max(totalWeightKg * RatePerKg, 4.99m); // minimum charge
    }
 
    public bool AppliesTo(Order order) =>
        order.Items.Any(i => i.Product.WeightKg > 0);
}
 
public class ZoneBasedStrategy(IZoneCalculator zoneCalc) : IShippingCostStrategy
{
    private static readonly Dictionary<int, decimal> ZoneRates = new()
        { [1] = 5.99m, [2] = 9.99m, [3] = 14.99m };
    public int Priority => 200;
 
    public decimal Calculate(Order order)
    {
        var zone = zoneCalc.GetZone(order.ShippingAddress);
        return ZoneRates.GetValueOrDefault(zone, 19.99m);
    }
 
    public bool AppliesTo(Order order) => order.ShippingAddress is not null;
}
 
public class FreeShippingStrategy : IShippingCostStrategy
{
    public int Priority => 400;
    public decimal Calculate(Order order) => 0m;
    public bool AppliesTo(Order order) =>
        order.Customer.Tier == CustomerTier.Gold || order.Total >= 100m;
}
 
// ✅ Adding same-day delivery = new class, zero changes to existing strategies
public class SameDayDeliveryStrategy : IShippingCostStrategy
{
    public int Priority => 300;
    public decimal Calculate(Order order) => 24.99m;
    public bool AppliesTo(Order order) =>
        order.ShippingAddress is { } address &&
        address.City == order.WarehouseCity &&
        DateTime.UtcNow.Hour < 14;
}
 
// Context — selects and applies the strategy
public class ShippingService(IEnumerable<IShippingCostStrategy> strategies)
{
    public decimal CalculateCost(Order order)
    {
        // ✅ Strategy selection via priority — no if/else
        var candidates = strategies
            .Where(s => s.AppliesTo(order))
            .OrderByDescending(s => s.Priority)
            .ToArray();
 
        var strategy = candidates.FirstOrDefault()
            ?? throw new InvalidOperationException("No shipping strategy applies");
 
        if (candidates.Skip(1).FirstOrDefault()?.Priority == strategy.Priority)
            throw new InvalidOperationException($"Multiple shipping strategies have priority {strategy.Priority}");
 
        return strategy.Calculate(order);
    }
}
 
// DI registration — adding a strategy = one new registration
builder.Services.AddScoped<IShippingCostStrategy, FreeShippingStrategy>();
builder.Services.AddScoped<IShippingCostStrategy, SameDayDeliveryStrategy>();
builder.Services.AddScoped<IShippingCostStrategy, ZoneBasedStrategy>();
builder.Services.AddScoped<IShippingCostStrategy, WeightBasedStrategy>();
builder.Services.AddScoped<IShippingCostStrategy, FlatRateStrategy>(); // fallback

The algorithm now lives in its own class. The selection policy still needs a deterministic precedence rule when several strategies apply. Extracting the calculations does not make that product decision disappear.

Familiar strategies

IComparer<T> supplies ordering behavior to sorting APIs. IComparable<T> is different: it places the default comparison on the value itself rather than injecting a separate strategy.

LINQ predicates pass filtering behavior as a delegate. A one-method strategy often needs no class at all.

JsonConverter<T> replaces the serialization behavior for a type while JsonSerializer keeps the surrounding pipeline.

IPasswordHasher<T> lets ASP.NET Core Identity depend on a password-hashing contract instead of one implementation.

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