ASP.NET Core Web API Tutorial
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Tutorials ASP.NET Core Web API Tutorial

Fluent API Custom Validators in ASP.NET Core Web API — Complete Guide

Fluent API Custom Validators in ASP.NET Core Web API — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of ASP.NET Core Web API Tutorial on Toolliyo Academy.

8 min read Updated 6/25/2026
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ASP.NET Core Web API Tutorial · Lesson 121 of 175

Fluent API Custom Validators in ASP.NET Core Web API

Beginner ✓ → Intermediate ✓ → Advanced → Professional

Advanced · 3 — Security & patterns · ~10 min · Module 11: FluentValidation

What is this?

Fluent API Custom Validators in ASP.NET Core Web API validates DTOs with FluentValidation rules — clearer than giant DataAnnotation attributes for complex ShopNest orders.

Why should you care?

Async and conditional rules (e.g. GST for India) are painful with annotations alone.

See it live — copy this example

Create a Web API (dotnet new webapi), paste the example, run dotnet run, test in Swagger.

public class CreateOrderValidator : AbstractValidator<CreateOrderDto>
{
    public CreateOrderValidator() { RuleFor(x => x.Lines).NotEmpty(); }
}

Run Example »

Edit the code and click Run — like W3Schools Try it Yourself.

Code
Result

What happened?

  • Study the example, run dotnet run, and test in Swagger.
  • Fluent API Custom Validators in ASP.NET Core Web API connects to earlier modules in this course.

Try it yourself

  1. Read what Fluent API Custom Validators in ASP.NET Core Web API means for ShopNest.API.
  2. Type the example — do not only copy-paste.
  3. Test in Swagger or Postman.
  4. Change a route URL or DTO property and save — test again in Swagger or curl.
  5. Return the wrong status code on purpose (404 instead of 200) and see what the client shows.

Remember

You understand Fluent API Custom Validators in ASP.NET Core Web API in plain language. You traced or ran working C# in ShopNest.API. Move on when you can teach this topic to a friend.

