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Inter-Process Communication in .NET Using Named Pipes, Part 1

, 14 Jun 2004
This article explores a way of implementing Named Pipes based Inter-Process Communication between .NET applications
using System;
using System.Runtime.InteropServices;

using AppModule.InterProcessComm;

namespace AppModule.NamedPipes {
	#region Comments
	/// <summary>
	/// A utility class that exposes named pipes operations.
	/// </summary>
	/// <remarks>
	/// This class uses the exposed exposed kernel32.dll methods by the 
	/// <see cref="AppModule.NamedPipes.NamedPipeNative">NamedPipeNative</see> class
	/// to provided controlled named pipe functionality.
	/// </remarks>
	#endregion
	public sealed class NamedPipeWrapper {
		#region Comments
		/// <summary>
		/// The number of retries when creating a pipe or connecting to a pipe.
		/// </summary>
		#endregion
		private const int ATTEMPTS = 2;
		#region Comments
		/// <summary>
		/// Wait time for the 
		/// <see cref="AppModule.NamedPipes.NamedPipeNative.WaitNamedPipe">NamedPipeNative.WaitNamedPipe</see> 
		/// operation.
		/// </summary>
		#endregion
		private const int WAIT_TIME = 5000;
		#region Comments
		/// <summary>
		/// Reads a string from a named pipe using the UTF8 encoding.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		/// <param name="maxBytes">The maximum bytes to read.</param>
		/// <returns>A UTF8 string.</returns>
		/// <remarks>This function uses 
		/// <see cref="AppModule.NamedPipes.NamedPipeWrapper.ReadBytes">AppModule.NamedPipes.ReadBytes</see> 
		/// to read the bytes from the pipe and then converts them to string.<br/><br/>
		/// The first four bytes of the pipe data are expected to contain 
		/// the data length of the message. This method first reads those four 
		/// bytes and converts them to integer. It then continues to read from the pipe using 
		/// the extracted data length.
		/// </remarks>
		#endregion
		public static string Read(PipeHandle handle, int maxBytes) {
			string returnVal = "";
			byte[] bytes = ReadBytes(handle, maxBytes);
			if (bytes != null) {
				returnVal = System.Text.Encoding.UTF8.GetString(bytes);
			}
			return returnVal;
		}
		#region Comments
		/// <summary>
		/// Reads the bytes from a named pipe.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		/// <param name="maxBytes">The maximum bytes to read.</param>
		/// <returns>An array of bytes.</returns>
		/// <remarks>This method expects that the first four bytes in the pipe define 
		/// the length of the data to read. If the data length is greater than 
		/// <b>maxBytes</b> the method returns null.<br/><br/>
		/// The first four bytes of the pipe data are expected to contain 
		/// the data length of the message. This method first reads those four 
		/// bytes and converts them to integer. It then continues to read from the pipe using 
		/// the extracted data length.
		/// </remarks>
		#endregion
		public static byte[] ReadBytes(PipeHandle handle, int maxBytes) {
			byte[] numReadWritten = new byte[4];
			byte[] intBytes = new byte[4];
			byte[] msgBytes = null;
			int len;
			
			// Set the Handle state to Reading
			handle.State = InterProcessConnectionState.Reading;
			// Read the first four bytes and convert them to integer
			if (NamedPipeNative.ReadFile(handle.Handle, intBytes, 4, numReadWritten, 0)) {
				len = BitConverter.ToInt32(intBytes, 0);
				msgBytes = new byte[len];
				// Read the rest of the data
				if (!NamedPipeNative.ReadFile(handle.Handle, msgBytes, (uint)len, numReadWritten, 0)) {
					handle.State = InterProcessConnectionState.Error;
					throw new NamedPipeIOException("Error reading from pipe. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
				}
			}
			else {
				handle.State = InterProcessConnectionState.Error;
				throw new NamedPipeIOException("Error reading from pipe. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
			}
			handle.State = InterProcessConnectionState.ReadData;
			if (len > maxBytes) {
				return null;
			}
			return msgBytes;
		}
		#region Comments
		/// <summary>
		/// Writes a string to a named pipe.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		/// <param name="text">The text to write to the pipe.</param>
		/// <remarks>This method converts the text into an array of bytes, using the 
		/// UTF8 encoding and the uses 
		/// <see cref="AppModule.NamedPipes.NamedPipeWrapper.WriteBytes">AppModule.NamedPipes.WriteBytes</see>
		/// to write to the pipe.<br/><br/>
		/// When writing to a pipe the method first writes four bytes that define the data length.
		/// It then writes the whole message.</remarks>
		#endregion
		public static void Write(PipeHandle handle, string text) {
			WriteBytes(handle, System.Text.Encoding.UTF8.GetBytes(text));
		}
		#region Comments
		/// <summary>
		/// Writes an array of bytes to a named pipe.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		/// <param name="bytes">The bytes to write.</param>
		/// <remarks>If we try bytes array we attempt to write is empty then this method write a space character to the pipe. This is necessary because the other end of the pipe uses a blocking Read operation so we must write someting.<br/><br/>
		/// The bytes length is restricted by the <b>maxBytes</b> parameter, which is done primarily for security reasons.<br/><br/>
		/// When writing to a pipe the method first writes four bytes that define the data length.
		/// It then writes the whole message.</remarks>
		#endregion
		public static void WriteBytes(PipeHandle handle, byte[] bytes) {
			byte[] numReadWritten = new byte[4];
			uint len;

