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 (aka ASP.NET 5 aka ASP.NET vNext)? 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;
    }
}