ASP.NET Core Web API Tutorial
Course syllabus
Module 1: Introduction and Environment Setup Introduction to ASP.NET Core Framework — Complete Guide .NET Core vs .NET Framework Code Execution Process — Complete Guide ASP.NET Core Environment Setup — Complete Guide Download and Install .NET Core SDK — Complete Guide Download and Install Visual Studio 2022 — Complete Guide Download and Install Visual Studio Code — Complete Guide Download and Install SQL Server 2022 — Complete Guide Download and Install SSMS — Complete Guide Download and Install Postman and Fiddler — Complete Guide
Module 2: Web API Basics Introduction to ASP.NET Core Web API — Complete Guide HTTP (HyperText Transport Protocol) — Complete Guide Creating ASP.NET Core Web API Project using .NET Core CLI — Complete Guide How to Test ASP.NET Core Web API Using Postman — Complete Guide Creating ASP.NET Core Web API Project in Visual Studio — Complete Guide Default ASP.NET Core Web API Files and Folders — Complete Guide ASP.NET Core Web API Project File — Complete Guide ASP.NET Core Web API Main Method — Complete Guide Controllers in ASP.NET Core Web API — Complete Guide Models in ASP.NET Core Web API — Complete Guide Services in ASP.NET Core Web API — Complete Guide ASP.NET Core Web API AppSettings.json file — Complete Guide ASP.NET Core In-Process Hosting Model — Complete Guide ASP.NET Core Kestrel Web Server — Complete Guide ASP.NET Core Out of Process Hosting Model — Complete Guide Hosting Model Interview Questions and Answers — Complete Guide Dependency Injection in ASP.NET Core Web API — Complete Guide Singleton vs Scoped vs Transient in ASP.NET Core Web API — Complete Guide Swagger API in ASP.NET Core Web API — Complete Guide
Module 3: Routing Routing in ASP.NET Core Web API — Complete Guide Route Parameters and Query Strings in Routing — Complete Guide Multiple URLs for a Single Resource in ASP.NET Core Web API — Complete Guide Token Replacement in ASP.NET Core Web API — Complete Guide Route Prefix in ASP.NET Core Web API — Complete Guide Route Constraints in ASP.NET Core Web API — Complete Guide How Routing Works in ASP.NET Core Web API — Complete Guide
Module 4: Return Types and Status Codes Action Return Types in ASP.NET Core Web API — Complete Guide HTTP Status Codes in ASP.NET Core Web API — Complete Guide 200 HTTP Status Code in ASP.NET Core Web API — Complete Guide 201 HTTP Status Code in ASP.NET Core Web API — Complete Guide 202 HTTP Status Code in ASP.NET Core Web API — Complete Guide 204 HTTP Status Code in ASP.NET Core Web API — Complete Guide 301 HTTP Status Code in ASP.NET Core Web API — Complete Guide 302 HTTP Status Code in ASP.NET Core Web API — Complete Guide 304 HTTP Status Code in ASP.NET Core Web API — Complete Guide 400 HTTP Status Code in ASP.NET Core Web API — Complete Guide 401 HTTP Status Code in ASP.NET Core Web API — Complete Guide 403 HTTP Status Code in ASP.NET Core Web API — Complete Guide 404 HTTP Status Code in ASP.NET Core Web API — Complete Guide 405 HTTP Status Code in ASP.NET Core Web API — Complete Guide Configure Allowed HTTP Methods Globally in ASP.NET Core Web API — Complete Guide 500 HTTP Status Code in ASP.NET Core Web API — Complete Guide 501 HTTP Status Code in ASP.NET Core Web API — Complete Guide 503 HTTP Status Code in ASP.NET Core Web API — Complete Guide 504 HTTP Status Code in ASP.NET Core Web API — Complete Guide
Module 5: Model Binding Model Binding in ASP.NET Core Web API — Complete Guide Model Binding using FromForm in ASP.NET Core Web API — Complete Guide Model Binding using FromQuery in ASP.NET Core Web API — Complete Guide Model Binding Using FromRoute in ASP.NET Core Web API — Complete Guide Model Binding Using FromHeader in ASP.NET Core Web API — Complete Guide Model Binding Using FromBody in ASP.NET Core Web API — Complete Guide Custom Model Binding in ASP.NET Core Web API — Complete Guide How to Apply Binding Attributes to Model Properties in ASP.NET Core Web API — Complete Guide Content Negotiation in ASP.NET Core Web API — Complete Guide Produces and Consumes Attributes in ASP.NET Core Web API — Complete Guide Exclude Properties from Model Binding in ASP.NET Core Web API — Complete Guide Validation using Data Annotations in ASP.NET Core Web API — Complete Guide
Module 6: Entity Framework Core A Brief Introduction to Entity Framework Core — Complete Guide Entity Framework Core in ASP.NET Core Web API with SQL Server — Complete Guide Entity States in Entity Framework Core — Complete Guide Relationships in Entity Framework Core — Complete Guide Lazy Loading in Entity Framework Core — Complete Guide Eager Loading in Entity Framework Core — Complete Guide Explicit Loading in Entity Framework Core — Complete Guide Default Conventions in EF Core — Complete Guide Data Annotation Attributes in EF Core — Complete Guide Fluent API Configurations in EF Core — Complete Guide Fluent API Global Configurations in EF Core — Complete Guide Fluent API Entity Configurations in EF Core — Complete Guide Fluent API Property Configuration in EF Core — Complete Guide EF Core Inheritance — Complete Guide EF Core Transactions — Complete Guide Entity Framework Core with Stored Procedures — Complete Guide EF Core Database First Approach — Complete Guide
Module 7: AutoMapper and Mapperly Automapper in ASP.NET Core Web API — Complete Guide Automapper Complex Type Mapping in ASP.NET Core Web API — Complete Guide Mapping Complex Type to Primitive Type using AutoMapper in ASP.NET Core Web API — Complete Guide Automapper Reverse Mapping in ASP.NET Core Web API — Complete Guide Automapper Conditional Mapping in ASP.NET Core Web API — Complete Guide Ignore Property Mapping using Automapper in ASP.NET Core Web API — Complete Guide Automapper Null Substitution in ASP.NET Core Web API — Complete Guide AutoMapper Advanced Concepts in ASP.NET Core Web API — Complete Guide AutoMapper Real-time Example in ASP.NET Core Web API — Complete Guide Mapperly in ASP.NET Core Web API — Complete Guide Mapperly Real-time Example in ASP.NET Core Web API — Complete Guide
