Need a SetConfig method for your Configuration Settings? What about an AppSettingsWriter?

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Reflection;
using System.Configuration;
using System.Xml;

namespace ConfigurationSettingsRWSample
{
	public class ConfigurationSettingsRWForm : System.Windows.Forms.Form
	{
		#region Fields
		private System.Windows.Forms.DataGrid grdAppSettings;
		private System.Data.DataTable tblAppSettings;
		private bool bSettingsLoaded;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;
		private System.Windows.Forms.Label lblAppSettings;
		private System.Windows.Forms.Label label1;
		private System.Windows.Forms.Button btnSave;
		private System.Windows.Forms.Button btnLoad;
		private System.Windows.Forms.DataGrid grdCustomSettings;
		private ConfigurationSettingsRW config = null;
		#endregion

		#region Constructor and Dispose
		public ConfigurationSettingsRWForm()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();
		
			string configFilePath = Assembly.GetCallingAssembly().Location + ".config";
			config = new ConfigurationSettingsRW(configFilePath);

			tblAppSettings = new DataTable("AppSettings");
			tblAppSettings.Columns.Add("key", typeof(System.String));
			tblAppSettings.Columns.Add("value", typeof(System.String));

			bSettingsLoaded = false;
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#endregion

		#region Windows Form 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()
		{
			this.grdAppSettings = new System.Windows.Forms.DataGrid();
			this.lblAppSettings = new System.Windows.Forms.Label();
			this.label1 = new System.Windows.Forms.Label();
			this.grdCustomSettings = new System.Windows.Forms.DataGrid();
			this.btnSave = new System.Windows.Forms.Button();
			this.btnLoad = new System.Windows.Forms.Button();
			((System.ComponentModel.ISupportInitialize)(this.grdAppSettings)).BeginInit();
			((System.ComponentModel.ISupportInitialize)(this.grdCustomSettings)).BeginInit();
			this.SuspendLayout();
			// 
			// grdAppSettings
			// 
			this.grdAppSettings.DataMember = "";
			this.grdAppSettings.HeaderForeColor = System.Drawing.SystemColors.ControlText;
			this.grdAppSettings.Location = new System.Drawing.Point(6, 24);
			this.grdAppSettings.Name = "grdAppSettings";
			this.grdAppSettings.Size = new System.Drawing.Size(280, 128);
			this.grdAppSettings.TabIndex = 0;
			// 
			// lblAppSettings
			// 
			this.lblAppSettings.AutoSize = true;
			this.lblAppSettings.Location = new System.Drawing.Point(8, 8);
			this.lblAppSettings.Name = "lblAppSettings";
			this.lblAppSettings.Size = new System.Drawing.Size(64, 16);
			this.lblAppSettings.TabIndex = 1;
			this.lblAppSettings.Text = "appSettings";
			// 
			// label1
			// 
			this.label1.AutoSize = true;
			this.label1.Location = new System.Drawing.Point(296, 8);
			this.label1.Name = "label1";
			this.label1.Size = new System.Drawing.Size(82, 16);
			this.label1.TabIndex = 3;
			this.label1.Text = "customSettings";
			// 
			// grdCustomSettings
			// 
			this.grdCustomSettings.DataMember = "";
			this.grdCustomSettings.HeaderForeColor = System.Drawing.SystemColors.ControlText;
			this.grdCustomSettings.Location = new System.Drawing.Point(296, 24);
			this.grdCustomSettings.Name = "grdCustomSettings";
			this.grdCustomSettings.Size = new System.Drawing.Size(280, 128);
			this.grdCustomSettings.TabIndex = 2;
			// 
			// btnSave
			// 
			this.btnSave.Location = new System.Drawing.Point(296, 160);
			this.btnSave.Name = "btnSave";
			this.btnSave.TabIndex = 4;
			this.btnSave.Text = "Save";
			this.btnSave.Click += new System.EventHandler(this.btnSave_Click);
			// 
			// btnLoad
			// 
			this.btnLoad.Location = new System.Drawing.Point(208, 160);
			this.btnLoad.Name = "btnLoad";
			this.btnLoad.TabIndex = 5;
			this.btnLoad.Text = "Load";
			this.btnLoad.Click += new System.EventHandler(this.btnLoad_Click);
			// 
			// ConfigurationSettingsRWForm
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(582, 191);
			this.Controls.Add(this.btnLoad);
			this.Controls.Add(this.btnSave);
			this.Controls.Add(this.label1);
			this.Controls.Add(this.grdCustomSettings);
			this.Controls.Add(this.lblAppSettings);
			this.Controls.Add(this.grdAppSettings);
			this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedDialog;
			this.Name = "ConfigurationSettingsRWForm";
			this.Text = "ConfigurationSettingsRW";
			((System.ComponentModel.ISupportInitialize)(this.grdAppSettings)).EndInit();
			((System.ComponentModel.ISupportInitialize)(this.grdCustomSettings)).EndInit();
			this.ResumeLayout(false);

