ISP Common Mistakes — Complete Guide

Introduction

ISP Common Mistakes — Complete Guide is essential for .NET developers building ShopNest Enterprise Clean Architecture Platform — Toolliyo's 100-article SOLID Design Principles master path covering SRP, OCP, LSP, ISP, DIP, refactoring, Clean Architecture, and enterprise projects. Every article includes minimum 2 detailed enterprise real-world examples with bad code before good code (banking, e-commerce, ERP, SaaS, healthcare).

In Indian delivery projects (TCS, Infosys, Wipro), interviewers expect isp common mistakes with real HDFC-style banking SRP fixes, Flipkart OCP payment strategies, TCS ERP LSP, Freshworks ISP, or Apollo hospital refactoring examples — not toy animal demos. This article delivers two mandatory enterprise examples on Audit Logs.

After this article you will

• Explain ISP Common Mistakes in plain English and in SOLID principles and maintainable OOP terms
• Implement isp common mistakes in ShopNest Enterprise Clean Architecture Platform (Audit Logs)
• Compare the wrong approach vs the production-ready enterprise approach
• Answer fresher, mid-level, and senior SOLID principles and clean architecture interview questions confidently
• Connect this lesson to Article 49 and the 100-article SOLID Principles roadmap

Prerequisites

• Software: .NET 8 SDK, VS 2022 or VS Code, SQL Server Express / LocalDB
• Knowledge: C# basics
• Previous: Article 47 — ISP in ASP.NET Core — Complete Guide
• Time: 24 min reading + 30–45 min hands-on

Concept deep-dive

Level 1 — Analogy

ISP is like menu sections — vegetarians should not need to read the steak page to order salad.

Level 2 — Technical

ISP Common Mistakes integrates with the LINQ query layer: write queries against IEnumerable or IQueryable, understand deferred execution, project to DTOs for ShopNest Clean Architecture reports. On ShopNest Clean Architecture this powers Audit Logs without coupling UI to database internals.

Level 3 — Architecture

[Browser] → [HTTPS/Kestrel] → [Middleware Pipeline]
  → [Routing] → [Controller Action] → [Service Layer]
  → [EF Core / Identity] → [Razor View Engine] → [HTML Response]

Project structure

ShopNest Clean Architecture/
├── ShopNest Clean Architecture/
├── src/
│   ├── ShopNest Clean Architecture.Api/       ← ASP.NET Core Web API
│   ├── ShopNest Clean Architecture.Core/      ← Repository interfaces
│   ├── ShopNest Clean Architecture.AdoNet/    ← SqlConnection, SPs, transactions
│   ├── ShopNest Clean Architecture.Reports/   ← Streaming readers, GL reports
│   └── ShopNest Clean Architecture.Tests/     ← Integration tests (Testcontainers SQL)
├── sql/
│   ├── migrations/
│   └── stored-procedures/
└── docker-compose.yml                 ← SQL Server 2022 + Redis

Step-by-Step Implementation — ShopNest (Audit Logs)

Follow the prompt template: create project → core classes → interfaces → pattern implementation → client code → run → enterprise refactor.

Step 1 — The wrong way

// ❌ BAD — god class violates SRP, tight coupling, untestable
public class OrderService {
    public void PlaceOrder(Order o) {
        Validate(o);
        _context.Orders.Add(o);
        _context.SaveChanges();
        SendEmail(o.CustomerEmail);
        GenerateInvoicePdf(o);
    }
}

Step 2 — The right way

// ✅ CORRECT — ISP Common Mistakes on ShopNest (Audit Logs) — SOLID applied
public sealed class PlaceOrderHandler(IOrderRepository repo, INotificationService notify)
    : IRequestHandler
{
    public async Task Handle(PlaceOrderCommand cmd, CancellationToken ct) {
        var order = Order.Create(cmd.CustomerId, cmd.Items);
        await repo.AddAsync(order, ct);
        await notify.OrderPlacedAsync(order, ct);
        return Result.Success(order.Id);
    }
}

Step 3 — Apply ISP Common Mistakes

public interface IProductRepository { Task<IReadOnlyList<Product>> GetAllAsync(); }
public class ProductRepository : IProductRepository { /* EF Core */ }

dotnet run --project ShopNest Clean Architecture.Api
# Verify ISP Common Mistakes — check Swagger UI and unit test pass rate and integration tests pass

The problem before SOLID

Without SOLID, ShopNest teams hit: tight coupling, god classes, untestable controllers, merge conflicts, and fear of refactoring. Indian IT projects (TCS, Infosys, Wipro) lose sprints when legacy code has no clear boundaries.

• Tight coupling — change SMS provider, break ledger posting
• Testing difficulty — cannot mock database from controller
• Scalability — monolith teams block each other
• Bug-prone — one class, five reasons to change

Real-World Example 1 — Freshworks CRM — ISP on Fat ICustomerService

MANDATORY enterprise scenario (SaaS CRM): ISP Common Mistakes applied in ShopNest Clean Architecture Audit Logs.

Business problem

ICustomerService had 18 methods; read-only report API was forced to implement DeleteCustomer and MergeDuplicates. ISP split into ICustomerReader, ICustomerWriter, ICustomerAdmin.

Before SOLID — bad design

public interface ICustomerService {
    Customer Get(int id); void Create(Customer c); void Delete(int id);
    void Merge(int a, int b); byte[] ExportPdf(); /* 13 more... */
}

After SOLID — production design

public interface ICustomerReader { Customer Get(int id); IReadOnlyList<Customer> Search(string q); }
public interface ICustomerWriter { void Create(Customer c); void Update(Customer c); }
public interface ICustomerAdmin { void Delete(int id); void Merge(int a, int b); }

Outcome

Report microservice depends only on ICustomerReader — security audit passed least-privilege review.

