The Error

Your controller action receives null:

[HttpPost]
public IActionResult Create(UserDto dto) // dto is null
{
    return Ok(dto.Name); // NullReferenceException
}

Quick Fix - 1 Minute

Add [FromBody] and check JSON content-type.

[HttpPost]
public IActionResult Create([FromBody] UserDto dto) // Works
{
    return Ok(dto.Name);
}

And send request with header: Content-Type: application/json

Why This Happens in.NET 8

1. Missing [FromBody]: Complex types need explicit attribute in Minimal APIs

2. Wrong Content-Type: Sending text/plain or form-data when API expects JSON

3. Case mismatch: JSON { "name": "John" } won't bind to public string Name unless case-insensitive is on

Step-by-Step Debug

Check request: Use Postman. Body = raw JSON. Header = Content-Type: application/json
Add [FromBody]: Required for complex types in Controllers
Check property setters: public string Name { get; } won't work. Need { get; set; }
Enable logging: builder.Logging.AddConsole(); shows model binding errors

Real-World Scenario: Minimal API vs Controller Binding

.NET 8 Minimal APIs broke millions of apps. Binding rules changed:

// Controllers: Needs [FromBody] unless [ApiController]
// Controllers\UsersController.cs
[HttpPost]
public IActionResult Create([FromBody] UserDto dto) // Required without [ApiController]
{
    return Ok(dto.Name);
}
// Minimal API: [FromBody] is inferred, but only for complex types
// Program.cs
app.MapPost("/users", (UserDto dto) => // [FromBody] inferred automatically
{
return Results.Ok(dto.Name);
});
// BREAKS: Primitives are [FromRoute] by default in Minimal API
app.MapPost("/users/{id}", (int id, UserDto dto) => // id from route, dto from body
{
return Results.Ok();
});

Gotcha: In Minimal APIs, string parameters come from route/query. If you send { "name": "John" } to (string name), name is null. Use (UserDto dto) or ([FromBody] string name).

Related Fixes You Should Know

Model binding null usually means these issues too:

Minimal API 400 Bad Request - Your DTO has required properties but JSON is missing them..NET 8 returns 400 with ProblemDetails instead of null.
JSON Deserialization Error - Case sensitivity or property name mismatch. { "name": "John" } won't bind to public string Name without PropertyNameCaseInsensitive = true.
System.Text.Json Required Property -.NET 8 added required keyword support. Missing required field = model is null or 400 error.
Minimal API JSON Options Not Applied - You set JsonSerializerOptions in AddControllers() but Minimal API ignores it. Use ConfigureHttpJsonOptions.

FAQ

Q: Do I need [FromBody] in.NET 8 Minimal APIs?

No for complex types. Yes for string, int, etc. Minimal APIs infer [FromBody] for classes/records, but [FromRoute] or [FromQuery] for primitives.

Q: Why is my DTO null but ModelState.IsValid is true?

You're missing [ApiController]. Without it, validation doesn't run and binding failures silently give you null. Add [ApiController] to get automatic 400 errors.

Best Practice for.NET 8

Use [ApiController] on controller. It auto-adds [FromBody] inference:

[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    [HttpPost]
    public IActionResult Create(UserDto dto) // [FromBody] inferred
    {
        if (!ModelState.IsValid) return BadRequest(ModelState);
        return Ok();
    }
}

Fix: Model binding returns null in.NET 8 API