ModelState

As explained in Building Web APIs with ASP.NET Core

ModelState is the ASP.NET Core representation of model-binding and validation state, including associated errors for an HTTP request.

ModelState is ASP.NET Core’s record of model-binding and validation results for an HTTP request. It becomes valid when no errors occur and invalid when binding or validation reports errors. section 6.2.1 section 6.4

Figure 6.3 Model binding and validation lifecycle with the [ApiController] attribute

Follow the request lifecycle from model binding through validation, automatic error handling, and action execution. section 6.2.1

With [ApiController], ModelState normally works automatically:

• Model binding converts request values into action parameters and complex types; binding failures produce an HTTP 400 response. section 6.2.1
• Validation checks built-in or custom validation attributes and IValidatableObject, recording the results in ModelState. section 6.2.1 section 6.4
• An invalid ModelState prevents the action method from running and returns HTTP 400 automatically. section 6.2.1

Customizing ModelState handling

Configure ApiBehaviorOptions with SuppressModelStateInvalidFilter = true when an endpoint must inspect invalid input itself. The action can then test ModelState.IsValid, create ValidationProblemDetails(ModelState), and return a status code such as HTTP 501 for an invalid pageSize. A ManualValidationFilterAttribute can apply the same opt-out selectively to a controller action while preserving [ApiController]. section 6.2.3 section 6.2.3 section 6.3.4

How Building Web APIs with ASP.NET Core covers it

How deep it goes

Explains the binding-validation lifecycle and manual configuration, but defers the internal structure of ModelState entries.

Examples

Uses custom validation messages, pageSize-specific HTTP 501 handling, and conditional HTTP 403 or HTTP 400 responses.

What it assumes

Assumes HTTP request handling and API actions, while introducing model binding, validation attributes, and automatic ApiController behavior.

What you can do after

Diagnose invalid request input automatically or configure manual inspection for customized validation responses.

How it connects

Connects model binding, data validation, validation attributes, IValidatableObject, ApiController, and HTTP error handling.

liveBookmanning.com

Read Building Web APIs with ASP.NET Core on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  ASP.NET Core in Action, 3eModelState is the ASP.NET Core request-state dictionary that stores bound values and validation errors associated with properties or the model.
• 

  liveBookmanning.com
  Pro ASP.NET Core 7, Tenth EditionModelStateDictionary is the ASP.NET Core MVC collection that stores bound model values and validation errors for request fields and properties.

Compare how 3 books explain ModelState →

Related concepts

• validation summaryused by · A validation summary is a UI component that displays collected validation error messages for multiple fields or controls in a form.
• manual model validationused by · Manual model validation is the explicit inspection of bound request data and validation errors within application code before producing a response.
• ModelState.AddModelErrorused by · ModelState.AddModelError is an ASP.NET Core MVC operation that adds a validation error associated with a model key or field.
• validationproduced by · The process of checking data, behavior, systems, models, or results against defined criteria to assess their correctness, suitability, or acceptability.

This explainer was generated by Manning's knowledge graph from the text of Building Web APIs with ASP.NET Core. The section links point to the parts of the book it draws on.