How to make connection strings available in a T4 template?

Injecting Your Web.Config Connection String Into Your T4 Template provides you a T4 template that reads connection string from web.config and app.config. I created my custom version based on it and that saved me a good bit of time.

<#@ assembly name="System.Configuration" #>
<#@ assembly name="EnvDTE" #>
<#@ assembly name="System.Core.dll" #>
<#@ import namespace="System" #>
<#@ import namespace="System.Configuration" #>
<#@ import namespace="System.Text.RegularExpressions" #>
<#+

/// <summary>
/// Provides strongly typed access to the hosting EnvDTE.Project and app.config/web.config
/// configuration file, if present.
///
/// Typical usage from T4 template:
/// <code>ConfigurationAccessor config = new ConfigurationAccessor((IServiceProvider)this.Host);</code>
///
/// </summary>
/// <author>Sky Sanders [[email protected], http://skysanders.net/subtext]</author>
/// <date>01-23-10</date>
/// <copyright>The contents of this file are a Public Domain Dedication.</copyright>
///
/// TODO: determine behaviour of ProjectItem.FileNames when referred to a linked file.
///
public class ConfigurationAccessor
{

 /// <summary>
 /// Typical usage from T4 template:
 /// <code>ConfigurationAccessor config = new ConfigurationAccessor((IServiceProvider)this.Host);</code>
 /// </summary>
 public ConfigurationAccessor(IServiceProvider host) : this(host, null)
 {  }

 /// <summary>
 /// Same as default constructor but it looks for a web.config/app.config in the passed config
 /// project location and not in the first startup project it finds. The configProjectLocation
 /// passed should be relative to the solution file.
 /// </summary>
 public ConfigurationAccessor(IServiceProvider host, string configProjectLocation)
 {
  // Get the instance of Visual Studio that is hosting the calling file
  EnvDTE.DTE env = (EnvDTE.DTE)host.GetService(typeof(EnvDTE.DTE));

  // Initialize configuration filename
  string configurationFilename=null;

  // Gets an array of currently selected projects. Since you are either in this file saving it or
  // right-clicking the item in solution explorer to invoke the context menu it stands to reason
  // that there is 1 ActiveSolutionProject and that it is the parent of this file....
  _project = (EnvDTE.Project)((Array)env.ActiveSolutionProjects).GetValue(0);

  // Try and find the configuration file in the active solution project
  configurationFilename = FindConfigurationFilename(_project);

  // If we didn't find the configuration file, check the startup project or passed config
  // project location in the constructor
  if (configurationFilename == null)
  {
   // We are going to get the first *STARTUP* project in the solution
   // Our startup projects should also have a valid web.config/app.config, however,
   // if for some reason we have more than one startup project in the solution, this
   // will just grab the first one it finds
   //
   // We can also supply a config project location to look for in the constructor. This solves
   // the problem in the case where we have multiple startup projects and this file is not
   // in the first, or the config file is not in either the startup project or the active solution
   // project.
   if (!string.IsNullOrEmpty(configProjectLocation))
   {
    _project = (EnvDTE.Project)env.Solution.Projects.Item(configProjectLocation);
   }
   else
   {
    foreach (String s in (Array)env.Solution.SolutionBuild.StartupProjects)
    {
     _project = (EnvDTE.Project)env.Solution.Projects.Item(s);
     break;
    }
   }

   // Try and find the configuration file in one of the projects we found
   configurationFilename = FindConfigurationFilename(_project);
  }

  // Return the configuration object if we have a configuration file name
  // If we do not have a configuration file name, throw an exception
  if(!string.IsNullOrEmpty(configurationFilename))
  {
   // found it, map it and expose salient members as properties
   ExeConfigurationFileMap configFile = null;
   configFile = new ExeConfigurationFileMap();
   configFile.ExeConfigFilename=configurationFilename;
   _configuration = System.Configuration.ConfigurationManager.OpenMappedExeConfiguration(configFile, ConfigurationUserLevel.None);
   }
  else
  {
   throw new ArgumentException("Unable to find a configuration file (web.config/app.config). If the config file is located in a different project, you must mark that project as either the Startup Project or pass the project location of the config file relative to the solution file.");
  }
 }

