A 400 status code is returned when the product description contains "XYZ Widget". The BadRequest convenience method is invoked as shorthand for return new BadRequestResult(); .

A 201 status code is generated by the CreatedAtAction convenience method when a product is created. The following code is an alternative to calling CreatedAtAction :

return new CreatedAtActionResult(nameof(CreateAsync), 
                                "Products", 
                                new { id = product.Id }, 
                                product);
In the preceding code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
For example, the following model indicates that requests must include the Name and Description properties. Failure to provide Name and Description in the request causes model validation to fail.
public class Product
    public int Id { get; set; }
    [Required]
    public string Name { get; set; } = string.Empty;
    [Required]
    public string Description { get; set; } = string.Empty;
    public bool IsOnSale { get; set; }
If the [ApiController] attribute is applied, model validation errors result in a 400 status code. For more information, see Automatic HTTP 400 responses.
ActionResult vs IActionResult
The following section compares ActionResult to  IActionResult
ActionResult<T> type
ASP.NET Core includes the ActionResult<T> return type for web API controller actions. It enables returning a type deriving from ActionResult or return a specific type. ActionResult<T> offers the following benefits over the IActionResult type:
The [ProducesResponseType] attribute's Type property can be excluded. For example, [ProducesResponseType(200, Type = typeof(Product))] is simplified to [ProducesResponseType(200)]. The action's expected return type is inferred from the T in ActionResult<T>.
Implicit cast operators support the conversion of both T and ActionResult to ActionResult<T>. T converts to ObjectResult, which means return new ObjectResult(T); is simplified to return T;.
C# doesn't support implicit cast operators on interfaces. Consequently, conversion of the interface to a concrete type is necessary to use ActionResult<T>. For example, use of IEnumerable in the following example doesn't work:
[HttpGet]
public ActionResult<IEnumerable<Product>> Get() =>
    _repository.GetProducts();
One option to fix the preceding code is to return _repository.GetProducts().ToList();.
Most actions have a specific return type. Unexpected conditions can occur during action execution, in which case the specific type isn't returned. For example, an action's input parameter may fail model validation. In such a case, it's common to return the appropriate ActionResult type instead of the specific type.
Synchronous action
Consider a synchronous action in which there are two possible return types:
[HttpGet("{id}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public ActionResult<Product> GetById_ActionResultOfT(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? NotFound() : product;
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist.
Asynchronous action
Consider an asynchronous action in which there are two possible return types:
[HttpPost()]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<ActionResult<Product>> CreateAsync_ActionResultOfT(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    return CreatedAtAction(nameof(CreateAsync_ActionResultOfT), new { id = product.Id }, product);
In the preceding action:
A 400 status code (BadRequest) is returned by the ASP.NET Core runtime when:
The [ApiController] attribute has been applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the CreatedAtAction method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
HttpResults type
In addition to the MVC-specific built-in result types (IActionResult and ActionResult<T>), ASP.NET Core includes the HttpResults types that can be used in both Minimal APIs and Web API.
Different than the MVC-specific result types, the HttpResults:
Are a results implementation that is processed by a call to IResult.ExecuteAsync.
Does not leverage the configured Formatters. Not leveraging the configured formatters means:
Some features like Content negotiation aren't available.
The produced Content-Type is decided by the HttpResults implementation.
The HttpResults can be useful when sharing code between Minimal APIs and Web API.




    

IResult type
The Microsoft.AspNetCore.Http.HttpResults namespace contains classes that implement the IResult interface. The IResult interface defines a contract that represents the result of an HTTP endpoint. The static Results class is used to create varying IResult objects that represent different types of responses.
The Built-in results table shows the common result helpers.
Consider the following code:
[HttpGet("{id}")]
[ProducesResponseType<Product>(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public IResult GetById(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? Results.NotFound() : Results.Ok(product);
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist, generated by the Results.Ok<T>().
Consider the following code:
[HttpPost]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType<Product>(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<IResult> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return Results.BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    var location = Url.Action(nameof(CreateAsync), new { id = product.Id }) ?? $"/{product.Id}";
    return Results.Created(location, product);
In the preceding action:
A 400 status code is returned when:
The [ApiController] attribute has been applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the Results.Create method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
Results<TResult1, TResultN> type
The static TypedResults class returns the concrete IResult implementation that allows using IResult as return type. The usage of the concrete IResult implementation offers the following benefit over the IResult type:
All the [ProducesResponseType] attributes can be excluded, since the HttpResult implementation contributes automatically to the endpoint metadata.