			if (bytes == null) {
				bytes = new byte[0];
			}
			if (bytes.Length == 0) {
				bytes = new byte[1];
				bytes = System.Text.Encoding.UTF8.GetBytes(" ");
			}
			// Get the message length
			len = (uint)bytes.Length;
			handle.State = InterProcessConnectionState.Writing;
			// Write four bytes that define the message length
			if (NamedPipeNative.WriteFile(handle.Handle, BitConverter.GetBytes(len), 4, numReadWritten, 0)) {
				// Write the whole message
				if (!NamedPipeNative.WriteFile(handle.Handle, bytes, len, numReadWritten, 0)) {
					handle.State = InterProcessConnectionState.Error;
					throw new NamedPipeIOException("Error writing to pipe. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
				}
			}
			else {
				handle.State = InterProcessConnectionState.Error;
				throw new NamedPipeIOException("Error writing to pipe. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
			}
			handle.State = InterProcessConnectionState.Flushing;
			Flush(handle);
			handle.State = InterProcessConnectionState.FlushedData;
		}
		#region Comments
		/// <summary>
		/// Tries to connect to a named pipe on the same machine.
		/// </summary>
		/// <param name="pipeName">The name of the pipe.</param>
		/// <param name="handle">The resulting pipe handle.</param>
		/// <returns>Return true if the attempt succeeds.</returns>
		/// <remarks>This method is used mainly when stopping the pipe server. It unblocks the existing pipes, which wait for client connection.</remarks>
		#endregion
		public static bool TryConnectToPipe(string pipeName, out PipeHandle handle) {
			return TryConnectToPipe(pipeName, ".", out handle);
		}
		#region Comments
		/// <summary>
		/// Tries to connect to a named pipe.
		/// </summary>
		/// <param name="pipeName">The name of the pipe.</param>
		/// <param name="serverName">The name of the server.</param>
		/// <param name="handle">The resulting pipe handle.</param>
		/// <returns>Return true if the attempt succeeds.</returns>
		/// <remarks>This method is used mainly when stopping the pipe server. It unblocks the existing pipes, which wait for client connection.</remarks>
		#endregion
		public static bool TryConnectToPipe(string pipeName, string serverName, out PipeHandle handle) {
			handle = new PipeHandle();
			// Build the pipe name string
			string name = @"\\" + serverName + @"\pipe\" + pipeName;
			handle.State = InterProcessConnectionState.ConnectingToServer;
			// Try to connect to a server pipe
			handle.Handle = NamedPipeNative.CreateFile(name, NamedPipeNative.GENERIC_READ | NamedPipeNative.GENERIC_WRITE, 0, null, NamedPipeNative.OPEN_EXISTING, 0, 0);
			if (handle.Handle.ToInt32() != NamedPipeNative.INVALID_HANDLE_VALUE) {
				handle.State = InterProcessConnectionState.ConnectedToServer;
				return true;
			} else {
				handle.State = InterProcessConnectionState.Error;
				return false;
			}
		}
		#region Comments
		/// <summary>
		/// Connects to a server named pipe on the same machine.
		/// </summary>
		/// <param name="pipeName">The pipe name.</param>
		/// <returns>The pipe handle, which also contains the pipe state.</returns>
		/// <remarks>This method is used by clients to establish a pipe connection with a server pipe.</remarks>
		#endregion
		public static PipeHandle ConnectToPipe(string pipeName) {
			return ConnectToPipe(pipeName, ".");
		}
		#region Comments
		/// <summary>
		/// Connects to a server named pipe.
		/// </summary>
		/// <param name="pipeName">The pipe name.</param>
		/// <param name="serverName">The server name.</param>
		/// <returns>The pipe handle, which also contains the pipe state.</returns>
		/// <remarks>This method is used by clients to establish a pipe connection with a server pipe.</remarks>
		#endregion
		public static PipeHandle ConnectToPipe(string pipeName, string serverName) {
			PipeHandle handle = new PipeHandle();
			// Build the name of the pipe.
			string name = @"\\" + serverName + @"\pipe\" + pipeName;