 /// <summary>
 /// Finds a web.config/app.config file in the passed project and returns the file name.
 /// If none are found, returns null.
 /// </summary>
 private string FindConfigurationFilename(EnvDTE.Project project)
 {
  // examine each project item's filename looking for app.config or web.config
  foreach (EnvDTE.ProjectItem item in project.ProjectItems)
  {
   if (Regex.IsMatch(item.Name,"(app|web).config",RegexOptions.IgnoreCase))
   {
    // TODO: try this with linked files. is the filename pointing to the source?
    return item.get_FileNames(0);
   }
  }

  // not found, return null
  return null;
 }

 private EnvDTE.Project _project;
 private System.Configuration.Configuration _configuration;

 /// <summary>
 /// Provides access to the host project.
 /// </summary>
 /// <remarks>see http://msdn.microsoft.com/en-us/library/envdte.project.aspx</remarks>
 public EnvDTE.Project Project
 {
  get { return _project; }
 }

 /// <summary>
 /// Convenience getter for Project.Properties.
 /// Examples:
 /// <code>string thisAssemblyName = config.Properties.Item("AssemblyName").Value.ToString();</code>
 /// <code>string thisAssemblyName = config.Properties.Item("AssemblyName").Value.ToString();</code>
 /// </summary>
 /// <remarks>see http://msdn.microsoft.com/en-us/library/envdte.project_properties.aspx</remarks>
 public EnvDTE.Properties Properties
 {
  get { return _project.Properties;}
 }

 /// <summary>
 /// Provides access to the application/web configuration file.
 /// </summary>
 /// <remarks>see http://msdn.microsoft.com/en-us/library/system.configuration.configuration.aspx</remarks>
 public System.Configuration.Configuration Configuration
 {
  get { return _configuration; }
 }

 /// <summary>
 /// Provides access to the appSettings section of the configuration file.
 /// Behavior differs from typical AppSettings usage in that the indexed
 /// item's .Value must be explicitly addressed.
 /// <code>string setting = config.AppSettings["MyAppSetting"].Value;</code>
 /// </summary>
 /// <remarks>see http://msdn.microsoft.com/en-us/library/system.configuration.configuration.appsettings.aspx</remarks>
 public  KeyValueConfigurationCollection AppSettings
 {
  get { return _configuration.AppSettings.Settings;}
 }

 /// <summary>
 /// Provides access to the connectionStrings section of the configuration file.
 /// Behavior is as expected; items are accessed by string key or integer index.
 /// <code>string northwindProvider = config.ConnectionStrings["northwind"].ProviderName;</code>
 /// </summary>
 /// <remarks>see http://msdn.microsoft.com/en-us/library/system.configuration.configuration.connectionstrings.aspx</remarks>
 public  ConnectionStringSettingsCollection ConnectionStrings
 {
  get { return _configuration.ConnectionStrings.ConnectionStrings;}
 }
}
#>

If anyone needs to resolve absolute path to a Web.config file (because e.g. a .tt file is included and used from multiple other .tt files in different relative paths):

<#@ template debug="true" hostspecific="true" language="C#" #>
<#@ assembly name="EnvDTE" #>
<#@ Assembly Name="System.Configuration" #>
<#@ import namespace="System.IO" #>
<#@ import namespace="System.Configuration" #>
<#+
string GetConnectionString()
{
    var map = new ExeConfigurationFileMap();
    var configPath = Path.Combine(
        Host.ResolveAssemblyReference("$(SolutionDir)"), "WwwFolderInSolution", "Web.config");

    map.ExeConfigFilename = configPath;

    var config = ConfigurationManager.
        OpenMappedExeConfiguration(map, ConfigurationUserLevel.None);

    return config.ConnectionStrings.ConnectionStrings["MyConnectionString"].ConnectionString;
}
#>

I accessed a connection string from App.config from T4 template in the following way:

<#@ template debug="false" hostspecific="true" language="C#" #>

ExeConfigurationFileMap configFileMap = new ExeConfigurationFileMap(this.Host.ResolvePath(@"..\ProjName\App.config"));
configFileMap.ExeConfigFilename = this.Host.ResolvePath(@"..\ProjName\App.config");
Configuration config = ConfigurationManager.OpenMappedExeConfiguration(configFileMap, ConfigurationUserLevel.None);
string connectionString = config.ConnectionStrings.ConnectionStrings[0].ConnectionString;