		}
		#endregion

		#region Main
		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.Run(new ConfigurationSettingsRWForm());
		}

		#endregion

		#region Specific methods
		private void LoadSettings()
		{
			#region appSettings
			// Loading the config.AppSettings from the configuration file.
			config.LoadAppSettings();
			// Clearing the DataSource of the appSettings DataGrid
			grdAppSettings.DataSource = null;
			// Creating a DataTable containing the config.AppSettings key-value data.
			tblAppSettings.Clear();
			for (int i = 0; i < config.AppSettings.Count; i++)
			{
				string key = config.AppSettings.GetKey(i);
				string val = config.AppSettings.Get(i);
			
				DataRow row = tblAppSettings.NewRow();
				row["key"] = key;
				row["value"] = val;
				tblAppSettings.Rows.Add(row);
			}
			// Assigning the created DataTable to the DataSource of the appSettings DataGrid.
			grdAppSettings.DataSource = tblAppSettings;
			#endregion

			#region customSettings
			// Clearing the DataSource of the customSettings DataGrid
			grdCustomSettings.DataSource = null;
			// Getting a DataTable containing the customSettings data.
			DataTable tblCustomSettings = (DataTable)config.GetConfig("customSettings");
			// Assigning the DataTable to the DataSource of the customSettings DataGrid.
			grdCustomSettings.DataSource = tblCustomSettings;
			#endregion
		}

		private void SaveSettings()
		{
			#region appSettings
			// Recreating the config.AppSettings collection with the data contained in appSettings DataGrid.
			config.AppSettings.Clear();
			foreach (DataRow row in tblAppSettings.Rows)
				config.AppSettings[row["key"].ToString()] = row["value"].ToString();
			// Saving the config.AppSettings data into the configuration file.
			config.SaveAppSettings();
			#endregion

			#region customSettings
			// Getting the DataTable that is the DataSource of the customSettings DataGrid.
			DataTable tblCustomSettings = (DataTable)grdCustomSettings.DataSource;
			// Setting the customSettings data to the config.
			config.SetConfig(tblCustomSettings, "customSettings");
			#endregion
		}
		#endregion

		#region Event handlers
		private void btnLoad_Click(object sender, System.EventArgs e)
		{
			LoadSettings();
			bSettingsLoaded = true;
		}

		private void btnSave_Click(object sender, System.EventArgs e)
		{
			if (bSettingsLoaded)
                SaveSettings();
		}
		#endregion
	}

	public class CustomSettingsSectionHandler: IConfigurationRWSectionHandler
	{
		#region IConfigurationRWSectionHandler Members
		public object Create(System.Xml.XmlNode section)
		{
			DataTable tblCustomSettings = new DataTable("CustomSettings");
			tblCustomSettings.Columns.Add("attr1", typeof(System.String));
			tblCustomSettings.Columns.Add("attr2", typeof(System.String));
			tblCustomSettings.Columns.Add("attr3", typeof(System.String));
			foreach (XmlNode field in section.SelectNodes("field"))
			{
				DataRow row = tblCustomSettings.NewRow();
				row["attr1"] = field.Attributes["attr1"].Value;
				row["attr2"] = field.Attributes["attr2"].Value;
				row["attr3"] = field.Attributes["attr3"].Value;
				tblCustomSettings.Rows.Add(row);
			}
			return tblCustomSettings;
		}

		public void Persist(object config, System.Xml.XmlNode section)
		{
			DataTable tblCustomSettings = (DataTable)config;
			section.RemoveAll();
			foreach (DataRow row in tblCustomSettings.Rows)
			{
				XmlNode node = section.OwnerDocument.CreateNode(XmlNodeType.Element, "field", "");

				XmlAttribute attr = section.OwnerDocument.CreateAttribute("attr1");
				attr.Value = row["attr1"].ToString();
				node.Attributes.Append(attr);

				attr = section.OwnerDocument.CreateAttribute("attr2");
				attr.Value = row["attr2"].ToString();
				node.Attributes.Append(attr);

				attr = section.OwnerDocument.CreateAttribute("attr3");
				attr.Value = row["attr3"].ToString();
				node.Attributes.Append(attr);

				section.AppendChild(node);
			}
		}
		#endregion
	}
}