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Lesson 5418 min read

Model Binding & Validation

Learn how ASP.NET Core automatically binds request data to C# objects, and how to validate it with data annotations.

What is Model Binding?

Model binding automatically maps incoming request data (route values, query strings, form fields, or JSON body) into strongly typed C# parameters and objects — you rarely need to parse raw request data by hand.

Binding Sources

AttributeSource
[FromRoute]URL route segments
[FromQuery]Query string (?key=value)
[FromBody]The JSON request body
[FromForm]Submitted form data
[HttpPost]
public IActionResult Create([FromBody] TaskDto dto)
{
    // dto is automatically populated from the JSON body
    return Ok(dto);
}

Validation with Data Annotations

Data annotation attributes describe validation rules directly on your model, and ASP.NET Core checks them automatically during model binding.

public class TaskDto
{
    [Required]
    [StringLength(100, MinimumLength = 3)]
    public string Title { get; set; } = "";


    [Range(1, 5)]
    public int Priority { get; set; }
}

Checking ModelState

[HttpPost]
public IActionResult Create([FromBody] TaskDto dto)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }


    return Ok("Task created!");
}
[ApiController] Simplifies This

Controllers marked with [ApiController] automatically return a 400 Bad Request with validation errors if the model is invalid — you often don't need to check ModelState.IsValid manually in a Web API.

FAQs

Yes — implement IValidatableObject on your model, or create a custom ValidationAttribute for reusable rules.

Summary

  • Model binding automatically populates C# objects from request data.
  • Data annotations (Required, StringLength, Range) declare validation rules on the model.
  • [ApiController] automatically handles invalid models with a 400 response.
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Layouts & Partial Views