Model Binders in ASP.NET Core

 1[Route("users/{id}", Name = "GetUserById")]
 2public async Task<ActionResult<User>> GetUser(int id)
 3{
 4    User user = await _userRepository.GetAsync(id);
 5    
 6    if ( user is null )
 7    {
 8        return NotFound();
 9    }
10
11    return Ok(user);
12}

 1public class UserModelBinder : IModelBinder
 2{
 3    private readonly IUserRepository _userRepository;
 4
 5    public UserModelBinder(IUserRepository userRepository) // supports dependency injection
 6    {
 7        _userRepository = userRepository;
 8    }
 9
10    public async Task BindModelAsync(ModelBindingContext bindingContext)
11    {
12        if (bindingContext == null)
13        {
14            throw new ArgumentNullException(nameof(bindingContext));
15        }
16
17        // Specify a default argument name if none is set by ModelBinderAttribute
18        string modelName = bindingContext.ModelName;
19
20        // Try to fetch the value of the argument by name
21        ValueProviderResult valueProviderResult =
22            bindingContext.ValueProvider.GetValue(modelName);
23
24        if (valueProviderResult != ValueProviderResult.None)
25        {
26            bindingContext.ModelState.SetModelValue(modelName,
27                valueProviderResult);
28
29            string value = valueProviderResult.FirstValue;
30
31            // Check if the argument value is null or empty
32            if (!string.IsNullOrEmpty(value))
33            {
34                // check if the value is an int as expected
35                if (!int.TryParse(value, out int id))
36                {
37                    bindingContext.ModelState.TryAddModelError(modelName,
38                        "User Id must be an integer.");
39                }
40                else
41                {
42                    // load the user from the database => repository
43                    User user = await _userRepository.GetAsync(id);
44
45                    // bind the user to the action
46                    bindingContext.Result = ModelBindingResult.Success(result);
47                }
48            }
49        }
50    }
51}

 1[Route("users/{id}", Name = "GetUserById")]
 2public async Task<ActionResult<User>> GetUser(
 3        [ModelBinder(Name = "id")] User user) // we need the attribut here to "listen" for the parameter "id"
 4{
 5    if ( user is null )
 6    {
 7        return NotFound();
 8    }
 9
10    return Ok(user);
11}

 1public class UserModelBinderProvider : IModelBinderProvider
 2{
 3    public IModelBinder GetBinder(ModelBinderProviderContext context)
 4    {
 5        if (context == null)
 6        {
 7            throw new ArgumentNullException(nameof(context));
 8        }
 9
10        if (context.Metadata.ModelType == typeof(User)) // we want to use our custom model binder when action parameter is of type User
11        {
12            return new BinderTypeModelBinder(typeof(UserModelBinder));
13        }
14        else
15        {
16            return null; // ignore all other types
17        }
18    }
19}

1services.AddMvc(options =>
2{ 
3    // Model Binder
4    options.ModelBinderProviders.Insert(0, new UserModelBinderProvider()); // register the binder to be the first one in the pipe
5});