Using [FromQuery] to parse enum values

Question

I'm using the [FromQuery] attribute to parse a Get requests arguments into a complex object. For example:

[HttpGet("some-get-request")]
public IActionResult DoSomething([FromQuery] SomeArguments someArgs)
{

}

One of the properties of the SomeArguments object is an enum.

public enum SomeEnum { EnumValue01, EnumValue02 }

public class SomeArguments 
{
    [FromQuery(Name = "enum_arg")]
    public SomeEnum EnumArgument { get; set; }
}

And I call the endpoint with something like:

http://localhost:1234/api/some-controller/some-get-request?enum_arg=EnumValue01

And this all works great. However, I want to be able to use a different enum value in the URL than in my C# enum value. For example, I want to call using a URL such as

http://localhost:1234/api/some-controller/some-get-request?enum_arg=eval01

How can I do this?

I thought I could use the [FromQuery] attribute, like I can with properties, but that doesnt seem to be possible:

'FromQuery' is not valid on this declaration type. It is only valid on 'property, indexer, parameter'

what do you mean by " I want to be able to use a different enum value in the URL than in my C# enum value" , what is your requirement — Nan Yu
– Nan Yu, Commented Jul 1, 2019 at 5:54
I would like the C# enum value to be one thing (EnumValue01, for example), but the value in the HTTP request to be something else (eval01, for example). — devklick
– devklick, Commented Jul 1, 2019 at 8:18
Then why not just use string as type , you can then map/modify that value to enum on server side . — Nan Yu
– Nan Yu, Commented Jul 1, 2019 at 8:20
Using a string is an option, but one I would like to avoid, if possible. Yes, it would allow any value for the parameter, which I can validate and handle in the code, however I dont want to do this if there's already a way to achieve what I'm trying to do — devklick
– devklick, Commented Jul 1, 2019 at 8:35

Oleksandr · Accepted Answer · 2021-05-04 09:57:49Z

13

You can use EnumMemberAttribute in conjunction with StringEnumConverter to achieve your goal. Define SomeEnum as following

[JsonConverter(typeof(StringEnumConverter))]
public enum SomeEnum
{
    [EnumMember(Value = "eval01")]
    EnumValue01,

    [EnumMember(Value = "eval02")]
    EnumValue02
}

At this point it will work as you need only when Newtonsoft json serializer is used. For example, when controller expects POST request and parameter is marked as [FromBody]. In your case it won't work yet because during binding of [FromQuery] parameter json serializer is not used. To solve this one create custom model binder

public class JsonModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        string rawData = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).FirstValue;
        rawData = JsonConvert.SerializeObject(rawData); //turns value to valid json
        try
        {
            SomeEnum result = JsonConvert.DeserializeObject<SomeEnum>(rawData); //manually deserializing value
            bindingContext.Result = ModelBindingResult.Success(result);
        }
        catch (JsonSerializationException ex)
        {
            //do nothing since "failed" result is set by default
        }


        return Task.CompletedTask;
    }
}

Update SomeEnum definition to use JsonModelBinder

[JsonConverter(typeof(StringEnumConverter))]
[ModelBinder(typeof(JsonModelBinder))]
public enum SomeEnum

edited May 4, 2021 at 9:57

answered Jul 1, 2019 at 22:06

Oleksandr

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9 Comments

Neyoh Over a year ago

Hello @Alexander , I have the same problem than this question owner, why when I have an enumvalue with a dash like [EnumMember(Value = "test-enum")] I need your jsonmodelbinder. But when I don't have a dash like [EnumMember(Value = "testenum")] the modelbinder isn't necessary ?

Oleksandr Over a year ago

@Neyoh What do you meand by the modelbinder isn't necessary in ceratain cases? It's always required to use the binder to make EnumMember work

Neyoh Over a year ago

In this exemple, when I remove the dash like Value = "testuno"), it works without the modelbinder. gist.github.com/Kyochi/c46afe423facb1c4fde37f4fed33e9f0

Oleksandr Over a year ago

@Neyoh What version of ASP.NET Core are you using? And do you mark your controller with [ApiController] attribute?

Oleksandr Over a year ago

@Neyoh If you remove dash from test-uno in you example the name will just match with TestUno member name that's why it works. Try [EnumMemberAttribute(Value = "uno")] which is without dash and see how it works

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