Сделал первый компонент

WinFormsControlLibrary1/ComboSingleAddControl.Designer.cs

@@ -0,0 +1,37 @@
+namespace WinFormsControlLibrary1
+{
+    partial class ComboSingleAddControl
+    {
+        /// <summary> 
+        /// Обязательная переменная конструктора.
+        /// </summary>
+        private System.ComponentModel.IContainer components = null;
+
+        /// <summary> 
+        /// Освободить все используемые ресурсы.
+        /// </summary>
+        /// <param name="disposing">истинно, если управляемый ресурс должен быть удален; иначе ложно.</param>
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing && (components != null))
+            {
+                components.Dispose();
+            }
+            base.Dispose(disposing);
+        }
+
+        #region Код, автоматически созданный конструктором компонентов
+
+        /// <summary> 
+        /// Требуемый метод для поддержки конструктора — не изменяйте 
+        /// содержимое этого метода с помощью редактора кода.
+        /// </summary>
+        private void InitializeComponent()
+        {
+            components = new System.ComponentModel.Container();
+            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
+        }
+
+        #endregion
+    }
+}

WinFormsControlLibrary1/ComboSingleAddControl.cs

@@ -0,0 +1,50 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Data;
+using System.Drawing;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Forms;
+
+namespace WinFormsControlLibrary1;
+
+[DefaultEvent(nameof(SelectedValueChanged))]
+public partial class ComboSingleAddControl : UserControl
+{
+    private readonly ComboBox _combo = new();
+    public EventHandler? SelectedValueChanged;
+    public ComboSingleAddControl()
+    {
+        InitializeComponent();
+        Controls.Add(_combo);
+        _combo.SelectedIndexChanged += (_, __) => SelectedValueChanged?.Invoke(this, EventArgs.Empty);
+    }
+
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
+    public string SelectedValue
+    {
+        get => _combo.SelectedIndex.ToString() ?? string.Empty; 
+        set
+        {
+            if(string.IsNullOrEmpty(value)) { _combo.SelectedIndex = -1; return; }
+            int id = _combo.Items.IndexOf(value);
+            if(id >= 0) _combo.SelectedIndex = id;
+        }
+    }
+
+    public void AddValue(string value)
+    {
+        if (string.IsNullOrEmpty(value)) return;
+        if (_combo.Items.Contains(value)) return;
+        _combo.Items.Add(value);
+        if(_combo.SelectedIndex < 0) _combo.SelectedIndex = 0;
+    }
+
+    public void ClearItems()
+    {
+        _combo.Items.Clear();
+        _combo.SelectedIndex = -1;
+    }
+}

WinFormsControlLibrary1/UserControl1.Designer.cs

@@ -1,38 +0,0 @@
-namespace WinFormsControlLibrary1
-{
-    partial class UserControl1
-    {
-        /// <summary>
-        /// Required designer variable.
-        /// </summary>
-        private System.ComponentModel.IContainer components = null;
-
-        /// <summary>
-        /// Clean up any resources being used.
-        /// </summary>
-        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
-        protected override void Dispose(bool disposing)
-        {
-            if (disposing && (components != null))
-            {
-                components.Dispose();
-            }
-            base.Dispose(disposing);
-        }
-
-        #region Component Designer generated code
-
-        /// <summary>
-        /// Required method for Designer support - do not modify 
-        /// the contents of this method with the code editor.
-        /// </summary>
-        private void InitializeComponent()
-        {
-            components = new System.ComponentModel.Container();
-            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
-            this.ClientSize = new System.Drawing.Size(800, 450);
-        }
-
-        #endregion
-    }
-}

WinFormsControlLibrary1/UserControl1.cs

@@ -1,10 +0,0 @@
-namespace WinFormsControlLibrary1
-{
-    public partial class UserControl1 : UserControl
-    {
-        public UserControl1()
-        {
-            InitializeComponent();
-        }
-    }
-}