C# Method parameter incorrectly updates

I have the following C# simple code :

public class Error
{
    public string ErrorDescription { get; set; }
    public string ErrorCode { get; set; }
}

public class Request
{
    public Error RequestError { get; set; }
}

public class Response
{
    public Error ResponseError { get; set; }
}

public Response Process(Request request)
    {
        var r = new Response
        {
            ResponseError = request.RequestError
        };

        r.ResponseError.ErrorDescription = "New Response Description";

        return r;
    }

I call Process through a console app with :

var request = new Request
{
    RequestError = new Error()
    {
        ErrorCode = "Request Error Code",
        ErrorDescription = "Request Error Description"
    }
};

var service = new Service();
var response = service.Process(request);
Debug.WriteLine("request ErrorDescription = " + request.RequestError.ErrorDescription);

Why request.ResponseError.ErrorDescription is equal to "New Response Description"; I have not updated request object and it's not called by out/ref. I only update response object that I return.

What am I doing wrong?

Thanks