Walkthrough: Creating a Custom Text Template Host

 

For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

A text templatehost provides an environment that enables the text template transformation engine to run. The host is responsible for managing the engine's interaction with the file system. The engine or directive processor that needs a file or an assembly can request a resource from the host. The host can then search directories and the global assembly cache to locate the requested resource. For more information, see The Text Template Transformation Process.

You can write a custom host if you want to use the text template transformation functionality from outside Visual Studio or if you want to integrate that functionality into custom tools. To create a custom host, you must create a class that inherits from ITextTemplatingEngineHost. For the documentation of the individual methods, see ITextTemplatingEngineHost.

System_CAPS_ICON_warning.jpg Warning

If you are writing a Visual Studio extension or package, consider using the text templating service instead of creating your own host. For more information, see Invoking Text Transformation in a VS Extension.

Tasks illustrated in this walkthrough include the following:

  • Creating a custom text template host.

  • Testing the custom host.

To complete this walkthrough, you must have the following:

  • Visual Studio 2010 or later

  • Visual Studio SDK

In this walkthrough, you create a custom host in an executable application that can be called from the command line. The application accepts a text template file as an argument, reads the template, calls the engine to transform the template, and displays any errors that occur in the command prompt window.

To create a custom host

  1. In Visual Studio, create a new Visual Basic or a C# console application named CustomHost.

  2. Add references to the following assemblies:

    • Microsoft.VisualStudio.TextTemplating.*.0

    • Microsoft.VisualStudio.TextTemplating.Interfaces.10.0 and later versions

  3. Replace the code in the Program.cs or Module1.vb file with the following code:

    using System;  
    using System.IO;  
    using System.CodeDom.Compiler;  
    using System.Collections.Generic;  
    using System.Text;  
    using Microsoft.VisualStudio.TextTemplating;  
    
    namespace CustomHost  
    {  
        //The text template transformation engine is responsible for running   
        //the transformation process.  
        //The host is responsible for all input and output, locating files,   
        //and anything else related to the external environment.  
        //-------------------------------------------------------------------------  
        class CustomCmdLineHost : ITextTemplatingEngineHost  
        {  
            //the path and file name of the text template that is being processed  
            //---------------------------------------------------------------------  
            internal string TemplateFileValue;  
            public string TemplateFile  
            {  
                get { return TemplateFileValue; }  
            }  
            //This will be the extension of the generated text output file.  
            //The host can provide a default by setting the value of the field here.  
            //The engine can change this value based on the optional output directive  
            //if the user specifies it in the text template.  
            //---------------------------------------------------------------------  
            private string fileExtensionValue = ".txt";  
            public string FileExtension  
            {  
                get { return fileExtensionValue; }  
            }  
            //This will be the encoding of the generated text output file.  
            //The host can provide a default by setting the value of the field here.  
            //The engine can change this value based on the optional output directive  
            //if the user specifies it in the text template.  
            //---------------------------------------------------------------------  
            private Encoding fileEncodingValue = Encoding.UTF8;  
            public Encoding FileEncoding  
            {  
                get { return fileEncodingValue; }  
            }  
            //These are the errors that occur when the engine processes a template.  
            //The engine passes the errors to the host when it is done processing,  
            //and the host can decide how to display them. For example, the host   
            //can display the errors in the UI or write them to a file.  
            //---------------------------------------------------------------------  
            private CompilerErrorCollection errorsValue;  
            public CompilerErrorCollection Errors  
            {  
                get { return errorsValue; }  
            }  
            //The host can provide standard assembly references.  
            //The engine will use these references when compiling and  
            //executing the generated transformation class.  
            //--------------------------------------------------------------  
            public IList<string> StandardAssemblyReferences  
            {  
                get  
                {  
                    return new string[]  
                    {  
                        //If this host searches standard paths and the GAC,  
                        //we can specify the assembly name like this.  
                        //---------------------------------------------------------  
                        //"System"  
    
