Modelbinding an optional array of a custom model bound type - asp.net-core

Question

I'm stuck with binding an optional array in an ASP.NET Core Controller. The array contains elements of a custom type. Single elements of this type are bound with a custom model binder and validated in it.
Sample repo here: https://github.com/MarcusKohnert/OptionalArrayModelBinding
I get only two tests out of three working in the sample test project:
https://github.com/MarcusKohnert/OptionalArrayModelBinding/blob/master/OptionalArrayModelBindingTest/TestOptionalArrayCustomModelBinder.cs
public class TestOptionalArrayCustomModelBinder
{
private readonly TestServer server;
private readonly HttpClient client;
public TestOptionalArrayCustomModelBinder()
{
server = new TestServer(new WebHostBuilder().UseStartup<Startup>());
client = server.CreateClient();
}
[Fact]
public async Task SuccessWithoutProvidingIds()
{
var response = await client.GetAsync("/api/values");
Assert.Equal(System.Net.HttpStatusCode.OK, response.StatusCode);
}
[Fact]
public async Task SuccessWithValidIds()
{
var response = await client.GetAsync("/api/values?ids=aaa001&ids=bbb002");
Assert.Equal(System.Net.HttpStatusCode.OK, response.StatusCode);
}
[Fact]
public async Task FailureWithOneInvalidId()
{
var response = await client.GetAsync("/api/values?ids=xaaa001&ids=bbb002");
Assert.Equal(System.Net.HttpStatusCode.BadRequest, response.StatusCode);
}
}
Controller:
[Route("api/[controller]")]
public class ValuesController : Controller
{
[HttpGet]
public IActionResult Get(CustomIdentifier[] ids)
{
if (this.ModelState.IsValid == false) return this.BadRequest();
return this.Ok(ids);
}
}
Startup:
public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
services.AddMvc(options =>
{
options.ModelBinderProviders.Insert(0, new CutomIdentifierModelBinderProvider());
//options.ModelBinderProviders.Add(new CutomIdentifierModelBinderProvider());
});
}
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
app.UseMvc();
}
}
ModelBinder:
public class CutomIdentifierModelBinderProvider : IModelBinderProvider
{
public IModelBinder GetBinder(ModelBinderProviderContext context)
{
//if (context.Metadata.ModelType.IsArray && context.Metadata.ModelType == typeof(CustomIdentifier[]))
//{
// return new ArrayModelBinder<CustomIdentifier>(new CustomIdentifierModelBinder());
//}
if (context.Metadata.ModelType == typeof(CustomIdentifier))
{
return new BinderTypeModelBinder(typeof(CustomIdentifierModelBinder));
}
return null;
}
}
public class CustomIdentifierModelBinder : IModelBinder
{
public Task BindModelAsync(ModelBindingContext bindingContext)
{
var attemptedValue = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).ToString();
var parseResult = CustomIdentifier.TryParse(attemptedValue);
if (parseResult.Failed)
{
bindingContext.Result = ModelBindingResult.Failed();
bindingContext.ModelState.AddModelError(bindingContext.ModelName, parseResult.Message.Message);
}
else
{
bindingContext.Model = parseResult.Value;
bindingContext.Result = ModelBindingResult.Success(parseResult.Value);
}
return Task.CompletedTask;
}
}
The MVC default ArrayModelBinder of T binds optional arrays correctly and sets ModelState.IsValid to true. If I use my own CustomIdentifierModelBinder however ModelState.IsValid will be false. Empty arrays are not recognized as valid.
How can I solve this problem? Thanks in advance.

Answer 1

You are very close. Just customize behavior of built-in ArrayModelBinder for the case of missing parameter. If extracted value is an empty string just fill the model with an empty array. In all other cases you could call usual ArrayModelBinder.
Here is a working sample that passes all your 3 tests:
public class CutomIdentifierModelBinderProvider : IModelBinderProvider
{
public IModelBinder GetBinder(ModelBinderProviderContext context)
{
if (context.Metadata.ModelType.IsArray && context.Metadata.ModelType == typeof(CustomIdentifier[]))
{
return new CustomArrayModelBinder<CustomIdentifier>(new CustomIdentifierModelBinder());
}
return null;
}
}
public class CustomArrayModelBinder<T> : IModelBinder
{
private readonly ArrayModelBinder<T> innerModelBinder;
public CustomArrayModelBinder(IModelBinder elemeBinder)
{
innerModelBinder = new ArrayModelBinder<T>(elemeBinder);
}
public Task BindModelAsync(ModelBindingContext bindingContext)
{
var attemptedValue = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).ToString();
if (String.IsNullOrEmpty(attemptedValue))
{
bindingContext.Model = new T[0];
bindingContext.Result = ModelBindingResult.Success(bindingContext.Model);
return Task.CompletedTask;
}
return innerModelBinder.BindModelAsync(bindingContext);
}
}

Answer 2

The solution is the following code change, reflected in this commit:
https://github.com/MarcusKohnert/OptionalArrayModelBinding/commit/552f4d35d8c33c002e1aa0c05acb407f1f962102
I've found the solution by inspecting MVC's source code again.
https://github.com/aspnet/Mvc/blob/35601f95b345d0ef938fb21ce1c51f5a67a1fb62/src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/SimpleTypeModelBinder.cs#L37
You'll need to check the valueProviderResult for None. If it's none then there is no parameter given and the ModelBinder binds correctly.
var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
if (valueProviderResult == ValueProviderResult.None)
And also you register the provided ArrayModelBinder of T with your custom ModelBinder:
if (context.Metadata.ModelType.IsArray && context.Metadata.ModelType == typeof(CustomIdentifier[]))
{
return new ArrayModelBinder<CustomIdentifier>(new CustomIdentifierModelBinder());
}