When multiple IResult return types are needed, returning Results<TResult1, TResultN> is preferred over returning IResult. Returning Results<TResult1, TResultN> is preferred because generic union types automatically retain the endpoint metadata.
The Results<TResult1, TResultN> union types implement implicit cast operators so that the compiler can automatically convert the types specified in the generic arguments to an instance of the union type. This has the added benefit of providing compile-time checking that a route handler actually only returns the results that it declares it does. Attempting to return a type that isn’t declared as one of the generic arguments to Results<> results in a compilation error.
Consider the following code:
[HttpGet("{id}")]
public Results<NotFound, Ok<Product>> GetById(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? TypedResults.NotFound() : TypedResults.Ok(product);
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist, generated by the TypedResults.Ok<T>.
[HttpPost]
public async Task<Results<BadRequest, Created<Product>>> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return TypedResults.BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    var location = Url.Action(nameof(CreateAsync), new { id = product.Id }) ?? $"/{product.Id}";
    return TypedResults.Created(location, product);
In the preceding action:
A 400 status code is returned when:
The [ApiController] attribute was applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the TypedResults.Created method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
Additional resources
Handle requests with controllers in ASP.NET Core MVC
Model validation in ASP.NET Core MVC
ASP.NET Core web API documentation with Swagger / OpenAPI
View or download sample code (how to download)
ASP.NET Core provides the following options for web API controller action return types:
Specific type
IActionResult
ActionResult<T>
HttpResults
This article explains when it's most appropriate to use each return type.
Specific type
The most basic action returns a primitive or complex data type, for example, string or a custom object. Consider the following action, which returns a collection of custom Product objects:
[HttpGet]
public Task<List<Product>> Get() =>
    _productContext.Products.OrderBy(p => p.Name).ToListAsync();
Without known conditions to safeguard against, returning a specific type could suffice. The preceding action accepts no parameters, so parameter constraints validation isn't needed.
When multiple return types are possible, it's common to mix an ActionResult return type with the primitive or complex return type. Either IActionResult or ActionResult<T> are necessary to accommodate this type of action. Several samples of multiple return types are provided in this article.
Return IEnumerable<T> or IAsyncEnumerable<T>
See Return IEnumerable<T> or IAsyncEnumerable<T> for performance considerations.
ASP.NET Core buffers the result of actions that return IEnumerable<T> before writing them to the response. Consider declaring the action signature's return type as IAsyncEnumerable<T> to guarantee asynchronous iteration. Ultimately, the iteration mode is based on the underlying concrete type being returned and the selected formatter affects how the result is processed:
When using System.Text.Json formatter, MVC relies on the support that System.Text.Json added to stream the result.
When using Newtonsoft.Json or with XML-based formatters the result is buffered.
Consider the following action, which returns sale-priced product records as IEnumerable<Product>:
[HttpGet("syncsale")]
public IEnumerable<Product> GetOnSaleProducts()
    var products = _productContext.Products.OrderBy(p => p.Name).ToList();
    foreach (var product in products)
        if (product.IsOnSale)
            yield return product;
The IAsyncEnumerable<Product> equivalent of the preceding action is:
[HttpGet("asyncsale")]
public async IAsyncEnumerable<Product> GetOnSaleProductsAsync()
    var products = _productContext.Products.OrderBy(p => p.Name).AsAsyncEnumerable();
    await foreach (var product in products)
        if (product.IsOnSale)
            yield return product;
IActionResult type
The IActionResult return type is appropriate when multiple ActionResult return types are possible in an action. The ActionResult types represent various HTTP status codes. Any non-abstract class deriving from ActionResult qualifies as a valid return type. Some common return types in this category are BadRequestResult (400), NotFoundResult (404), and OkObjectResult (200). Alternatively, convenience methods in the ControllerBase class can be used to return ActionResult types from an action. For example, return BadRequest(); is a shorthand form of return new BadRequestResult();.
Because there are multiple return types and paths in this type of action, liberal use of the [ProducesResponseType] attribute is necessary. This attribute produces more descriptive response details for web API help pages generated by tools like Swagger. [ProducesResponseType] indicates the known types and HTTP status codes to be returned by the action.
