How to add Boolean validation for a model property WebApi .NET Core using data annotation

Question

I am trying to validate below property using annotation, either it should be true or false

public bool Info { get; set; }

I will get a invalid data validation error if i pass json like below

{  
  "info": trues
}

But strange if i pass like below, no data validation.

{  
  "info": 12345
}

I had tried with ValidationAttribute like below, but value is always true even if val is 12345

public class IsBoolAttribute : ValidationAttribute
{
    //public override bool RequiresValidationContext => true;

    public override bool IsValid(object value)
    {

        if (value == null) return false;
        if (value.GetType() != typeof(bool)) return false;
        return (bool)value;
    }
}

Try nullable boolean using ? Operator. In your case if mvc is not able to parse value it wiuld be false. But if yomake it nullable as follows public bool? Info { get; set; } value will be null if invalid data is passed. — Hemant
– Hemant, Commented Jan 20, 2020 at 7:54

Xueli Chen · Accepted Answer · 2020-01-21 10:03:42Z

If you use Newtonsoft.Json in Startup.cs , it seems to convert the random integer to true by design . You could write a custom JsonConverter like below:

public class CustomBoolConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return true;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var value = reader.Value;

        if (value.GetType() != typeof(bool))
        {
            throw new JsonReaderException("The JSON value could not be converted to System.Boolean.");
        }
            return value;
    }

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

Startup.cs

services.AddControllers()
            .AddNewtonsoftJson();

Alternative method , you could use System.Text.Json which is by default since ASP.NET Core 3.0 , Startup.cs like below:

services.AddControllers();
        //.AddNewtonsoftJson();

It will return the below error when you input incorrect value:

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