Sample code committed as start of client code.

AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.CompilerServices;
+
+// Information about this assembly is defined by the following attributes. 
+// Change them to the values specific to your project.
+
+[assembly: AssemblyTitle("SmartFileAPI")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("")]
+[assembly: AssemblyCopyright("")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// The assembly version has the format "{Major}.{Minor}.{Build}.{Revision}".
+// The form "{Major}.{Minor}.*" will automatically update the build and revision,
+// and "{Major}.{Minor}.{Build}.*" will update just the revision.
+
+[assembly: AssemblyVersion("1.0.*")]
+
+// The following attributes are used to specify the signing key for the assembly, 
+// if desired. See the Mono documentation for more information about signing.
+
+//[assembly: AssemblyDelaySign(false)]
+//[assembly: AssemblyKeyFile("")]

Main.cs

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+using System;
+
+namespace SmartFileAPI {
+	class MainClass {
+		// A short function to ask the user a question and
+		// return their response.
+		private static string Prompt(string prompt) {
+			Console.Write(prompt);
+			return Console.ReadLine();
+		}
+
+		// Start things off in Main()
+		public static void Main(string[] args) {
+			// Ask the user for the required parameters. These will be
+    		// passed to the API via an HTTP POST request.
+			string fullname = Prompt("Please enter a full name: ");
+			string username = Prompt("Please enter a username: ");
+			string password = Prompt("Please enter a password: ");
+			string email = Prompt("Please enter an email address: ");
+			try {
+				// Try to create the new user...
+				SmartFileAPI.CreateUser(fullname, username, password, email);
+				Console.WriteLine("Successfully created user {0}.", username);
+			}
+			catch (SmartFileException e) {
+				// Print the server response on failure.
+				Console.WriteLine("Error creating user {0}: {1}", username, e.Message);
+			}
+		}
+	}
+}

SmartFileAPI.cs

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+using System;
+using System.IO;
+using System.Net;
+using System.Web;
+using System.Xml;
+using System.Text;
+using System.Collections;
+
+namespace SmartFileAPI {
+	class SmartFileAPI {
+	    // These constants are needed to access the API.
+		private static string API_URL = "http://app.smartfile.com/api/1";
+		private static string API_KEY = "api-key";
+		private static string API_PWD = "api-password";
+
+		// This function does the bulk of the work by performing
+		// the HTTP request and raising an exception for any HTTP
+		// status code other than 201.
+		private static void httpRequest(string uri, Hashtable data, string method) {
+			// We use the XML format for C# because there is no native JSON decoder.
+			string url = String.Format("{0}{1}?format=xml", API_URL, uri);
+			string auth = String.Format("{0}:{1}", API_KEY, API_PWD);
+			auth = Convert.ToBase64String(new ASCIIEncoding().GetBytes(auth));
+			HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
+			request.Headers.Add("Authorization", String.Format("Basic {0}", auth));
+			request.ContentType = "application/x-www-form-urlencoded";
+			request.UserAgent = ".NET SmartFile API Sample Client";
+			request.KeepAlive = false;
+			request.Method = method;
+			if (method == "POST") {
+				string pre = "";
+				StringBuilder post = new StringBuilder();
+				foreach (string key in data.Keys) {
+					string val = HttpUtility.UrlEncode(data[key].ToString());
+					post.Append(String.Format("{0}{1}={2}", pre, key, val));
+					pre = "&";
+				}
+				byte[] byteData = UTF8Encoding.UTF8.GetBytes(post.ToString());
+				request.ContentLength = byteData.Length;
+				using (Stream requestStream = request.GetRequestStream()) {
+					requestStream.Write(byteData, 0, byteData.Length);
+				}
+			}
+			try {
+				using (HttpWebResponse response = request.GetResponse() as HttpWebResponse) {
+					return;
+				}
+			}
+			catch (WebException e) {
+				// Any HTTP status other than 2XX will land us here.
+				using (HttpWebResponse response = (HttpWebResponse)e.Response) {
+					StreamReader responseStream = new StreamReader(response.GetResponseStream());
+					string message = responseStream.ReadToEnd();
+					try {
+						using (XmlTextReader reader = new XmlTextReader(new System.IO.StringReader(message)))
+						{
+							bool inMessage = false;
+							while (reader.Read())
+								if (reader.Name == "message" && reader.NodeType == XmlNodeType.Element)
+									inMessage = true;
+								else if (inMessage && reader.NodeType == XmlNodeType.Text)
+								{
+									message = reader.Value;
+									break;
+								}
+						}
+					}
+					catch (Exception)
+					{ }
+					throw new SmartFileException(response.StatusCode, message);
+				}
+			}
+		}
+
+		// This function makes the User add API call. It uses the httpRequest
+		// function to handle the transport. Additional API calls could be supported
+		// simply by writing additional wrappers that create the data Hashtable and
+		// use httpRequest to do the grunt work.
+		public static void CreateUser(string fullname, string username, string password, string email) {
+			Hashtable data = new Hashtable();
+			data["name"] = fullname;
+			data["username"] = username;
+			data["password"] = password;
+			data["email"] = email;
+			httpRequest("/users/add/", data, "POST");
+		}
+	}
+}

