ASP.NET Core 1 Web API Model Binding Array

Question

How do you model bind an array from the URI with GET in ASP.NET Core 1 Web API (implicitly or explicitly)?

In ASP.NET Web API pre Core 1, this worked:

[HttpGet]
public void Method([FromUri] IEnumerable<int> ints) { ... }

How do you do this in ASP.NET Web API Core 1? The docs have nothing.

Answer 1

The FromUriAttribute class combines the FromRouteAttribute and FromQueryAttribute classes. Depending the configuration of your routes / the request being sent, you should be able to replace your attribute with one of those.

However, there is a shim available which will give you the FromUriAttribute class. Install the "Microsoft.AspNet.Mvc.WebApiCompatShim" NuGet package through the package explorer, or add it directly to your project.json file:

"dependencies": {
  "Microsoft.AspNet.Mvc.WebApiCompatShim": "6.0.0-rc1-final"
}

While it is a little old, I've found that this article does a pretty good job of explaining some of the changes.

Binding

If you're looking to bind comma separated values for the array ("/api/values?ints=1,2,3"), you will need a custom binder just as before. This is an adapted version of Mrchief's solution for use in ASP.NET Core.

public class CommaDelimitedArrayModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelMetadata.IsEnumerableType)
        {
            var key = bindingContext.ModelName;
            var value = bindingContext.ValueProvider.GetValue(key).ToString();

            if (!string.IsNullOrWhiteSpace(value))
            {
                var elementType = bindingContext.ModelType.GetTypeInfo().GenericTypeArguments[0];
                var converter = TypeDescriptor.GetConverter(elementType);

                var values = value.Split(new[] { "," }, StringSplitOptions.RemoveEmptyEntries)
                    .Select(x => converter.ConvertFromString(x.Trim()))
                    .ToArray();

                var typedValues = Array.CreateInstance(elementType, values.Length);

                values.CopyTo(typedValues, 0);
                
                bindingContext.Result = ModelBindingResult.Success(typedValues);
            }
            else
            {
                // change this line to null if you prefer nulls to empty arrays 
                bindingContext.Result = ModelBindingResult.Success(Array.CreateInstance(bindingContext.ModelType.GetElementType(), 0));
            }

            return TaskCache.CompletedTask;
        }

        return TaskCache.CompletedTask;
    }
}

You can either specify the model binder to be used for all collections in Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc().AddMvcOptions(opts =>
        {
            opts.ModelBinders.Insert(0, new CommaDelimitedArrayModelBinder());
        });
}

Or specify it once in your API call:

[HttpGet]
public void Method([ModelBinder(BinderType = typeof(CommaDelimitedArrayModelBinder))] IEnumerable<int> ints)

Answer 2

ASP.NET Core 1.1 Answer

@WillRay's answer is a little outdated. I have written an 'IModelBinder' and 'IModelBinderProvider'. The first can be used with the [ModelBinder(BinderType = typeof(DelimitedArrayModelBinder))] attribute, while the second can be used to apply the model binder globally as I've show below.

.AddMvc(options =>
{
    // Add to global model binders so you don't need to use the [ModelBinder] attribute.
    var arrayModelBinderProvider = options.ModelBinderProviders.OfType<ArrayModelBinderProvider>().First();
    options.ModelBinderProviders.Insert(
        options.ModelBinderProviders.IndexOf(arrayModelBinderProvider),
        new DelimitedArrayModelBinderProvider());
})

public class DelimitedArrayModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }

        if (context.Metadata.IsEnumerableType && !context.Metadata.ElementMetadata.IsComplexType)
        {
            return new DelimitedArrayModelBinder();
        }

        return null;
    }
}

public class DelimitedArrayModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
        {
            throw new ArgumentNullException(nameof(bindingContext));
        }

        var modelName = bindingContext.ModelName;
        var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
        var values = valueProviderResult
            .ToString()
            .Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
        var elementType = bindingContext.ModelType.GetTypeInfo().GenericTypeArguments[0];

        if (values.Length == 0)
        {
            bindingContext.Result = ModelBindingResult.Success(Array.CreateInstance(elementType, 0));
        }
        else
        {
            var converter = TypeDescriptor.GetConverter(elementType);
            var typedArray = Array.CreateInstance(elementType, values.Length);

            try
            {
                for (int i = 0; i < values.Length; ++i)
                {
                    var value = values[i];
                    var convertedValue = converter.ConvertFromString(value);
                    typedArray.SetValue(convertedValue, i);
                }
            }
            catch (Exception exception)
            {
                bindingContext.ModelState.TryAddModelError(
                    modelName,
                    exception,
                    bindingContext.ModelMetadata);
            }

            bindingContext.Result = ModelBindingResult.Success(typedArray);
        }

        return Task.CompletedTask;
    }
}

Answer 3

There are some changes in the .NET Core 3.

Microsoft has split out the functionality from the AddMvc method (source).

As AddMvc also includes support for View Controllers, Razor Views and etc. If you don't need to use them in your project (like in an API), you might consider using services.AddControllers() which is for Web API controllers.

So, updated code will look like this:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()
            .AddMvcOptions(opt =>
            {
                var mbp = opt.ModelBinderProviders.OfType<ArrayModelBinderProvider>().First();
                    opt.ModelBinderProviders.Insert(opt.ModelBinderProviders.IndexOf(mbp), new DelimitedArrayModelBinderProvider());
            });
}