Generating async WCF OperationContract signatures with a T4 template

From time to time it happens that people want to use the async programming model in WCF based on the IAsyncResult approach - either on the client or the service side. Writing all the asynchronous operation contract signatures is quite a tedious and error-prone task. Given this fact, design-time code generation with a T4 template may come in very handy for this purpose.
The following is my first try to create such a T4 template and it seems to work for the scenarios I need it for. Please refer to code below to see how to use the template: it is easy and straight-forward.

<#
/*
   Copyright (c) 2009, thinktecture (http://www.thinktecture.com).
   All rights reserved.

   Permission is hereby granted to use this software, for both commercial
   and non-commercial purposes, without limitations and free of charge.
   Permission is hereby granted to copy and distribute the software for
   non-commercial purposes. A commercial distribution is NOT allowed without
   prior written permission of the authors.

   This software is supplied "AS IS". The authors disclaim all warranties,
   expressed or implied, including, without limitation, the warranties of
   merchantability and of fitness for any purpose. The authors assume no
   liability for direct, indirect, incidental, special, exemplary, or
   consequential damages, which may result from the use of this software,
   even if advised of the possibility of such damage.
*/
#>
<#@ template debug="true" hostspecific="true" #>
<#@ output extension="cs" #>
<#@ assembly name="EnvDTE" #>
<#@ import namespace="EnvDTE" #>
<#@ import namespace="System.Diagnostics" #>
<#@ import namespace="System.Collections.Generic" #>
<#   
    // USAGE:
    // 1. Drop this file into your VS project
    // 2. Rename this file to match this pattern: {YOUR_SERVICECONTRACT_FILE}Async.tt
    // 3. The async version of your original service contract gets generated as
    //    {YOUR_SERVICECONTRACT_FILE}Async.cs
   
    EnvDTE.DTE dte = GetEnvDTE();

    string sourceFileName = dte.Solution.FindProjectItem(Host.TemplateFile).Name;
    sourceFileName = sourceFileName.Replace("Async.tt", ".cs");
  
    ProjectItem enumProjectItem = dte.Solution.FindProjectItem(sourceFileName);
    FileCodeModel codeModel = enumProjectItem.FileCodeModel;

    CodeNamespace codeNamespace = FindNamespace(codeModel.CodeElements);
    CodeInterface codeInterface = FindInterface(codeModel.CodeElements);
    List<CodeFunction> codeFunctions = FindMethods(codeInterface.Children);
       
    //System.Diagnostics.Debugger.Break();
#>
using System;
using System.ServiceModel;

namespace <#= codeNamespace.Name #>
{
    [ServiceContract]
    public interface <#= codeInterface.Name #>Channel :
        <#= codeInterface.Name#>
    {                   
        <#
            PushIndent("        ");
           
            int methodCount = 0;
           
            foreach (CodeFunction method in codeFunctions)
            {
                if(methodCount > 0)
                {
                    WriteLine(String.Empty);
                    WriteLine(String.Empty);
                }
               
                WriteAsyncOperationContract(method);
                WriteLine(string.Empty);
               
                methodCount ++;
            }
           
            ClearIndent();
        #>
    }
}

<#+
    private void WriteAsyncOperationContract(CodeFunction method)
    {
        string operationContractValue = TryGetOperationContractValue(method);
       
        WriteLine("[OperationContract(");
       
        if(String.IsNullOrEmpty(operationContractValue))
        {           
            Write("    Action = \"");
            Write(method.Name + "\",");
            WriteLine(String.Empty);
            Write("    ReplyAction = \"");
            Write(method.Name + "Reply\",");
            WriteLine(String.Empty);
        }
        else
        {
            ClearIndent();
            WriteLine("            " + operationContractValue + ",");
            PushIndent("        ");
        }
       
        WriteLine("    AsyncPattern = true)]");
       
        Write("IAsyncResult Begin");
        Write(method.Name);
        Write("(");
               
        foreach(CodeElement element in method.Parameters)
        {
            int count = 0;           
            CodeParameter parameter = element as CodeParameter;
       
            if (parameter != null)
            {
                Write(parameter.Type.AsString + " ");
                Write(parameter.Name);
               
                if(count < method.Parameters.Count)
                    Write(", ");
               
                count++;
            }
        }
       
        Write("AsyncCallback callback, object asyncState);");
       
        WriteLine(String.Empty);
        WriteLine(String.Empty);
       
        Write(method.Type.AsString + " ");
        Write("End");
        Write(method.Name);
        Write("(IAsyncResult result);");       
    }
   
    private string TryGetOperationContractValue(CodeFunction method)
    {
        string attributeValue = String.Empty;
       
        foreach(CodeElement attributeElement in method.Attributes)
        {
            CodeAttribute attribute = attributeElement as CodeAttribute;
           
            if(attribute != null)
            {       
                if(attribute.Name == "OperationContract" || attribute.Name == "OperationContractAttribute")
                {
                    attributeValue = attribute.Value;
                    break;
                }
            }
        }
       
        return attributeValue;
    }
   
    private CodeNamespace FindNamespace(CodeElements elements)
    {
        foreach (CodeElement element in elements)
        {
            CodeNamespace ns = element as CodeNamespace;
       
            if (ns != null)
                return ns;
        }
   
        return null;
    }
   
    private CodeInterface FindInterface(CodeElements elements)
    {
        foreach (CodeElement element in elements)
        {
            CodeInterface codeInterface = element as CodeInterface;
       
            if (codeInterface != null)
                return codeInterface;
   
            codeInterface = FindInterface(element.Children);
   
            if (codeInterface != null)
                return codeInterface;
        }
   
        return null;
    }
   
    private List<CodeFunction> FindMethods(CodeElements elements)
    {
        List<CodeFunction> methods = new List<CodeFunction>();
       
        foreach (CodeElement element in elements)
        {
            CodeFunction method = element as CodeFunction;
       
            if (method != null)
                methods.Add(method);           
        }
   
        return methods;
    }
   
    private EnvDTE.DTE GetEnvDTE()
    {
        IServiceProvider hostServiceProvider = (IServiceProvider)Host;
       
        if (hostServiceProvider == null)
               throw new Exception("Host property returned unexpected value (null)");
       
        EnvDTE.DTE dte = (EnvDTE.DTE)hostServiceProvider.GetService(typeof(EnvDTE.DTE));
       
        if (dte == null)
               throw new Exception("Unable to retrieve EnvDTE.DTE");
   
        return dte;
    }
#>

This is it - T4 programming reminds me of my very old days as a classical COM and ASP programmer... :)

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if(String.IsNullOrEmpty(operationContractValue))
{
Write(" Action = \"");
Write(method.Name + "\",");
WriteLine(String.Empty);
Write(" ReplyAction = \"");
Write(method.Name + "Reply\",");
WriteLine(String.Empty);
}
else
{
ClearIndent();
WriteLine(" " + operationContractValue + ",");
PushIndent(" ");
}
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