                        //Because this host only resolves assemblies from the   
                        //fully qualified path and name of the assembly,  
                        //this is a quick way to get the code to give us the  
                        //fully qualified path and name of the System assembly.  
                        //---------------------------------------------------------  
                        typeof(System.Uri).Assembly.Location  
                    };  
                }  
            }  
            //The host can provide standard imports or using statements.  
            //The engine will add these statements to the generated   
            //transformation class.  
            //--------------------------------------------------------------  
            public IList<string> StandardImports  
            {  
                get  
                {  
                    return new string[]  
                    {  
                        "System"  
                    };  
                }  
            }  
            //The engine calls this method based on the optional include directive  
            //if the user has specified it in the text template.  
            //This method can be called 0, 1, or more times.  
            //---------------------------------------------------------------------  
            //The included text is returned in the context parameter.  
            //If the host searches the registry for the location of include files,  
            //or if the host searches multiple locations by default, the host can  
            //return the final path of the include file in the location parameter.  
            //---------------------------------------------------------------------  
            public bool LoadIncludeText(string requestFileName, out string content, out string location)  
            {  
                content = System.String.Empty;  
                location = System.String.Empty;  
    
                //If the argument is the fully qualified path of an existing file,  
                //then we are done.  
                //----------------------------------------------------------------  
                if (File.Exists(requestFileName))  
                {  
                    content = File.ReadAllText(requestFileName);  
                    return true;  
                }  
                //This can be customized to search specific paths for the file.  
                //This can be customized to accept paths to search as command line  
                //arguments.  
                //----------------------------------------------------------------  
                else  
                {  
                    return false;  
                }  
            }  
            //Called by the Engine to enquire about   
            //the processing options you require.   
            //If you recognize that option, return an   
            //appropriate value.   
            //Otherwise, pass back NULL.  
            //--------------------------------------------------------------------  
            public object GetHostOption(string optionName)  
            {  
            object returnObject;  
            switch (optionName)  
            {  
            case "CacheAssemblies":  
                        returnObject = true;  
         break;  
            default:  
            returnObject = null;  
            break;  
            }  
            return returnObject;  
            }  
            //The engine calls this method to resolve assembly references used in  
            //the generated transformation class project and for the optional   
            //assembly directive if the user has specified it in the text template.  
            //This method can be called 0, 1, or more times.  
            //---------------------------------------------------------------------  
            public string ResolveAssemblyReference(string assemblyReference)  
            {  
                //If the argument is the fully qualified path of an existing file,  
                //then we are done. (This does not do any work.)  
                //----------------------------------------------------------------  
                if (File.Exists(assemblyReference))  
                {  
                    return assemblyReference;  
                }  
                //Maybe the assembly is in the same folder as the text template that   
                //called the directive.  
                //----------------------------------------------------------------  
                string candidate = Path.Combine(Path.GetDirectoryName(this.TemplateFile), assemblyReference);  
                if (File.Exists(candidate))  
                {  
                    return candidate;  
                }  
                //This can be customized to search specific paths for the file  
                //or to search the GAC.  
                //----------------------------------------------------------------  
                //This can be customized to accept paths to search as command line  
                //arguments.  
                //----------------------------------------------------------------  
                //If we cannot do better, return the original file name.  
                return "";  
            }  
            //The engine calls this method based on the directives the user has   
            //specified in the text template.  
            //This method can be called 0, 1, or more times.  
            //---------------------------------------------------------------------  
            public Type ResolveDirectiveProcessor(string processorName)  
            {  
                //This host will not resolve any specific processors.  
                //Check the processor name, and if it is the name of a processor the   
                //host wants to support, return the type of the processor.  
                //---------------------------------------------------------------------  
                if (string.Compare(processorName, "XYZ", StringComparison.OrdinalIgnoreCase) == 0)  
                {  
                    //return typeof();  
                }  
                //This can be customized to search specific paths for the file  
                //or to search the GAC  
                //If the directive processor cannot be found, throw an error.  
                throw new Exception("Directive Processor not found");  
            }  
            //A directive processor can call this method if a file name does not   
            //have a path.  
            //The host can attempt to provide path information by searching   
            //specific paths for the file and returning the file and path if found.  
            //This method can be called 0, 1, or more times.  
            //---------------------------------------------------------------------  
            public string ResolvePath(string fileName)  
            {  
                if (fileName == null)  
                {  
                    throw new ArgumentNullException("the file name cannot be null");  
                }  
                //If the argument is the fully qualified path of an existing file,  
