Why default Model binder behaving differently?

Question

I am creating API in which has following url

/api/abc/?q=1&a=2&b=3&b=4
                  ^^^^^^^

Input.cs(class used in ModelBinding)

...
public string A { get; set; }
public string B { get; set; }
public string Q { get; set; }
...

I am using default ModelBinding of .NET, But problem with that is when I passed above url, following values are assigned to the property

obj.A = "2"   // here obj is object of Input class
obj.B = "3"
obj.Q = "1"

I am expecting obj.B = "3,4"(when I am doing Request.QueryString["b"], it giving output as "3,4"), but it is binding the first value only.

Why is this happening?(I don't know internals of default ModelBinding but I am guessing it somewhere using Request.QueryString for binding).

Can anyone tell me why it is happening and How can I get "3,4" as obj.B value?

My approach for getting "3,4" for B is

using custom Model Binder I have done following

public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
    object model = base.BindModel(controllerContext, bindingContext);
    var obj = model as Input;
    obj.B = Request.QueryString["b"];
}

score 2 · Answer 1 · answered Jul 31 '18 at 08:22

You are sending multiple values for B which means that you need to make B a collection. Note the DefaultModelBinder binds the first matching name/value pair and ignores the rest if your property is not IEnumerable.

Change the property to

public IEnumerable<string> B { get; set; }

and it will contain both values (if you then actually want 3,4, you can use String.Join on the array).

Why default Model binder behaving differently?

1 Answers1