C#: Call External DLL Library

person Jason Huangfolder_open.NET, C#, Codelocal_offer, , access_time September 16, 2009

To import external DLL library file and be able to use the function in the library.
(That is the way to make use of old windows dll libraries. To use a .NET assembly you would simply add the reference to .NET assembly and add the using clause.)

You need to:

  • Add the using System.Runtime.InteropServices (this .NET class handles interoperation between libraries).
  • Before the function you declare, use [DllImport(“library.dll file name”)].
  • Then declare the function explicitly (declare the function in the library that you want to call with parameters etc).

In the below simple example. I want to use the a function called InternetGetConnectedState from the dll file wininet.dll.

using System;
using System.Collections.Generic;
using System.Text;
// DllImport
using System.Runtime.InteropServices; 
namespace DatabaseFunctions
	class ConnectionManager
                //define the external function
		private extern static bool InternetGetConnectedState( out int connectionDescription, int reservedValue ) ;
		private bool bIsOnLine = false;
		public ConnectionManager()
		private bool IsOnLine()
			this.bIsOnLine = queryConnetionStatus();
			return this.bIsOnLine;
		private bool queryConnetionStatus()
			int connectionDescription = 0;
			return InternetGetConnectedState(out connectionDescription, 0);


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