			for (int i = 1; i<=ATTEMPTS; i++) {
				handle.State = InterProcessConnectionState.ConnectingToServer;
				// Try to connect to the server
				handle.Handle = NamedPipeNative.CreateFile(name, NamedPipeNative.GENERIC_READ | NamedPipeNative.GENERIC_WRITE, 0, null, NamedPipeNative.OPEN_EXISTING, 0, 0);
				if (handle.Handle.ToInt32() != NamedPipeNative.INVALID_HANDLE_VALUE) {
					// The client managed to connect to the server pipe
					handle.State = InterProcessConnectionState.ConnectedToServer;
					// Set the read mode of the pipe channel
					uint mode = NamedPipeNative.PIPE_READMODE_MESSAGE;
					if (NamedPipeNative.SetNamedPipeHandleState(handle.Handle, ref mode, IntPtr.Zero, IntPtr.Zero)) {
						break;
					}	
					if (i >= ATTEMPTS) {
						handle.State = InterProcessConnectionState.Error;
						throw new NamedPipeIOException("Error setting read mode on pipe " + name + " . Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
					}
				}
				if (i >= ATTEMPTS) {
					if (NamedPipeNative.GetLastError() != NamedPipeNative.ERROR_PIPE_BUSY) {
						handle.State = InterProcessConnectionState.Error;
						// After a certain number of unsuccessful attempt raise an exception
						throw new NamedPipeIOException("Error connecting to pipe " + name + " . Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
					} else {
						handle.State = InterProcessConnectionState.Error;
						throw new NamedPipeIOException("Pipe " + name + " is too busy. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
					}
				} else {
					// The pipe is busy so lets wait for some time and try again
					if (NamedPipeNative.GetLastError() == NamedPipeNative.ERROR_PIPE_BUSY) 
						NamedPipeNative.WaitNamedPipe(name, WAIT_TIME);
				}
			}

			return handle;
		}
		#region Comments
		/// <summary>
		/// Creates a server named pipe.
		/// </summary>
		/// <param name="name">The name of the pipe.</param>
		/// <param name="outBuffer">The size of the outbound buffer.</param>
		/// <param name="inBuffer">The size of the inbound buffer.</param>
		/// <returns>The pipe handle.</returns>
		#endregion
		public static PipeHandle Create(string name, uint outBuffer, uint inBuffer) {
			return Create(name, outBuffer, inBuffer, true);
		}
		#region Comments
		/// <summary>
		/// Creates a server named pipe.
		/// </summary>
		/// <param name="name">The name of the pipe.</param>
		/// <param name="outBuffer">The size of the outbound buffer.</param>
		/// <param name="inBuffer">The size of the inbound buffer.</param>
		/// <param name="secure">Specifies whether to make the pipe secure.</param>
		/// <returns>The pipe handle.</returns>
		/// <remarks>You can specify a security descriptor for a named pipe when you call the CreateNamedPipe function. The security descriptor controls access to both client and server ends of the named pipe. If NULL is specified, the named pipe gets a default security descriptor. The ACLs in the default security descriptor for a named pipe grant full control to the LocalSystem account, administrators, and the creator owner. They also grant read access to members of the Everyone group and the anonymous account.
		/// <br/><br/>
		/// If the <b>secure</b> parameter is false the method creates a security descriptor that grants full access to Everyone.
		/// </remarks>
		#endregion
		public static PipeHandle Create(string name, uint outBuffer, uint inBuffer, bool secure) {
			name = @"\\.\pipe\" + name;
			PipeHandle handle = new PipeHandle();
			IntPtr secAttr = IntPtr.Zero;
			SECURITY_ATTRIBUTES sa = new SECURITY_ATTRIBUTES();

			if (!secure) {
				SECURITY_DESCRIPTOR sd;
				GetNullDaclSecurityDescriptor(out sd);
				sa.lpSecurityDescriptor = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(SECURITY_DESCRIPTOR)));
				Marshal.StructureToPtr(sd, sa.lpSecurityDescriptor, false);
				sa.bInheritHandle = false;
				sa.nLength = Marshal.SizeOf(typeof(SECURITY_ATTRIBUTES));
				secAttr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(SECURITY_ATTRIBUTES)));
				Marshal.StructureToPtr(sa, secAttr, false);
			}
			