Synchronous action
Consider the following synchronous action in which there are two possible return types:
[HttpGet("{id}")]
[ProducesResponseType(StatusCodes.Status200OK, Type = typeof(Product))]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public IActionResult GetById_IActionResult(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? NotFound() : Ok(product);
In the preceding action:
A 404 status code is returned when the product represented by id doesn't exist in the underlying data store. The NotFound convenience method is invoked as shorthand for return new NotFoundResult();.
A 200 status code is returned with the Product object when the product does exist. The Ok convenience method is invoked as shorthand for return new OkObjectResult(product);.
Asynchronous action
Consider the following asynchronous action in which there are two possible return types:
[HttpPost()]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<IActionResult> CreateAsync_IActionResult(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    return CreatedAtAction(nameof(GetById_IActionResult), new { id = product.Id }, product);
In the preceding action:
A 400 status code is returned when the product description contains "XYZ Widget". The BadRequest convenience method is invoked as shorthand for return new BadRequestResult();.
A 201 status code is generated by the CreatedAtAction convenience method when a product is created. The following code is an alternative to calling CreatedAtAction:
return new CreatedAtActionResult(nameof(GetById), 
                                "Products", 
                                new { id = product.Id }, 
                                product);
In the preceding code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
For example, the following model indicates that requests must include the Name and Description properties. Failure to provide Name and Description in the request causes model validation to fail.
public class Product
    public int Id { get; set; }
    [Required]
    public string Name { get; set; } = string.Empty;
    [Required]
    public string Description { get; set; } = string.Empty;
    public bool IsOnSale { get; set; }
If the [ApiController] attribute is applied, model validation errors result in a 400 status code. For more information, see Automatic HTTP 400 responses.
ActionResult vs IActionResult
The following section compares ActionResult to  IActionResult
ActionResult<T> type
ASP.NET Core includes the ActionResult<T> return type for web API controller actions. It enables returning a type deriving from ActionResult or return a specific type. ActionResult<T> offers the following benefits over the IActionResult type:
The [ProducesResponseType] attribute's Type property can be excluded. For example, [ProducesResponseType(200, Type = typeof(Product))] is simplified to [ProducesResponseType(200)]. The action's expected return type is inferred from the T in ActionResult<T>.
Implicit cast operators support the conversion of both T and ActionResult to ActionResult<T>. T converts to ObjectResult, which means return new ObjectResult(T); is simplified to return T;.
C# doesn't support implicit cast operators on interfaces. Consequently, conversion of the interface to a concrete type is necessary to use ActionResult<T>. For example, use of IEnumerable in the following example doesn't work:




    

[HttpGet]
public ActionResult<IEnumerable<Product>> Get() =>
    _repository.GetProducts();
One option to fix the preceding code is to return _repository.GetProducts().ToList();.
Most actions have a specific return type. Unexpected conditions can occur during action execution, in which case the specific type isn't returned. For example, an action's input parameter may fail model validation. In such a case, it's common to return the appropriate ActionResult type instead of the specific type.
Synchronous action
Consider a synchronous action in which there are two possible return types:
[HttpGet("{id}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public ActionResult<Product> GetById_ActionResultOfT(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? NotFound() : product;
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist.
Asynchronous action
Consider an asynchronous action in which there are two possible return types:
[HttpPost()]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<ActionResult<Product>> CreateAsync_ActionResultOfT(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    return CreatedAtAction(nameof(GetById_ActionResultOfT), new { id = product.Id }, product);
In the preceding action:
A 400 status code (BadRequest) is returned by the ASP.NET Core runtime when:
The [ApiController] attribute has been applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the CreatedAtAction method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
HttpResults type
In addition to the MVC-specific built-in result types (IActionResult and ActionResult<T>), ASP.NET Core includes the HttpResults types that can be used in both Minimal APIs and Web API.
Different than the MVC-specific result types, the HttpResults:
Are a results implementation that is processed by a call to IResult.ExecuteAsync.
Does not leverage the configured Formatters. Not leveraging the configured formatters means:
Some features like Content negotiation aren't available.
The produced Content-Type is decided by the HttpResults implementation.
The HttpResults can be useful when sharing code between Minimal APIs and Web API.
IResult type
The Microsoft.AspNetCore.Http.HttpResults namespace contains classes that implement the IResult interface. The IResult interface defines a contract that represents the result of an HTTP endpoint. The static Results class is used to create varying IResult objects that represent different types of responses.
The Built-in results table shows the common result helpers.
Consider the following code:
[HttpGet("{id}")]
[ProducesResponseType(typeof(Product), StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public IResult GetById(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? Results.NotFound() : Results.Ok(product);
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist, generated by the Results.Ok<T>().
