Calling MapInfo Professional from C# and Borland Delphi 8 is very similar. This code should work with Delphi with some minor tweaks. Borland Delphi 2005 is very different. The following instructions are based on Microsoft Visual Studio 2003.

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Step 1. Create a new C# Class Library File->New->Visual C# Projects->Class Library.

using System;
using MapBasic;
using MapInfo;


namespace MI_CS_Sample
{
	/// 
	/// Sample DLL for MapInfo Professional 8
	/// 
	public class csMapInfo 
	{
		private bool bTest = false;
		public csMapInfo()
		{

			//
			// TODO: Add constructor logic here
			//
			csMIForm shForm;
			shForm = new csMIForm();
			shForm.ShowDialog();
		}

		/// 
		/// //This is a test property
		/// 
		public bool Extra
		{
			get
			{
				return true;
			}
			set
			{
			}
		}
	}
}

Step 2. Add Additional Forms to your project File->Add New Item->Windows Form

using System;
using System.Text;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;

// MapInfo Specific Uses
using System.Runtime.InteropServices;
using MapInfo;
using MapBasic;


namespace MI_CS_Sample
{
	/// 
	/// Summary description for csMIForm.
	/// 
	public class csMIForm : System.Windows.Forms.Form
	{
		// MapInfo OLE Connection
		private MapInfo.MapInfoApplication oleMI;

		private System.Windows.Forms.Button btnConnect;
		private System.Windows.Forms.ListBox lbMapInfo;
		/// 
		/// Required designer variable.
		/// 
		private System.ComponentModel.Container components = null;

		public csMIForm()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after InitializeComponent call
			//
		}

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

		#region Windows Form Designer generated code
		/// 
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// 
		private void InitializeComponent()
		{
			this.btnConnect = new System.Windows.Forms.Button();
			this.lbMapInfo = new System.Windows.Forms.ListBox();
			this.SuspendLayout();
			// 
			// btnConnect
			// 
			this.btnConnect.Location = new System.Drawing.Point(8, 8);
			this.btnConnect.Name = "btnConnect";
			this.btnConnect.Size = new System.Drawing.Size(408, 56);
			this.btnConnect.TabIndex = 0;
			this.btnConnect.Text = "Connect to MapInfo Professional";
			this.btnConnect.Click += new System.EventHandler(this.btnConnect_Click);
			// 
			// lbMapInfo
			// 
			this.lbMapInfo.Location = new System.Drawing.Point(8, 72);
			this.lbMapInfo.Name = "lbMapInfo";
			this.lbMapInfo.Size = new System.Drawing.Size(400, 121);
			this.lbMapInfo.TabIndex = 1;
			// 
			// csMIForm
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(424, 266);
			this.Controls.Add(this.lbMapInfo);
			this.Controls.Add(this.btnConnect);
			this.Name = "csMIForm";
			this.Text = "csMIForm";
			this.Closing += new System.ComponentModel.CancelEventHandler(this.csMIForm_Closing);
			this.ResumeLayout(false);

		}
		#endregion

		private void btnConnect_Click(object sender, System.EventArgs e)
		{
			string sRes = "";  // Used to store MapInfo results
			string sCmd = "";  // Used to issue commands to MapInfo

			#region Connect to MapInfo Professional
			oleMI = new MapInfo.MapInfoApplicationClass();
			try
			{
				oleMI = (MapInfo.MapInfoApplication) Marshal.GetActiveObject("MapInfo.Application");
			}
			catch 
			{
				MessageBox.Show("Unable to connect to MapInfo Professional");
				Application.Exit();
			}
			#endregion

			#region Get Stuff From MapInfo
			try
			{
				// Display a Note and Print Command
				sCmd = "OLE Connection Created";
				oleMI.Do("Print " + '\u0022' + sCmd + '\u0022');

				// Get the Number of Windows
				sCmd = "NumWindows()";
				sRes = oleMI.Eval(sCmd);
				lbMapInfo.Items.Add("Open Windows = " + sRes);

                // Get the Number of Tables
				sCmd = "NumTables()";
				sRes = oleMI.Eval(sCmd);
				lbMapInfo.Items.Add("Open Tables = " + sRes);
			}
			catch
			{
				MessageBox.Show("An Error Has Occured.");
				Application.Exit();
			}
			#endregion
	
			#region Close Connection to MapInfo
			oleMI.RunMenuCommand(MapBasic.Constants.M_TOOLS_SELECTOR);
			oleMI = null;
			#endregion
		}

		private void csMIForm_Closing(object sender, System.ComponentModel.CancelEventArgs e)
		{
			Application.Exit();
		}
	}
}

Step 3. Add the Interop.MapInfo.DLL and MapBasic.CS files as references Project->Add References->Projects->Browse (browse to Interop.MapInfo.DLL)->Select->OK and File->Add Existing Item->(Browse to MapBasic.CS)->Open

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Key Parts

using System.Runtime.InteropServices; Provides COM interoperability.
using MapInfo; Allows you to use functions in the Interop.MapInfo.DLL.
using MapBasic; Allows you to use constants defined in the MapBasic.CS.
private MapInfo.MapInfoApplication oleMI; Creates a variable "oleMI" with OLE properties and methods exposed by the Interoperability DLL.
oleMI = new MapInfo.MapInfoApplicationClass(); Initializes the OLE variable.
oleMI = (MapInfo.MapInfoApplication) Marshal.GetActiveObject("MapInfo.Application"); Creates the actual OLE connection to a running instance of MapInfo Professional.
From this point if you want to run a menu command, you use .RunMenuCommand

oleMI.RunMenuCommand(MapBasic.Constants.M_TOOLS_SELECTOR);

Return a value back to your DLL, you use .Eval. A string is returned which you must test and cast to what you require.

// Get the Number of Windows
sCmd = "NumWindows()";
sRes = oleMI.Eval(sCmd);
lbMapInfo.Items.Add("Open Windows = " + sRes);

Just do something (execute a SQL Query) .Do.

// Display a Print Command
sCmd = "OLE Connection Created";
oleMI.Do("Print " + '\u0022' + sCmd + '\u0022');

oleMI = null; Don't forget to close your OLE connection when you have finished.

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OK, this was the simple bit. This will build a managed C# DLL, the only problem is that MapInfo Professional cannot use it. To be able to call the DLL you must create a bridge DLL. But this code shows you how to establish an OLE call to MapInfo Professional.