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Invoke text transformation in a Visual Studio extension

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If you are writing a Visual Studio extension such as a menu command or domain-specific language, you can use the text templating service to transform text templates. Get the STextTemplating service and cast it to ITextTemplating.

Get the text templating service

using Microsoft.VisualStudio.TextTemplating;
using Microsoft.VisualStudio.TextTemplating.VSHost;
...
// Get a service provider - how you do this depends on the context:
IServiceProvider serviceProvider = ...; // An instance of EnvDTE, for example

// Get the text template service:
ITextTemplating t4 = serviceProvider.GetService(typeof(STextTemplating)) as ITextTemplating;

// Process a text template:
string result = t4.ProcessTemplate(filePath, System.IO.File.ReadAllText(filePath));

Pass parameters to the template

You can pass parameters into the template. Inside the template, you can get the parameter values by using the <#@parameter#> directive.

For the type of a parameter, you must use a type that is serializable or that can be marshaled. That is, the type must be declared with SerializableAttribute, or it must be derived from MarshalByRefObject. This restriction is necessary because the text template is executed in a separate AppDomain. All built-in types such as System.String and System.Int32 are serializable.

To pass parameter values, the calling code can place values either in the Session dictionary, or in the CallContext.

The following example uses both methods to transform a short test template:

using Microsoft.VisualStudio.TextTemplating;
using Microsoft.VisualStudio.TextTemplating.VSHost;
...
// Get a service provider - how you do this depends on the context:
IServiceProvider serviceProvider = dte;

// Get the text template service:
ITextTemplating t4 = serviceProvider.GetService(typeof(STextTemplating)) as ITextTemplating;
ITextTemplatingSessionHost sessionHost = t4 as ITextTemplatingSessionHost;

// Create a Session in which to pass parameters:
sessionHost.Session = sessionHost.CreateSession();
sessionHost.Session["parameter1"] = "Hello";
sessionHost.Session["parameter2"] = DateTime.Now;

// Pass another value in CallContext:
System.Runtime.Remoting.Messaging.CallContext.LogicalSetData("parameter3", 42);

// Process a text template:
string result = t4.ProcessTemplate("",
   // This is the test template:
   "<#@parameter type=\"System.String\" name=\"parameter1\"#>"
 + "<#@parameter type=\"System.DateTime\" name=\"parameter2\"#>"
 + "<#@parameter type=\"System.Int32\" name=\"parameter3\"#>"
 + "Test: <#=parameter1#>    <#=parameter2#>    <#=parameter3#>");

// This test code yields a result similar to the following line:
//     Test: Hello    07/06/2010 12:37:45    42

Error reporting and the output directive

Any errors that arise during processing will be displayed in the Visual Studio error window. In addition, you can be notified of errors by specifying a callback that implements ITextTemplatingCallback.

If you want to write the result string to a file, you might want to know what file extension and encoding have been specified in the <#@output#> directive in the template. This information will also be passed to your callback. For more information, see T4 Output Directive.

void ProcessMyTemplate(string MyTemplateFile)
{
  string templateContent = File.ReadAllText(MyTemplateFile);
  T4Callback cb = new T4Callback();
  // Process a text template:
  string result = t4.ProcessTemplate(MyTemplateFile, templateContent, cb);
  // If there was an output directive in the MyTemplateFile,
  // then cb.SetFileExtension() will have been called.
  // Determine the output file name:
  string resultFileName =
    Path.Combine(Path.GetDirectoryName(MyTemplateFile),
        Path.GetFileNameWithoutExtension(MyTemplateFile))
      + cb.fileExtension;
  // Write the processed output to file:
  File.WriteAllText(resultFileName, result, cb.outputEncoding);
  // Append any error messages:
  if (cb.errorMessages.Count > 0)
  {
    File.AppendAllLines(resultFileName, cb.errorMessages);
  }
}

class T4Callback : ITextTemplatingCallback
{
  public List<string> errorMessages = new List<string>();
  public string fileExtension = ".txt";
  public Encoding outputEncoding = Encoding.UTF8;

  public void ErrorCallback(bool warning, string message, int line, int column)
  { errorMessages.Add(message); }

  public void SetFileExtension(string extension)
  { fileExtension = extension; }

  public void SetOutputEncoding(Encoding encoding, bool fromOutputDirective)
  { outputEncoding = encoding; }
}

The code can be tested with a template file similar to the following:

<#@output extension=".htm" encoding="ASCII"#>
<# int unused;  // Compiler warning "unused variable"
#>
Sample text.

The compiler warning will appear in the Visual Studio error window, and it will also generate a call to ErrorCallback.

Reference parameters

You can pass values out of a text template by using a parameter class that is derived from MarshalByRefObject.

To generate text from a preprocessed text template: Call the TransformText() method of the generated class. For more information, see Run-Time Text Generation with T4 Text Templates.

To generate text outside a Visual Studio extension: Define a custom host. For more information, see Processing Text Templates by using a Custom Host.

To generate source code that can later be compiled and executed: Call the PreprocessTemplate method of ITextTemplating.