Model Validation: Controller-Based API vs Minimal API

This document highlights the key differences in model validation between Controller-Based APIs and Minimal APIs, focusing on how ModelState behaves.

Sample Model

public class Employee
{
    [Required(ErrorMessage = "ID is required.")]
    [Range(1, int.MaxValue, ErrorMessage = "ID must be greater than 0.")]
    public int Id { get; set; }

    [Required(ErrorMessage = "Name is required.")]
    public string? Name { get; set; }
}

Controller-Based API

In a Controller-Based API, ModelState is available and validation is automatic when using the [ApiController] attribute.

Example:

[ApiController]
[Route("api/[controller]")]
public class EmployeeController : ControllerBase
{
    [HttpPost]
    public IActionResult AddEmployee(Employee employee)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState); // Automatically populated.
        }

        return Ok(employee);
    }
}

Minimal API

In Minimal APIs, ModelState is not available. Validation must be performed manually using tools like Validator.TryValidateObject.

Example:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/employee", (Employee employee) =>
{
    var validationResults = new List<ValidationResult>();
    var validationContext = new ValidationContext(employee);

    if (!Validator.TryValidateObject(employee, validationContext, validationResults, true))
    {
        return Results.BadRequest(validationResults);
    }

    return Results.Ok(employee);
});

app.Run();

Comparison

Feature	Controller-Based API	Minimal API
ModelState Behavior	Available and automatic validation	Not available; manual validation needed
Ease of Use	Easier with [ApiController]	Requires custom validation logic

Summary

• Controller-Based API: Leverage automatic ModelState validation for simplicity.
• Minimal API: Handle validation manually as ModelState is not available.