How to validate type of collections?

Question

How can I validate and catch collection type conversion (JSON string array to C# long collection) for my System.Web.Http.ApiController class (before the model is initialized if possible)?

I want to validate and catch any non-numeric elements in the JSON array to be return as a bad request response (maybe somehow with data annotation).

When non-numeric JSON elements are included (to be converted to long collection), they fail to parse and get stripped before the model is passed to the ApiController method. Given the classes below, a valid input should contain only numeric values for "PerferredNutTypes" and "GeographyIDs".

Classes

public class SquirrelController : ApiController
{
    [HttpPost]
    [Route("api/squirrels/search")]
    [SwaggerResponse(HttpStatusCode.OK, Type = typeof(SquirrelsResponse))]
    public HttpResponseMessage Squirrels(SquirrelsRequest model)
    {
        // model already parsed by the time breakpoint reaches here and non-convertable elements already stripped

        ...
        ...
        ...

        SquirrelsResponse results = Targeting.SearchForSquirrels(model);
        return Request.CreateResponse(HttpStatusCode.OK, results);
    }
}

public class SquirrelsRequest
{
    public SquirrelsRequest() {}

    public List<long> PreferredNutTypes { get; set; } = new List<long>();
    public GeographySearch geographySearch { get; set; } = new GeographySearch();
}

public class GeographySearch
{
    public GeographySearch() {}

    public BooleanOperator Operator { get; set; } = BooleanOperator.OR;
    public List<long> GeographyIDs { get; set; } = new List<long>();
}

public enum BooleanOperator
{
    AND,
    OR
}

Examples:

//"Toronto" sould be an invalid input when converting from JSON string array to c# long collection.
{
  "PreferredNutTypes": [34,21],
  "GeographySearch": {
    "Operator": 1,
    "GeographyIDs": ["Toronto"]
  },
}

// This is what the model currently looks like in public HttpResponseMessage Squirrels(SquirrelsRequest model)
new SquirrelsRequest()
{
    PreferredNutTypes = new List<long>() { 34, 21 },
    GeographySearch = new GeographySearch()
    {
        Operator = 1
        GeographyIDs = new List<long>()
    }
}

Expectations:

Ideally catch any non-numeric values and return them as a bad request. Similar to how you can validate to how you use use data annotations to validate range.

Data annotation solution acceptable.

Ideally a clearer approach than accessing ModelState and parsing the error's message/key.

Ideally can be generically applied to any collection.

Things I've attempted:

Tried custom data annotation validator, but can only access the values after it's been parsed.

Tried accessing validation errors via HttpActionContext's ModelState but at best, I can only get these values...

System.Web.Http.Controllers.HttpActionContext actionContext.ModelState.["model.GeographySearch.GeographyIDs[0]"].Errors[0].Exception.Message => "Error converting value \"sonali7678687\" to type 'System.Int64'. Path 'subjectSearch.writingAbout[0]', line 6, position 36."
System.Web.Http.Controllers.HttpActionContext actionContext.ModelState.["model.GeographySearch.GeographyIDs[0]"].Errors[0].Exception.InnerException.Message => "Input string was not in a correct format."

... surely there must be a better way of validating?

UPDATE 1: Rephrased question to make explanation and intent more clear.

Answer 1

You can try JSON Schema validator, modify your method to receive JSON body, valid it first then convert it to model.

public class SquirrelController : ApiController
{
    [HttpPost]
    [Route("api/squirrels/search")]
    public SquirrelsResponse Squirrels(PostBody model)
    {
        var generator = new JSchemaGenerator();
        var schema = generator.Generate(typeof(SquirrelsRequest));
        var body = JObject.Parse(model.Body);

        bool valid = body.IsValid(schema, out IList<string> messages);
        if (!valid)
        {
            // Fail, do something
        }

        // Success
    }
}

public class PostBody
{
    public string Body { get; set; }
}

Get more info...
Validating JSON with JSON Schema
Json.NET Schema

Answer 2

Why is your type long? Also, are you going to be working with decimals? If not, what you need is int.TryParse(). Wherever you have int.parse (or long.parse() in your case) Replace it with int.TryParse(). TryParse returns a boolean (True or False) that will let you know if the string to be parsed was a number or not.

example:

bool isNumber = int.TryParse("Data I'm Trying to parse but it's a string", variableTheResultWillGetStoredToifSuccessful);

This will come back false, and my super long variable name that you should never use is going to stay empty or rather unchanged. Therefore, you can do

if(isNumber == false){
//skip storing the number
}else{
//keep doing what you're doing;
}

You can also do, more concisely a rewrite of the above:

if(isNumber){
//keep doing what you're doing
}

More or less, that's the summary of it. TryParse will return a true or false based on if the parsing was successful or not without crashing your code, then you can use that information to continue how you seem fit.

If you're going to be working with decimals use double instead of int.