The various constructor overloads for [`ReportParameter`][1] only take in a string or string array as acceptable input.

[![ReportParameter Constructor][2]][1]


And the [`ReportParameter.Values`][3] property itself is actually a `StringCollection` in order to force the serialization to happen at compile time.

[![ReportParameter Properties][4]][3]

But you *can pass a null value* with string typing per this thread on [Passing NULL parameter from aspx page to Report Server][5] like this:

    var rp = new ReportParameter("ServiceType_cd", new string[] { null });

Or per this question [Report Viewer: Set null value to Report Parameters having allow null true][6], you can pass in a value like this:

    string str = null;
    var rp = new ReportParameter("ServiceType_cd", str));

  [1]: https://msdn.microsoft.com/en-us/library/Microsoft.Reporting.WebForms.ReportParameter#Constructors "Microsoft.Reporting.WebForms.ReportParameter"
  [2]: https://i.stack.imgur.com/TV060.png
  [3]: https://msdn.microsoft.com/en-us/library/Microsoft.Reporting.WebForms.ReportParameter#Properties "Microsoft.Reporting.WebForms.ReportParameter"
  [4]: https://i.stack.imgur.com/1QhTr.png
  [5]: https://forums.asp.net/t/1113545.aspx?Passing+NULL+parameter+from+aspx+page+to+Report+Server
  [6]: https://stackoverflow.com/q/22625842/1366033

How can I put a null value on datetime variable type?
 
I tried to make it nullable value, but I need to use it in `ReportParameter()` method to send it to my report but `ReportParameter()` constructor cannot take a nullable value and the ReportParameter() take just a string values!