How to use implicit operators in class for passing parameter in web api c#

Question

I have Web API which throws an error if one of the parameters passed in a URL query is null. Let's say I cant overwrite anything to the above validation.

So that, I want to have a Web API as below, the parameter c is string and the API call may pass an empty string which instantiates parameter c as null and not "".

    [Route("{id}")]
    [HttpGet]
    public IdDTO GetIdDetails(
        int Id,
        [FromUri] int a,
        [FromUri] string b,
        [FromUri] string c,
    {
         //doing something here...
    }

A typical API call is localhost:3000/123?a=1&b=abc&c=

I also changed the data type of c to a custom datatype ReqString ([FromUri] ReqString <string> c) as below

public struct ReqString<T>
{
    private T _value;

    public ReqString(T s)
    {
        _value = s;
    }

    public static implicit operator ReqString<T>(T s) => new ReqString<T>(s);

    public static implicit operator string(ReqString<T> s)
    {
        if(typeof(T).Equals(typeof(string)))
        {
            return s._value as string == null ? "" : s._value as string;
        }
        else
        {
            return s._value.ToString();
        }
    }
}

The problem now is, the value for c is "". But if I pass values for parameter c in API URL query, it is still "" and not the value passed

for eg: localhost:3000/123?a=1&b=abc&c=def

ie, the _value is still null

So my question is How can I use implicit operators to instantiate _value with query value?

UPDATE

The custom datatype works when instantiated in a statement as below, but I want to get similar result in a function call parameter

ReqString rStr = "testing";

Is a custom model binder an option for you? (stackoverflow.com/questions/62277271/…) — Michael
– Michael, Commented Aug 13, 2021 at 10:34

Shereif SRF · Accepted Answer · 2021-08-13 14:20:18Z

As per Michael comment, I took a look into ModalBinder. I was able to resolve my issue. Below is the sample solution I used

public class ReqStringModelBinder : IModelBinder
{
    public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType != typeof(ReqString))
        {
            return false;
        }

        ValueProviderResult val = bindingContext.ValueProvider.GetValue(
            bindingContext.ModelName);
        if (val == null)
        {
            return false;
        }

        string key = val.RawValue as string;
        if (key == null)
        {
            bindingContext.ModelState.AddModelError(
                bindingContext.ModelName, "Wrong value type");
            return false;
        }

        ReqString result;
        if (ReqString.TryParse(key, out result))
        {
            bindingContext.Model = result;
            return true;
        }

        bindingContext.ModelState.AddModelError(
            bindingContext.ModelName, "Cannot convert value to ReqString");
        return false;
    }
}

[ModelBinder(typeof(ReqStringModelBinder))]
public class ReqString
{
    public string value { get; set; }
    public static bool TryParse(string s, out ReqString result)
    {
        result = null;
        result = new ReqString() { value = String.IsNullOrEmpty(s) ? "" : s };
        return true;
    }

    public static implicit operator string(ReqString reqString) => reqString.value;
}

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