                //then we are done  
                //----------------------------------------------------------------  
                if (File.Exists(fileName))  
                {  
                    return fileName;  
                }  
                //Maybe the file is in the same folder as the text template that   
                //called the directive.  
                //----------------------------------------------------------------  
                string candidate = Path.Combine(Path.GetDirectoryName(this.TemplateFile), fileName);  
                if (File.Exists(candidate))  
                {  
                    return candidate;  
                }  
                //Look more places.  
                //----------------------------------------------------------------  
                //More code can go here...  
                //If we cannot do better, return the original file name.  
                return fileName;  
            }  
            //If a call to a directive in a text template does not provide a value  
            //for a required parameter, the directive processor can try to get it  
            //from the host by calling this method.  
            //This method can be called 0, 1, or more times.  
            //---------------------------------------------------------------------  
            public string ResolveParameterValue(string directiveId, string processorName, string parameterName)  
            {  
                if (directiveId == null)  
                {  
                    throw new ArgumentNullException("the directiveId cannot be null");  
                }  
                if (processorName == null)  
                {  
                    throw new ArgumentNullException("the processorName cannot be null");  
                }  
                if (parameterName == null)  
                {  
                    throw new ArgumentNullException("the parameterName cannot be null");  
                }  
                //Code to provide "hard-coded" parameter values goes here.  
                //This code depends on the directive processors this host will interact with.  
                //If we cannot do better, return the empty string.  
                return String.Empty;  
            }  
            //The engine calls this method to change the extension of the   
            //generated text output file based on the optional output directive   
            //if the user specifies it in the text template.  
            //---------------------------------------------------------------------  
            public void SetFileExtension(string extension)  
            {  
                //The parameter extension has a '.' in front of it already.  
                //--------------------------------------------------------  
                fileExtensionValue = extension;  
            }  
            //The engine calls this method to change the encoding of the   
            //generated text output file based on the optional output directive   
            //if the user specifies it in the text template.  
            //----------------------------------------------------------------------  
            public void SetOutputEncoding(System.Text.Encoding encoding, bool fromOutputDirective)  
            {  
                fileEncodingValue = encoding;  
            }  
            //The engine calls this method when it is done processing a text  
            //template to pass any errors that occurred to the host.  
            //The host can decide how to display them.  
            //---------------------------------------------------------------------  
            public void LogErrors(CompilerErrorCollection errors)  
            {  
                errorsValue = errors;  
            }  
            //This is the application domain that is used to compile and run  
            //the generated transformation class to create the generated text output.  
            //----------------------------------------------------------------------  
            public AppDomain ProvideTemplatingAppDomain(string content)  
            {  
                //This host will provide a new application domain each time the   
                //engine processes a text template.  
                //-------------------------------------------------------------  
                return AppDomain.CreateDomain("Generation App Domain");  
                //This could be changed to return the current appdomain, but new   
                //assemblies are loaded into this AppDomain on a regular basis.  
                //If the AppDomain lasts too long, it will grow indefintely,   
                //which might be regarded as a leak.  
                //This could be customized to cache the application domain for   
                //a certain number of text template generations (for example, 10).  
                //This could be customized based on the contents of the text   
                //template, which are provided as a parameter for that purpose.  
            }  
        }  
        //This will accept the path of a text template as an argument.  
        //It will create an instance of the custom host and an instance of the  
        //text templating transformation engine, and will transform the  
        //template to create the generated text output file.  
        //-------------------------------------------------------------------------  
        class Program  
        {  
            static void Main(string[] args)  
            {  
                try  
                {  
                    ProcessTemplate(args);  
                }  
                catch (Exception ex)  
                {  
                    Console.WriteLine(ex.Message);  
                }  
            }  
            static void ProcessTemplate(string[] args)  
            {  
                string templateFileName = null;  
                if (args.Length == 0)  
                {  
                    throw new System.Exception("you must provide a text template file path");  
                }  
                templateFileName = args[0];  
                if (templateFileName == null)  
                {  
                    throw new ArgumentNullException("the file name cannot be null");  
                }  
                if (!File.Exists(templateFileName))  
                {  
                    throw new FileNotFoundException("the file cannot be found");  
                }  
                CustomCmdLineHost host = new CustomCmdLineHost();  
                Engine engine = new Engine();  
                host.TemplateFileValue = templateFileName;  
                //Read the text template.  
                string input = File.ReadAllText(templateFileName);  
                //Transform the text template.  
                string output = engine.ProcessTemplate(input, host);  
                string outputFileName = Path.GetFileNameWithoutExtension(templateFileName);  
                outputFileName = Path.Combine(Path.GetDirectoryName(templateFileName), outputFileName);  
                outputFileName = outputFileName + "1" + host.FileExtension;  
                File.WriteAllText(outputFileName, output, host.FileEncoding);  
    
