passing an array to a asp net core web api action method HttpGet

I create a new web api class, with only one action.

[Produces("application/json")]
[Route("api/accounts")]
public class AccountsController : Controller
{
    [HttpGet]
    [Route("servicesbycategoryids")]
    public IActionResult ServicesByCategoryIds([FromQuery] int[] ids)
    {
        return Ok();
    }
}

Then use the same url as yours:

http://localhost:2443/api/accounts/servicesbycategoryids?ids=1&ids=2

It is working.

A slight variation on Plamen's answer.

• Arrays seem to have an empty GenericTypeArguments so added GetElementType()
• Renamed class to avoid clashing with the framework class ArrayModelBinder.
• Added a check on the element type as it's required.
• More options for surrounding the array with brackets.

public class CustomArrayModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (!bindingContext.ModelMetadata.IsEnumerableType)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return Task.CompletedTask;
        }

        var value = bindingContext.ValueProvider
            .GetValue(bindingContext.ModelName)
            .ToString();

        if (string.IsNullOrWhiteSpace(value))
        {
            bindingContext.Result = ModelBindingResult.Success(null);
            return Task.CompletedTask;
        }

        var elementType = bindingContext.ModelType.GetElementType() ??
            bindingContext.ModelType.GetTypeInfo().GenericTypeArguments.FirstOrDefault();

        if (elementType == null)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return Task.CompletedTask;
        }

        var converter = TypeDescriptor.GetConverter(elementType);

        var values = value.Split(',', StringSplitOptions.RemoveEmptyEntries)
            .Select(x => converter.ConvertFromString(Clean(x)))
            .ToArray();

        var typedValues = Array.CreateInstance(elementType, values.Length);
        values.CopyTo(typedValues, 0);
        bindingContext.Model = typedValues;

        bindingContext.Result = ModelBindingResult.Success(bindingContext.Model);
        return Task.CompletedTask;
    }

    private static string Clean(string str)
    {
        return str.Trim('(', ')').Trim('[', ']').Trim();
    }
}

Then use with an IEnumerable<T>, IList<T> or array T[]

[ModelBinder(BinderType = typeof(CustomArrayModelBinder))] IEnumerable<T> ids
                                                       ... T[] ids
                                                       ... IList<T> ids

The parameter could be in path or query with optional brackets.

[Route("resources/{ids}")]

resource/ids/1,2,3
resource/ids/(1,2,3)
resource/ids/[1,2,3]

[Route("resources")]

resource?ids=1,2,3
resource?ids=(1,2,3)
resource?ids=[1,2,3]

Binding failed for Array parameter is a known issue under Asp.Net Core 2.1 which has been recorded Array or List in query string does not get parsed #7712.

For a tempory workaround, you could set the FromQuery Name Property like below:

        [HttpGet()]
    [Route("ServicesByCategoryIds")]
    public async Task<IActionResult> ServicesByCategoryIds([FromQuery(Name = "ids")]int[] ids)
    {            
        return Ok();
    }

You can implement custom model binder and the ids to be part of the URI, not in the query string.

Your endpoint could look like this: /api/accounts/servicesbycategoryids/(1,2)

public class ArrayModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        // Our binder works only on enumerable types
        if (!bindingContext.ModelMetadata.IsEnumerableType)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return Task.CompletedTask;
        }

        // Get the inputted value through the value provider
        var value = bindingContext.ValueProvider
            .GetValue(bindingContext.ModelName).ToString();

        // If that value is null or whitespace, we return null
        if (string.IsNullOrWhiteSpace(value))
        {
            bindingContext.Result = ModelBindingResult.Success(null);
            return Task.CompletedTask;
        }

        // The value isn't null or whitespace,
        // and the type of the model is enumerable.
        // Get the enumerable's type, and a converter
        var elementType = bindingContext.ModelType.GetTypeInfo().GenericTypeArguments[0];
        var converter = TypeDescriptor.GetConverter(elementType);

        // Convert each item in the value list to the enumerable type
        var values = value.Split(new[] { "," }, StringSplitOptions.RemoveEmptyEntries)
            .Select(x => converter.ConvertFromString(x.Trim()))
            .ToArray();

        // Create an array of that type, and set it as the Model value
        var typedValues = Array.CreateInstance(elementType, values.Length);
        values.CopyTo(typedValues, 0);
        bindingContext.Model = typedValues;

        // return a successful result, passing in the Model
        bindingContext.Result = ModelBindingResult.Success(bindingContext.Model);
        return Task.CompletedTask;
    }
}

Then use it in your action:

[HttpGet("({ids})", Name="GetAuthorCollection")]
public IActionResult GetAuthorCollection(
    [ModelBinder(BinderType = typeof(ArrayModelBinder))] IEnumerable<int> ids)
{
    //enter code here
}

Learned this from a pluralsight course: Building RESTful API with ASP.NET Core