How to update abstract class(used both in the request parameter and response parameters) property value at run time by derived class in C#

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C#

MVC4

How to update abstract class property value at run time by derived class in c#

I have following abstract class

public abstract class BaseResponse
{
 public string TestId{ get; set; }
}

I have two concrete class(TestRequest and TestResponse) derived from above abstract class

public class TestResponse: BaseResponse
    {
        
    }

public class TestRequest: BaseResponse
    {
        //some properties

    }

I have passed some value for TestId in the request parameters. As TestId is used for TestRequest and TestResponse both. when i tried to update the TestId got exception "

object reference not set to an instance of an object 

" .Following are the code

private async Task<IEnumerable<testresponse>> GetTestResponse(){

TestResponse testObject = new TestResponse();
testObject.TestId = "some data here";

}

I want to update TestId using TestResponse class, how to achieve it?

Thanks in advance!

What I have tried:

<pre>TestResponse testObject = new TestResponse();
testObject.TestId = "some data here";