Custom Model Binders in ASP.NET Core

1// users/1
2public IActionResult GetById(int id) ..

 1
 2public IActionResult MyAction()
 3{
 4    int? userId;
 5
 6    var user = this.User;
 7    var claims = user.Claims;
 8    var idClaim = claims.SingleOrDefault(claim => claim.Type == ClaimTypes.NameIdentifier);
 9
10    if(idClaim != null)
11    {
12        if (int.TryParse(idClaim.Value, out int claimIdValue)
13        {
14            userId = claimIdValue;
15        }
16    }
17
18    // you can use userId here
19    // ...
20}
21
22## Binder Class
23
24In order for a ModelBinder to become active at all, we need a separate class for it.
25Thanks to the type, ASP.NET Core can later recognize which ModelBinder must be used.
26
27Since our user id is always an integer, I gave the model some operators to make it easier to use against methods with int parameters.
28
29```csharp
30    public class PortalUserId
31    {
32        public PortalUserId(int value)
33        {
34            Value = value;
35        }
36
37        public int Value { get; }
38
39        public static implicit operator PortalUserId(int value)
40        {
41            return new PortalUserId(value);
42        }
43
44        public static implicit operator int?(PortalUserId userId)
45        {
46            return userId?.Value;
47        }
48
49        public static implicit operator int(PortalUserId userId)
50        {
51            return userId.Value;
52        }
53    }

1public IActionResult MyAction(PortalUserId userId)
2{
3    // you can use userId here
4    // ...
5}

 1public class PortalUserIdBinder : IModelBinder
 2{
 3    public Task BindModelAsync(ModelBindingContext bindingContext)
 4    {
 5        if (bindingContext == null)
 6        {
 7            throw new ArgumentNullException(nameof(bindingContext));
 8        }
 9
10        PortalUserId portalUserId = null;
11
12        int? userId = null;
13
14        // find user and the claims
15        var user = bindingContext.ActionContext.HttpContext.User;
16        var claims = user.Claims;
17        var idClaim = claims.SingleOrDefault(claim => claim.Type == ClaimTypes.NameIdentifier);
18
19        // find claim and read value
20        if (idClaim != null)
21        {
22            if (int.TryParse(idClaim.Value, out int claimIdValue)
23            {
24                userId = claimIdValue;
25            }
26        }
27
28        // create object if id is valid
29        if (userId.HasValue)
30        {
31            portalUserId = PortalUserId.Create(userId.Value);
32        }
33
34        // if user id was found, the filled object is passed
35        //      if userId was not found, null will be passed to the actions
36        bindingContext.Result = ModelBindingResult.Success(portalUserId);
37        return Task.CompletedTask;
38    }
39}

 1    public class PortlModelBinderProvider : IModelBinderProvider
 2    {
 3        public IModelBinder GetBinder(ModelBinderProviderContext context)
 4        {
 5            if (context is null) throw new ArgumentNullException(nameof(context));
 6
 7            // our binders here
 8            if (context.Metadata.ModelType == typeof(PortalUserId))
 9            {
10                return new BinderTypeModelBinder(typeof(PortalUserIdBinder));
11            }
12
13            // your maybe have more binders?
14            // ....
15
16            // this provider does not provide any binder for given type
17            //   so we return null
18            return null;
19        }
20    }

 1public void ConfigureServices(IServiceCollection services)
 2{
 3    // ....
 4
 5    services.AddControllersWithViews(o =>
 6    {
 7        // adds custom binder at first place
 8        o.ModelBinderProviders.Insert(0, new PortlModelBinderProvider());
 9
10    }).AddRazorRuntimeCompilation();
11
12    // ....
13}