Problem to get property value from user Control in C#

I've gone back over your original question and realized I didn't understand what your goal is here. Here's a working template you can use to experiment with ... hopefully learn from.

The challenge, in this case, is interesting: you are kind of "bending" the RadioButton to a task it wasn't designed for. In this case ... a single-answer-is-correct questionnaire ... you want a group of UserControls to behave as if their individual RadioButtons are part of a group of RadioButtons in the same container.

I created a UserControl named 'ucQuestion that has one TextBox, and one RadioButton.

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace Nov30_QuestionnaireProtoType
{
    public partial class ucQuestion : UserControl
    {
        public ucQuestion()
        {
            InitializeComponent();
        }

        // expose the TextBox
        public TextBox theUCTextBox { get; private set; }

        // expose the CheckState of the RadioButton
        public CheckState theUCCheckState { get; private set; }

        // keep a List of all ucQuestions created
        private static List<ucQuestion> ucQuestionList; 

        // use of a boolean variable here is unfortunately
        // required to deal with the quirks of having a single
        // RadioButton in a Control
        private bool isRadioButtonChecked = false;

        private void ucQuestion_Load(object sender, EventArgs e)
        {
            // initialize the static List<ucQuestion> only once
            if(ucQuestionList == null) ucQuestionList = new List<ucQuestion>();

            // add the current instance of this UserControl to the List of ucQuestion
            ucQuestionList.Add(this);

            // set the Public Property that exposes the internal TextBox
            theUCTextBox = textBox1;
            
            // initialize the Public Property that exposes the internal RadioButton
            // CheckState: note that's an Enumeration, not a boolean value !
            theUCCheckState = CheckState.Unchecked;
            
            // for "safety" ... probably unnecessary
            radioButton1.Checked = false;
        }

        private void radioButton1_Click(object sender, EventArgs e)
        {
            isRadioButtonChecked = ! isRadioButtonChecked;
            radioButton1.Checked = isRadioButtonChecked;
            theUCCheckState = isRadioButtonChecked ? CheckState.Checked : CheckState.Unchecked;
            
            // if the RadioButton is Checked, make sure all other
            // ucQuestion Controls in the List<ucQuestion>
            // have their RadioButtons Unchecked
            if (isRadioButtonChecked)
            {
                foreach (ucQuestion theUCQuestion in ucQuestionList)
                {
                    if (theUCQuestion == this) continue;

                    theUCQuestion.isRadioButtonChecked = false;
                    theUCQuestion.radioButton1.Checked = false;
                    theUCQuestion.theUCCheckState = CheckState.Unchecked;
                }
            }
        }
    }
}

This was done in a hurry (fifteen minutes), and I'm out of time for today, so it's not fully tested. An obvious limitation is that using a static List of ucQuestions means you can't create multiple groups of ucQuestions: they'd all reference the same List. That may, or may not, "play well" with what you are trying to accomplish.

If you needed to create multiple "independent" groups of ucQuestions, a logical next step might be to create a compound-UserControl designed to host a group of ucQuestions.