HttpListenerPrefixCollection.Add(String) Method


Adds a Uniform Resource Identifier (URI) prefix to the collection.

 virtual void Add(System::String ^ uriPrefix);
public void Add (string uriPrefix);
abstract member Add : string -> unit
override this.Add : string -> unit
Public Sub Add (uriPrefix As String)



A String that identifies the URI information that is compared in incoming requests. The prefix must be terminated with a forward slash ("/").



uriPrefix is null.

uriPrefix does not use the http:// or https:// scheme. These are the only schemes supported for HttpListener objects.


uriPrefix is not a correctly formatted URI prefix. Make sure the string is terminated with a "/".

The HttpListener associated with this collection is closed.

A Windows function call failed. Check the exception's ErrorCode property to determine the cause of the exception. This exception is thrown if another HttpListener has already added the prefix uriPrefix.


The following code example creates an HttpListener and adds user-specified prefixes to its HttpListenerPrefixCollection.

// This example requires the System and System.Net namespaces.
public static void SimpleListenerExample(string[] prefixes)
    if (!HttpListener.IsSupported)
        Console.WriteLine ("Windows XP SP2 or Server 2003 is required to use the HttpListener class.");
    // URI prefixes are required,
    // for example "".
    if (prefixes == null || prefixes.Length == 0)
      throw new ArgumentException("prefixes");

    // Create a listener.
    HttpListener listener = new HttpListener();
    // Add the prefixes.
    foreach (string s in prefixes)
    // Note: The GetContext method blocks while waiting for a request.
    HttpListenerContext context = listener.GetContext();
    HttpListenerRequest request = context.Request;
    // Obtain a response object.
    HttpListenerResponse response = context.Response;
    // Construct a response.
    string responseString = "<HTML><BODY> Hello world!</BODY></HTML>";
    byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);
    // Get a response stream and write the response to it.
    response.ContentLength64 = buffer.Length;
    System.IO.Stream output = response.OutputStream;
    // You must close the output stream.
Public Shared Sub SimpleListenerExample(prefixes As String())
    If Not HttpListener.IsSupported Then
        Console.WriteLine("Windows XP SP2 or Server 2003 is required to use the HttpListener class.")
    End If
    ' URI prefixes are required,
    ' for example "".
    If prefixes Is Nothing Or prefixes.Length = 0 Then
        Throw New ArgumentException("prefixes")
    End If

    ' Create a listener
    Dim listener = New HttpListener()

    For Each s As String In prefixes
    ' Note: The GetContext method blocks while waiting for a request.
    Dim context As HttpListenerContext = listener.GetContext()
    ' Obtain a response object
    Dim request As HttpListenerRequest = context.Request
    ' Construct a response.
    Dim response As HttpListenerResponse = context.Response
    Dim responseString As String = "<HTML><BODY> Hello world!</BODY></HTML>"
    Dim buffer As Byte() = System.Text.Encoding.UTF8.GetBytes(responseString)
    ' Get a response stream and write the response to it.
    response.ContentLength64 = buffer.Length
    Dim output As System.IO.Stream = response.OutputStream
    output.Write(buffer, 0, buffer.Length)
    'You must close the output stream.
End Sub


This method adds a URI prefix to the set of prefixes managed by the associated HttpListener object. When checking uriPrefix to ensure it is valid, case is ignored.

A URI prefix string is composed of a scheme (http or https), a host, an optional port, and an optional path, for example, "". The prefix must be terminated with a forward slash ("/"). The HttpListener with the prefix that most closely matches a requested URI responds to the request. Multiple HttpListener objects cannot add the same prefix. An HttpListenerException exception is thrown if an HttpListener adds a prefix that is already in use.

When a port is specified, the host element can be replaced with "*" to indicate that the HttpListener accepts requests sent to the port if the requested URI does not match any other prefix. For example, to receive all requests sent to port 8080 when the requested URI is not handled by any other HttpListener, the prefix is "http://*:8080/". Similarly, to specify that the HttpListener accepts all requests sent to a port, replace the host element with the "+" character, "https://+:8080/". The "*" and "+" characters can be present in prefixes that include paths.

Starting with .NET 4.5.3 and Windows 10, wildcard subdomains are supported in URI prefixes that are managed by an HttpListener object. To specify a wildcard subdomain, use the "*" character as part of the hostname in a URI prefix: for example, http://*, and pass this as the argument to the HttpListenerPrefixCollection.Add method. This will work on .NET 4.5.3 and Windows 10; in earlier versions, this would generate an HttpListenerException

Applies to

See also