SmartFileAPI.csproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="3.5">
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">x86</Platform>
+    <ProductVersion>9.0.21022</ProductVersion>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <RootNamespace>SmartFileAPI</RootNamespace>
+    <AssemblyName>SmartFileAPI</AssemblyName>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|x86' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug</OutputPath>
+    <DefineConstants>DEBUG</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+    <PlatformTarget>x86</PlatformTarget>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|x86' ">
+    <DebugType>none</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Release</OutputPath>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+    <PlatformTarget>x86</PlatformTarget>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Web" />
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="Main.cs" />
+    <Compile Include="AssemblyInfo.cs" />
+    <Compile Include="SmartFileException.cs" />
+    <Compile Include="SmartFileAPI.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
+</Project>

SmartFileAPI.sln

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+
+Microsoft Visual Studio Solution File, Format Version 10.00
+# Visual Studio 2008
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SmartFileAPI", "SmartFileAPI.csproj", "{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|x86 = Debug|x86
+		Release|x86 = Release|x86
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}.Debug|x86.ActiveCfg = Debug|x86
+		{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}.Debug|x86.Build.0 = Debug|x86
+		{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}.Release|x86.ActiveCfg = Release|x86
+		{0B790D9A-11A9-48F8-9617-5EF5C41E44C4}.Release|x86.Build.0 = Release|x86
+	EndGlobalSection
+	GlobalSection(MonoDevelopProperties) = preSolution
+		StartupItem = SmartFileAPI.csproj
+	EndGlobalSection
+EndGlobal

SmartFileAPI.userprefs

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+<Properties>
+  <MonoDevelop.Ide.Workspace ActiveConfiguration="Debug|x86" ctype="Workspace" />
+  <MonoDevelop.Ide.Workbench ActiveDocument="SmartFileAPI.cs" ctype="Workbench">
+    <Files>
+      <File FileName="Main.cs" Line="31" Column="2" />
+      <File FileName="SmartFileException.cs" Line="1" Column="1" />
+      <File FileName="SmartFileAPI.cs" Line="12" Column="44" />
+    </Files>
+    <Pads>
+      <Pad Id="ProjectPad">
+        <State expanded="True">
+          <Node name="SmartFileAPI" expanded="True">
+            <Node name="References" expanded="True" />
+            <Node name="SmartFileAPI.cs" selected="True" />
+          </Node>
+        </State>
+      </Pad>
+      <Pad Id="ClassPad">
+        <State expanded="True" selected="True" />
+      </Pad>
+    </Pads>
+  </MonoDevelop.Ide.Workbench>
+</Properties>

SmartFileException.cs

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+using System;
+using System.Net;
+
+namespace SmartFileAPI {
+	// A simple Exception class that can handle the HTTP
+	// status.
+	class SmartFileException : Exception {
+		private HttpStatusCode status;
+
+		public SmartFileException(HttpStatusCode status, string error) : base(error) {
+			this.status = status;
+		}
+
+		public override string ToString() {
+			return this.Message;
+		}
+
+		public HttpStatusCode StatusCode {
+			get {
+				return this.status;
+			}
+		}
+	}
+}