How to do custom model binding for string to enum without comma separation in web api

Question

I want a user to be able to query GET /api/mycontroller?enums=ABC without using commas for the enums parameter. I know I can pass a comma separated parameter but using it without commas returns 'ABC' is not a valid value for type MyEnum. In my database, this field is stored as combination of characters without a comma. Is there a custom model binding attribute I can use and add it to the EnumVal property in MyRequest?

public enum MyEnum 
{
   A=1,
   B=2,
   C=4
}

public class MyRequest
{
   public MyEnum EnumVal {get; set;}
}

[HttpGet("mycontroller")]
public async Task<ActionResult> MyController([FromQuery] MyRequest request)
{
   //query db for row containing resuest.myEnum string combination...
   // ...
}

I've looked into overriding the ValidationAttribute but it still returns an error response.

Answer 1

I was able to figure it out using a custom model binder

public class MyEnumTypeEntityBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
        {
            throw new ArgumentNullException(nameof(bindingContext));
        }
        var modelName = bindingContext.ModelName;

        // Try to fetch the value of the argument by name
        var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);

        if (valueProviderResult == ValueProviderResult.None)
        {
            return Task.CompletedTask;
        }

        int len = valueProviderResult.FirstValue.Length;
        string query = valueProviderResult.FirstValue;
        char[] charlist = query.ToCharArray( );
        string enumConversionString = string.Join(",", charlist);

        if (!Enum.TryParse(enumConversionString, out MyEnum model))
        {
            bindingContext.ModelState.TryAddModelError(modelName, string.Format("{0} is not a valid value for type {1}", valueProviderResult.FirstValue, modelName));

            return Task.CompletedTask;

        }

        bindingContext.Result = ModelBindingResult.Success(model);

        return Task.CompletedTask;
    }

}

and adding the attribute above the MyEnum request prop:

[ModelBinder(BinderType = typeof(MyEnumTypeEntityBinder))]
public MyEnum? Type { get; set; }

public enum MyEnum 
{
   A=1,
   B=2,
   C=4
}