Model Binder for a custom property type

Question

On an ASP.NET Core project I have the following action:

public async Task<IActionResult> Get(ProductModel model) {
}

public class ProductModel {
  public Filter<Double> Price { get; set; }
}    

I have a Filter base class and a RangeFilter as follows:

public class Filter<T> { }

public class RangeFilter<T> : Filter<T> { 
  public abstract Boolean TryParse(String value, out Filter<T> filter);  
}

I am passing a String ("[3.34;18.75]") to the Action as Price.

I need to create a ModelBinder where I use the TryParse method to try to convert that String into a RangeFilter<Double> to define the ProductModel.Price property.

If TryParse fails, e.g., returns false then the ProductModel.Price property becomes null.

How can this be done?

Answer 1

If you're willing to move the parsing method into a static class, this would become a bit more feasible.

Parser

public static class RangeFilterParse
{
    public static Filter<T> Parse<T>(string value)
    {
        // parse the stuff!
        return new RangeFilter<T>();
    }
}

public abstract class Filter<T> { }

public class RangeFilter<T> : Filter<T> { }

Model

public class ProductModel
{
    [ModelBinder(BinderType = typeof(RangeModelBinder))]
    public Filter<double> Price { get; set; }
}

Binder

public class RangeModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        try
        {
            var input = bindingContext.ValueProvider.GetValue(bindingContext.ModelName).ToString();
            var inputType = bindingContext.ModelType.GetTypeInfo().GenericTypeArguments[0];

            // invoke generic method with proper type
            var method = typeof(RangeFilterParse).GetMethod(nameof(RangeFilterParse.Parse), BindingFlags.Static);
            var generic = method.MakeGenericMethod(inputType);
            var result = generic.Invoke(this, new object[] { input });

            bindingContext.Result = ModelBindingResult.Success(result);
            return Task.CompletedTask;
        }
        catch(Exception) // or catch a more specific error related to parsing
        {
            bindingContext.Result = ModelBindingResult.Success(null);
            return Task.CompletedTask;
        }
    }
}