                foreach (CompilerError error in host.Errors)  
                {  
                    Console.WriteLine(error.ToString());  
                }  
            }  
        }  
    }  
    
    
  4. For Visual Basic only, open the Project menu, and click CustomHost Properties. In the Startup object list, click CustomHost.Program.

  5. On the File menu, click Save All.

  6. On the Build menu, click Build Solution.

To test the custom host, you write a text template, then you run the custom host, pass it the name of the text template, and verify that the template is transformed.

To create a text template to test the custom host

  1. Create a text file, and name it TestTemplate.tt.

    You can use any text editor (for example, Notepad) to create the file.

  2. Add the following to the file:

    System_CAPS_ICON_note.jpg Note

    The programming language of the text template does not have to match that of the custom host.

    Text Template Host Test  
    
    <#@ template debug="true" #>  
    
    <# //Uncomment this line to test that the host allows the engine to set the extension. #>  
    <# //@ output extension=".htm" #>  
    
    <# //Uncomment this line if you want to debug the generated transformation class. #>  
    <# //System.Diagnostics.Debugger.Break(); #>  
    
    <# for (int i=0; i<3; i++)  
       {  
           WriteLine("This is a test");  
       }   
    #>  
    
    
  3. Save and close the file.

To test the custom host

  1. Open the Command Prompt window.

  2. Type the path of the executable file for the custom host, but do not press ENTER yet.

    For example, type:

    <YOUR PATH>CustomHost\bin\Debug\CustomHost.exe

    System_CAPS_ICON_note.jpg Note

    Instead of typing the address, you can browse to the file CustomHost.exe in Windows Explorer and then drag the file into the Command Prompt window.

  3. Type a space.

  4. Type the path of the text template file, and then press ENTER.

    For example, type:

    C:\<YOUR PATH>TestTemplate.tt

    System_CAPS_ICON_note.jpg Note

    Instead of typing the address, you can browse to the file TestTemplate.tt in Windows Explorer and then drag the file into the Command Prompt window.

    The custom host application runs and completes the text template transformation process.

  5. In Windows Explorer, browse to the folder that contains the file TestTemplate.tt.

    That folder also contains the file TestTemplate1.txt.

  6. Open this file to see the results of the text template transformation.

    The generated text output appears and looks like this:

    Text Template Host Test  
    
    This is a test  
    This is a test  
    This is a test  
    
    

In this walkthrough, you created a text template transformation host that supports the basic transformation functionality. You can expand your host to support text templates that call custom or generated directive processors. For more information, see Walkthrough: Connecting a Host to a Generated Directive Processor.

ITextTemplatingEngineHost

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