How i can get the arguments of my Json and validate before of make a post int web api 2 .net?

Question

For example, the next json, I want validate the amount before of make a post

{
    "amount":"10"
}

Could I create a filter class and get this argument and validate it, I have to make a cast?

public class ValidateModelTransaction : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var posted = actionContext.Request.Content.ReadAsStringAsync().Result;
        TransactionModel amount = JsonConvert.DeserializeObject<TransactionModel>(posted);
    }
}

Answer 1

You can create a custom function that uses a JsonTextReader to read the value of the amount property, and takes a delegate as a parameter to validate the amount:

private static bool ValidateAmount(string json, Func<int, bool> validateFunc)
{
    using (JsonTextReader reader = new JsonTextReader(new StringReader(json)))
    {
        reader.CloseInput = true;

        while (reader.Read())
        {
            if (reader.TokenType == JsonToken.PropertyName && reader.Value.Equals("amount"))
            {
                int? amount = reader.ReadAsInt32();

                if (!amount.HasValue)
                {
                    return false;
                }

                bool isValid = validateFunc(amount.Value);
                return isValid;
            }
        }
    }

    return false;
}

Using a delegate as a parameter ( Func<int, bool> validateFunc ) allows you to reuse the method in different scenarios - example:

string json = @"{""amount"":""10""}";

bool isPositiveAmount = ValidateAmount(json, x => x > 0); // returns true
bool isBetween100And200 = ValidateAmount(json, x => x > 100 && x < 200); // returns false

EDIT

The ValidateAmount method can eventually be used in an ActionFilterAttribute .

Example - check if the amount is positive:

public class ValidateModelTransaction : ActionFilterAttribute
{
    public override void OnActionExecuting(HttpActionContext actionContext)
    {
        var json = actionContext.Request.Content.ReadAsStringAsync().Result;
        bool isPositiveAmount = ValidateAmount(json, x => x > 0);

        if (isPositiveAmount)
        {
            // ....
        }
    }
}