			try {
				for (int i = 1; i<=ATTEMPTS; i++) {
					handle.State = InterProcessConnectionState.Creating;
					handle.Handle = NamedPipeNative.CreateNamedPipe(
						name,
						NamedPipeNative.PIPE_ACCESS_DUPLEX,
						NamedPipeNative.PIPE_TYPE_MESSAGE | NamedPipeNative.PIPE_READMODE_MESSAGE | NamedPipeNative.PIPE_WAIT,
						NamedPipeNative.PIPE_UNLIMITED_INSTANCES,
						outBuffer,
						inBuffer,
						NamedPipeNative.NMPWAIT_WAIT_FOREVER,
						secAttr);
					if (handle.Handle.ToInt32() != NamedPipeNative.INVALID_HANDLE_VALUE) {
						handle.State = InterProcessConnectionState.Created;
						break;
					}
					if (i >= ATTEMPTS) {
						handle.State = InterProcessConnectionState.Error;
						throw new NamedPipeIOException("Error creating named pipe " + name + " . Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
					}
				}
			}
			finally {
				if (!secure) {
					Marshal.FreeHGlobal(sa.lpSecurityDescriptor);
					Marshal.FreeHGlobal(secAttr);
				}
			}
			return handle;
		}
		#region Comments
		/// <summary>
		/// Creates a SECURITY_DESCRIPTOR with DACL = null, which allows full access to Everyone.
		/// </summary>
		/// <param name="sd">The SECURITY_DESCRIPTOR structure.</param>
		#endregion
		public static void GetNullDaclSecurityDescriptor(out SECURITY_DESCRIPTOR sd) {
			if(NamedPipeNative.InitializeSecurityDescriptor(out sd, 1)) {
				if(!NamedPipeNative.SetSecurityDescriptorDacl(ref sd, true, IntPtr.Zero, false)) {
					throw new NamedPipeIOException("Error setting SECURITY_DESCRIPTOR attributes. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
				}
			}
			else {
				throw new NamedPipeIOException("Error setting SECURITY_DESCRIPTOR attributes. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
			}
		}
		#region Comments
		/// <summary>
		/// Starts waiting for client connections.
		/// </summary>
		/// <remarks>
		/// Blocks the current execution until a client pipe attempts to establish a connection.
		/// </remarks>
		/// <param name="handle">The pipe handle.</param>
		#endregion
		public static void Connect(PipeHandle handle) {
			handle.State = InterProcessConnectionState.WaitingForClient;
			bool connected = NamedPipeNative.ConnectNamedPipe(handle.Handle, null);
			handle.State = InterProcessConnectionState.ConnectedToClient;
			if (!connected && NamedPipeNative.GetLastError() != NamedPipeNative.ERROR_PIPE_CONNECTED) {
				handle.State = InterProcessConnectionState.Error;
				throw new NamedPipeIOException("Error connecting pipe. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
			}
		}
		#region Comments
		/// <summary>
		/// Returns the number of instances of a named pipe.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		/// <returns>The number of instances.</returns>
		#endregion
		public static uint NumberPipeInstances(PipeHandle handle) {
			uint curInstances = 0;

			if (NamedPipeNative.GetNamedPipeHandleState(handle.Handle, IntPtr.Zero, ref curInstances, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero)) {
				return curInstances;
			} 
			else {
				throw new NamedPipeIOException("Error getting the pipe state. Internal error: " + NamedPipeNative.GetLastError().ToString(), NamedPipeNative.GetLastError());
			}
		}
		#region Comments
		/// <summary>
		/// Closes a named pipe and releases the native handle.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		#endregion
		public static void Close(PipeHandle handle) {
			handle.State = InterProcessConnectionState.Closing;
			NamedPipeNative.CloseHandle(handle.Handle);
			handle.Handle = IntPtr.Zero;
			handle.State = InterProcessConnectionState.Closed;
		}	
		#region Comments
		/// <summary>
		/// Flushes all the data in a named pipe.
		/// </summary>
		/// <param name="handle">The pipe handle.</param>
		#endregion
		public static void Flush(PipeHandle handle) {
			handle.State = InterProcessConnectionState.Flushing;
			NamedPipeNative.FlushFileBuffers(handle.Handle);
			handle.State = InterProcessConnectionState.FlushedData;
		}
		#region Comments
		/// <summary>
		/// Disconnects a server named pipe from the client.
		/// </summary>
		/// <remarks>
		/// Server pipes can be reused by first disconnecting them from the client and then
		/// calling the <see cref="AppModule.NamedPipes.NamedPipeWrapper.Connect">Connect</see>
		/// method to start listening. This improves the performance as it is not necessary
		/// to create new pipe handles.
		/// </remarks>
		/// <param name="handle">The pipe handle.</param>
		#endregion
		public static void Disconnect(PipeHandle handle) {
			handle.State = InterProcessConnectionState.Disconnecting;
			NamedPipeNative.DisconnectNamedPipe(handle.Handle);
			handle.State = InterProcessConnectionState.Disconnected;
		}
		#region Comments
		/// <summary>
		/// Private constructor.
		/// </summary>
		#endregion
		private NamedPipeWrapper() {}
	}
}

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About the Author

Ivan L
Web Developer
United Kingdom United Kingdom
Ivan Latunov is a Software Architect with long term commercial experience in the areas of software architecture and engineering, design and development of web and distributed applications and business and technical analysis. Technical blog. Website: ivanweb.com.

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