Apply Custom Model Binder to Object Property in asp.net core

Question

I am trying to apply custom model binder for DateTime type property of model. Here is the IModelBinder and IModelBinderProvider implementations.

public clas         


        

Answer 1

1. Reason

According to the [FromBody]Time time in your action, I guess you're sending a payload with Content-Type of application/json. In that case, when a josn payload received, the Model Binding System will inspect the parameter time and then try to find a proper binder for it. Because the context.Metadata.ModelType equals typeof(Time) instead of the typeof(DateTime), and there's no custom ModelBinder for typeof(Time) , your GetBinder(context) method will return a null :

public class DateTimeModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }

        if (context.Metadata.ModelType == typeof(DateTime))     // not typeof(Time)
        {
            return new BinderTypeModelBinder(typeof(DateTime));  
        }

        return null;
    }
}

Thus it falls back to the default model binder for application/json. The default json model binder uses Newtonsoft.Json under the hood and will simply deserialize the hole payload as an instance of Time. As a result, your DateTimeModelBinder is not invoked.

2. Quick Fix

One approach is to use application/x-www-form-urlencoded (avoid using the application/json)

Remove the [FromBody] attribute:

[HttpPost("/test2")]
public IActionResult test2(Time time)
{
    return Ok(time);
}

and send the payload in the format of application/x-www-form-urlencoded

POST https://localhost:5001/test2
Content-Type: application/x-www-form-urlencoded

validFrom=2018-01-01&validTo=2018-02-02

It should work now.

3. Working with JSON

Create a custom converter as below :

public class CustomDateConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
         return true;
    }
    public static string[] _formats = new string[] { 
        "yyyyMMdd", "yyyy-MM-dd", "yyyy/MM/dd"
        , "yyyyMMddHHmm", "yyyy-MM-dd HH:mm", "yyyy/MM/dd HH:mm"
        , "yyyyMMddHHmmss", "yyyy-MM-dd HH:mm:ss", "yyyy/MM/dd HH:mm:ss"
    };

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var dt= reader.Value;
        if (DateTime.TryParseExact(dt as string, _formats, new CultureInfo("en-US"), DateTimeStyles.None, out DateTime dateTime)) 
            return dateTime;
        else 
            return null;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        serializer.Serialize(writer, value as string);
    }
}

I simply copy your code to format date.

Change your Model as below :

public class Time
{
    [ModelBinder(BinderType = typeof(DateTimeModelBinder))]
    [JsonConverter(typeof(CustomDateConverter))]
    public DateTime? validFrom { get; set; }

    [ModelBinder(BinderType = typeof(DateTimeModelBinder))]
    [JsonConverter(typeof(CustomDateConverter))]
    public DateTime? validTo { get; set; }
}

And now you can receive the time using [FromBody]

    [HttpPost("/test")]
    public IActionResult test([FromBody]Time time)
    {

        return Ok(time);
    }