Module 8: HTTP Methods HTTP Methods in ASP.NET Core Web API — Complete Guide HTTP GET Method in ASP.NET Core Web API — Complete Guide HTTP Post Method in ASP.NET Core Web API — Complete Guide HTTP PUT Method in ASP.NET Core Web API — Complete Guide HTTP PATCH Method in ASP.NET Core Web API — Complete Guide HTTP DELETE Method in ASP.NET Core Web API — Complete Guide HTTP HEAD Method in ASP.NET Core Web API — Complete Guide HTTP OPTIONS Method in ASP.NET Core Web API — Complete Guide
Module 9: Logging Logging in ASP.NET Core Web API — Complete Guide Custom Logging Provider in ASP.NET Core Web API — Complete Guide How to Implement Serilog in ASP.NET Core Web API — Complete Guide Serilog, Elasticsearch, and Kibana in ASP.NET Core Web API — Complete Guide How to Implement Logging using nLog in ASP.NET Core Web API — Complete Guide Logging to Database using NLog in ASP.NET Core Web API — Complete Guide Serilog vs NLog in ASP.NET Core Web API — Complete Guide
Module 10: Caching Caching in ASP.NET Core Web API — Complete Guide In-Memory Caching in ASP.NET Core Web API — Complete Guide Custom In-Memory Cache in ASP.NET Core — Complete Guide How to Implement Redis Cache in ASP.NET Core — Complete Guide In-Memory Caching vs Distributed Caching in ASP.NET Core — Complete Guide NCache Cluster and Its Types in Distributed Caching — Complete Guide How to Download and Install NCache in Windows — Complete Guide How to Implement NCache in ASP.NET Core Web API — Complete Guide Response Caching in ASP.NET Core Web API — Complete Guide
Module 11: FluentValidation Fluent API Validation in ASP.NET Core Web API — Complete Guide Fluent API Async Validators in ASP.NET Core Web API — Complete Guide Fluent API Custom Validators in ASP.NET Core Web API — Complete Guide Fluent API Conditional Validations in ASP.NET Core Web API — Complete Guide Validate Nested Complex or Collection Property using Fluent API — Complete Guide
Module 12: Filters Filters in ASP.NET Core Web API — Complete Guide Authorization Filters in ASP.NET Core Web API — Complete Guide Custom Authorization Filter in ASP.NET Core Web API — Complete Guide Resource Filters in ASP.NET Core Web API — Complete Guide Action Filters in ASP.NET Core Web API — Complete Guide Result Filters in ASP.NET Core Web API — Complete Guide Exception Filters in ASP.NET Core Web API — Complete Guide ServiceFilter vs TypeFilter in ASP.NET Core Web API — Complete Guide ASP.NET Core Request Processing Life Cycle — Complete Guide
Module 13: Security How to Store Password in Hash Format in ASP.NET Core Web API — Complete Guide HMAC Authentication in ASP.NET Core Web API — Complete Guide Encryption and Decryption in ASP.NET Core Web API — Complete Guide Asymmetric Encryption in ASP.NET Core Web API — Complete Guide Authentication and Authorization in Web APIs — Complete Guide Basic Authentication in ASP.NET Core Web API — Complete Guide Role-Based Basic Authentication in ASP.NET Core Web API — Complete Guide JWT Authentication in ASP.NET Core — Complete Guide CORS in ASP.NET Core Web API — Complete Guide JWT – ASP.NET Core Web API — Complete Guide JWT Authentication in ASP.NET Core Web API — Complete Guide Resource Server and Client Applications with JWT Authentication — Complete Guide Refresh Token in ASP.NET Core Web API using JWT Authentication — Complete Guide Revoke Refresh Tokens in JWT-Based Token Authentication — Complete Guide Role Based JWT Authentication in ASP.NET Core Web API — Complete Guide SSO Implementation — Complete Guide SSO Authentication in ASP.NET Core Web API — Complete Guide Authentication Server using ASP.NET Core Web API — Complete Guide Resource Server using ASP.NET Core Web API — Complete Guide Client Application One using ASP.NET Core MVC — Complete Guide Client Application Two using ASP.NET Core MVC — Complete Guide
Module 14: API Versioning ASP.NET Core Web API Versioning — Complete Guide ASP.NET Core Web API Versioning using Query String — Complete Guide ASP.NET Core Web API Versioning using URL Path — Complete Guide Header Versioning in ASP.NET Core Web API — Complete Guide Media Type Versioning in ASP.NET Core Web API — Complete Guide
Module 15: Repository Pattern Introduction to Repository Pattern in ASP.NET Core Web API — Complete Guide Start Without Using Repository Pattern in ASP.NET Core Web API — Complete Guide Implementing Non-Generic Repository Pattern in ASP.NET Core Web API — Complete Guide Implementing Generic Repository Pattern in ASP.NET Core Web API — Complete Guide Combining Generic and Non-Generic Repositories in ASP.NET Core Web API — Complete Guide Unit of Work with Repository Pattern in ASP.NET Core Web API — Complete Guide
Module 16: E-Commerce Real-Time Application Real-time Ecommerce Application using ASP.NET Core Web API and EF Core — Capstone Customer Module in ECommerce Application — ShopNest.API Address Module in ECommerce Application — ShopNest.API Category Module in ECommerce Application — ShopNest.API Product Module in ECommerce Application — ShopNest.API Shopping Cart Module in ECommerce Application — ShopNest.API Order Module in ECommerce Application — ShopNest.API Payment Module in ECommerce Application — ShopNest.API Cancellation Module in ECommerce Application — ShopNest.API Refund Module in Ecommerce Application — ShopNest.API User Feedback Module in Ecommerce Application — ShopNest.API
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