Consider the following code:
[HttpPost]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(typeof(Product), StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<IResult> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return Results.BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    var location = Url.Action(nameof(GetById), new { id = product.Id }) ?? $"/{product.Id}";
    return Results.Created(location, product);
In the preceding action:
A 400 status code is returned when:
The [ApiController] attribute has been applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the Results.Create method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
Results<TResult1, TResultN> type
The static TypedResults class returns the concrete IResult implementation that allows using IResult as return type. The usage of the concrete IResult implementation offers the following benefit over the IResult type:
All the [ProducesResponseType] attributes can be excluded, since the HttpResult implementation contributes automatically to the endpoint metadata.
When multiple IResult return types are needed, returning Results<TResult1, TResultN> is preferred over returning IResult. Returning Results<TResult1, TResultN> is preferred because generic union types automatically retain the endpoint metadata.
The Results<TResult1, TResultN> union types implement implicit cast operators so that the compiler can automatically convert the types specified in the generic arguments to an instance of the union type. This has the added benefit of providing compile-time checking that a route handler actually only returns the results that it declares it does. Attempting to return a type that isn’t declared as one of the generic arguments to Results<> results in a compilation error.
Consider the following code:
[HttpGet("{id}")]
public Results<NotFound, Ok<Product>> GetById(int id)
    var product = _productContext.Products.Find(id);
    return product == null ? TypedResults.NotFound() : TypedResults.Ok(product);
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist, generated by the TypedResults.Ok<T>.
[HttpPost]
public async Task<Results<BadRequest, Created<Product>>> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return TypedResults.BadRequest();
    _productContext.Products.Add(product);
    await _productContext.SaveChangesAsync();
    var location = Url.Action(nameof(GetById), new { id = product.Id }) ?? $"/{product.Id}";
    return TypedResults.Created(location, product);
In the preceding action:
A 400 status code is returned when:
The [ApiController] attribute was applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the TypedResults.Create method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
Additional resources
Handle requests with controllers in ASP.NET Core MVC
Model validation in ASP.NET Core MVC
ASP.NET Core web API documentation with Swagger / OpenAPI
View or download sample code (how to download)
ASP.NET Core offers the following options for web API controller action return types:
Specific type
IActionResult
ActionResult<T>
This document explains when it's most appropriate to use each return type.
Specific type
The simplest action returns a primitive or complex data type (for example, string or a custom object type). Consider the following action, which returns a collection of custom Product objects:
[HttpGet]
public List<Product> Get() =>
    _repository.GetProducts();
Without known conditions to safeguard against during action execution, returning a specific type could suffice. The preceding action accepts no parameters, so parameter constraints validation isn't needed.
When multiple return types are possible, it's common to mix an ActionResult return type with the primitive or complex return type. Either IActionResult or ActionResult<T> are necessary to accommodate this type of action. Several samples of multiple return types are provided in this document.
Return IEnumerable<T> or IAsyncEnumerable<T>
ASP.NET Core buffers the result of actions that return IEnumerable<T> before writing them to the response. Consider declaring the action signature's return type as IAsyncEnumerable<T> to guarantee asynchronous iteration. Ultimately, the iteration mode is based on the underlying concrete type being returned. MVC automatically buffers any concrete type that implements IAsyncEnumerable<T>.
Consider the following action, which returns sale-priced product records as IEnumerable<Product>:
[HttpGet("syncsale")]
public IEnumerable<Product> GetOnSaleProducts()
    var products = _repository.GetProducts();
    foreach (var product in products)
        if (product.IsOnSale)
            yield return product;
The IAsyncEnumerable<Product> equivalent of the preceding action is:
[HttpGet("asyncsale")]
public async IAsyncEnumerable<Product> GetOnSaleProductsAsync()
    var products = _repository.GetProductsAsync();
    await foreach (var product in products)
        if (product.IsOnSale)
            yield return product;
IActionResult type
The IActionResult return type is appropriate when multiple ActionResult return types are possible in an action. The ActionResult types represent various HTTP status codes. Any non-abstract class deriving from ActionResult qualifies as a valid return type. Some common return types in this category are BadRequestResult (400), NotFoundResult (404), and OkObjectResult (200). Alternatively, convenience methods in the ControllerBase class can be used to return ActionResult types from an action. For example, return BadRequest(); is a shorthand form of return new BadRequestResult();.