Real-World Example 2 — Flipkart Checkout — OCP with Payment Strategies

MANDATORY enterprise scenario (E-Commerce): ISP Common Mistakes applied in ShopNest Clean Architecture Audit Logs.

Business problem

Checkout had if/else for UPI, card, COD, wallet — every new payment method required editing CheckoutService. OCP: IPaymentStrategy + new strategy class per method; open for extension, closed for modification.

Before SOLID — bad design

public void Pay(Order order, string method) {
    if (method == "UPI") { /* 50 lines */ }
    else if (method == "CARD") { /* 60 lines */ }
    else if (method == "COD") { /* 40 lines */ }
}

After SOLID — production design

public interface IPaymentStrategy {
    Task<PaymentResult> PayAsync(Order order, CancellationToken ct);
}
public class UpiPaymentStrategy : IPaymentStrategy { /* ... */ }
public class CheckoutService(IPaymentStrategyFactory factory) {
    public Task PayAsync(Order o, PaymentMethod m, CancellationToken ct) =>
        factory.Get(m).PayAsync(o, ct);
}

Outcome

BNPL and EMI added in 2 sprints without touching CheckoutService core — zero regression on existing methods.

SOLID in ASP.NET Core — ISP Common Mistakes

Register abstractions in Program.cs as Scoped. Keep controllers thin — delegate to MediatR handlers or application services. ShopNest Clean Architecture: Domain → Application → Infrastructure → Api.

builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(PlaceOrderHandler).Assembly));

When NOT to over-apply SOLID

Small console tools, one-off scripts, and prototypes do not need five interfaces. Apply SOLID when team size, codebase lifetime, and change frequency justify abstraction cost.

SOLID and design patterns

SRP enables focused classes; OCP pairs with Strategy and Factory; LSP guards inheritance; ISP splits fat interfaces; DIP powers DI and Repository pattern. SOLID is the foundation — patterns are the tools.

Unit testing with SOLID

var mock = new Mock<IOrderRepository>();
mock.Setup(r => r.GetAsync(1, default)).ReturnsAsync(new Order(1, 100m));
var handler = new GetOrderHandler(mock.Object);
var result = await handler.Handle(new GetOrderQuery(1), default);
Assert.Equal(100m, result.Total);

Common errors & fixes

🔴 Mistake 1: God classes with 10+ responsibilities (SRP violation)
✅ Fix: Extract focused services — one reason to change per class.

🔴 Mistake 2: Adding if/else chains for every new feature (OCP violation)
✅ Fix: Use Strategy or Factory; extend via new classes, not edits.

🔴 Mistake 3: Subclass throws NotImplementedException (LSP violation)
✅ Fix: Prefer composition and role-specific interfaces over broken inheritance.

🔴 Mistake 4: Controllers new-ing concrete repositories (DIP violation)
✅ Fix: Inject interfaces via constructor DI in ASP.NET Core.

Best practices

• 🟢 Use async/await end-to-end for database and I/O calls
• 🟢 Register DbContext as Scoped; avoid capturing it in singletons
• 🟡 Use IQueryable until the last moment; avoid multiple enumeration; project with Select before ToList
• 🟡 Prefer method syntax for complex chains; use query syntax for joins when readability wins
• 🔴 Log structured data with Serilog — include OrderId, UserId, not passwords
• 🔴 Use HTTPS, secure cookies, and authorization policies in production

Interview questions

Fresher level

Q1: What is ISP Common Mistakes in ASP.NET Core MVC?
A: ISP Common Mistakes is a core MVC capability used in ShopNest Clean Architecture for Audit Logs. Explain in one sentence, then describe controller/view/service placement.

Q2: How would you implement ISP Common Mistakes on a TCS-style delivery project?
A: Deferred execution, IQueryable pipelines, Select projection, Skip/Take pagination, and SQL logging in development.

Q3: IEnumerable vs IQueryable — when to use which?
A: IEnumerable for in-memory collections; IQueryable for EF Core database queries that translate to SQL.

Mid / senior level

Q4: Explain LINQ deferred execution and query translation briefly.
A: LINQ → Expression Tree → IQueryProvider → SQL (EF) or Iterator (in-memory) → Results.

Q5: Common production mistake with this topic?
A: Skipping validation, exposing secrets in Git, or untested edge cases (null model, unauthorized user).

Q6: .NET LINQ vs SQL — when to push logic to database?
A: Core is cross-platform, faster, cloud-ready; Framework is maintenance mode on Windows/IIS.

Coding round

Implement ISP Common Mistakes for ShopNest Audit Logs: show interface, concrete class, DI registration, and xUnit test with mock.

public class ISPCommonMistakesPatternTests
{
    [Fact]
    public async Task ExecuteAsync_ReturnsSuccess()
    {
        var mock = new Mock();
        mock.Setup(s => s.ExecuteAsync(It.IsAny(), default))
            .ReturnsAsync(Result.Success("test-id"));
        var result = await mock.Object.ExecuteAsync(new Request("test-id"));
        Assert.True(result.IsSuccess);
    }
}

Summary & next steps

• Article 48: ISP Common Mistakes — Complete Guide
• Module: Module 5: Interface Segregation Principle (ISP) · Level: INTERMEDIATE
• Applied to ShopNest Clean Architecture — Audit Logs

Previous: ISP in ASP.NET Core — Complete Guide
Next: ISP Performance Considerations — Complete Guide

Practice: Add one small feature using today's pattern — commit with feat(solid): article-48.

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