How to bind optional querystring string parameter on POST - asp.net-web-api2

Question

I have the following controller in ASP.NET WebApi 2:
[RoutePrefix("Validations")]
public partial class ValidationsController
{
[HttpPost, Route("Bsb")]
public IHttpActionResult ValidateBsb(string value)
{
var validator = new BankStateBranchValidator(DbContext.BankStateBranches);
var data = new ValidationsResult
{
IsValid = validator.IsValid(value ?? string.Empty)
};
data.Error = data.IsValid
? null
: "The BSB you have entered does not appear to be valid. Please check the value and try again.";
return Ok(data);
}
}
For historical reasons, the value parameter needs to be in the querystring, rather than the form body, which should be empty. So the expected API call would be POST /Validate/Bsb?value=012345.
That all works fine, and I get the expected result; however, sometimes we are getting clients calling the API with POST /Validate/Bsb or POST /Validate/Bsb?value=, and that is resulting in a 400 Bad Request response from WebAPI itself, because, as far as I can tell, the model binder is failing to bind the missing value to the parameter. If I put a breakpoint inside the method, it never gets hit.
So, given that I can't change the API contract, how can I handle this scenario? I've tried adding a [ValueProvider(typeof(RouteDataValueProviderFactory))] attribute to the parameter, and my test case for the missing value works, but then the valid value test cases break since the value isn't in the route but in the querystring.
Update
Based on Craig H's suggestion, I've added a default value to the value parameter. So the various scenarios are:
POST /Validate/Bsb?value=012345 - pass (valid value)
POST /Validate/Bsb?value=000000 - pass (invalid value)
POST /Validate/Bsb?value= - fail (empty value)
POST /Validate/Bsb - pass (missing value)

Answer 1

You should be able to make the parameter optional by specifying a default value in the method signature.
e.g.
[HttpPost, Route("Bsb")]
public IHttpActionResult ValidateBsb(string value = null)
Your question says that a query with ?value= was throwing a bad request.
When I tried this locally my breakpoint was hit and value was null.
If I omitted the QS parameter completely, then I received a method not allowed response.
This page makes mention of optional route parameters with attribute routing, although you are not specifying the parameter like that here.
I cannot find the document which describes the other options with regards to routing and optional parameters. I have seen one which indicates the differences between defining it as optional in the route definition, and optional in the method signature. If I find it, I will update this answer!