Because there are multiple return types and paths in this type of action, liberal use of the [ProducesResponseType] attribute is necessary. This attribute produces more descriptive response details for web API help pages generated by tools like Swagger. [ProducesResponseType] indicates the known types and HTTP status codes to be returned by the action.
Synchronous action
Consider the following synchronous action in which there are two possible return types:
[HttpGet("{id}")]
[ProducesResponseType(StatusCodes.Status200OK, Type = typeof(Product))]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public IActionResult GetById(int id)
    if (!_repository.TryGetProduct(id, out var product))
        return NotFound();
    return Ok(product);
In the preceding action:
A 404 status code is returned when the product represented by id doesn't exist in the underlying data store. The NotFound convenience method is invoked as shorthand for return new NotFoundResult();.
A 200 status code is returned with the Product object when the product does exist. The Ok convenience method is invoked as shorthand for return new OkObjectResult(product);.
Asynchronous action
Consider the following asynchronous action in which there are two possible return types:
[HttpPost]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<IActionResult> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return BadRequest();
    await _repository.AddProductAsync(product);
    return CreatedAtAction(nameof(GetById), new { id = product.Id }, product);
In the preceding action:
A 400 status code is returned when the product description contains "XYZ Widget". The BadRequest convenience method is invoked as shorthand for return new BadRequestResult();.
A 201 status code is generated by the CreatedAtAction convenience method when a product is created. An alternative to calling CreatedAtAction is return new CreatedAtActionResult(nameof(GetById), "Products", new { id = product.Id }, product);. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
For example, the following model indicates that requests must include the Name and Description properties. Failure to provide Name and Description in the request causes model validation to fail.
public class Product
    public int Id { get; set; }
    [Required]
    public string Name { get; set; }
    [Required]
    public string Description { get; set; }
    public bool IsOnSale { get; set; }
If the [ApiController] attribute is applied, model validation errors result in a 400 status code. For more information, see Automatic HTTP 400 responses.
ActionResult vs IActionResult
The following section compares ActionResult to  IActionResult
ActionResult<T> type
ASP.NET Core includes the ActionResult<T> return type for web API controller actions. It enables you to return a type deriving from ActionResult or return a specific type. ActionResult<T> offers the following benefits over the IActionResult type:
The [ProducesResponseType] attribute's Type property can be excluded. For example, [ProducesResponseType(200, Type = typeof(Product))] is simplified to [ProducesResponseType(200)]. The action's expected return type is instead inferred from the T in ActionResult<T>.
Implicit cast operators support the conversion of both T and ActionResult to ActionResult<T>. T converts to ObjectResult, which means return new ObjectResult(T); is simplified to return T;.
C# doesn't support implicit cast operators on interfaces. Consequently, conversion of the interface to a concrete type is necessary to use ActionResult<T>. For example, use of IEnumerable in the following example doesn't work:
[HttpGet]
public ActionResult<IEnumerable<Product>> Get() =>
    _repository.GetProducts();
One option to fix the preceding code is to return _repository.GetProducts().ToList();.
Most actions have a specific return type. Unexpected conditions can occur during action execution, in which case the specific type isn't returned. For example, an action's input parameter may fail model validation. In such a case, it's common to return the appropriate ActionResult type instead of the specific type.
Synchronous action
Consider a synchronous action in which there are two possible return types:
[HttpGet("{id}")]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public ActionResult<Product> GetById(int id)
    if (!_repository.TryGetProduct(id, out var product))
        return NotFound();
    return product;
In the preceding action:
A 404 status code is returned when the product doesn't exist in the database.
A 200 status code is returned with the corresponding Product object when the product does exist.
Asynchronous action
Consider an asynchronous action in which there are two possible return types:
[HttpPost]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<ActionResult<Product>> CreateAsync(Product product)
    if (product.Description.Contains("XYZ Widget"))
        return BadRequest();
    await _repository.AddProductAsync(product);
    return CreatedAtAction(nameof(GetById), new { id = product.Id }, product);
In the preceding action:
A 400 status code (BadRequest) is returned by the ASP.NET Core runtime when:
The [ApiController] attribute has been applied and model validation fails.
The product description contains "XYZ Widget".
A 201 status code is generated by the CreatedAtAction method when a product is created. In this code path, the Product object is provided in the response body. A Location response header containing the newly created product's URL is provided.
Additional resources
Handle requests with controllers in ASP.NET Core MVC
Model validation in ASP.NET Core MVC
ASP.NET Core web API documentation with Swagger / OpenAPI
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