初次认识并尝试使用T4生成代码的时候，相关学习资料似乎比较少。不过现在VS2010 的MSDN里已有相关章节，可参看《代码生成和文本模板》章节。可以用C#的语法写模板，实在舒服很多。
很快就发现T4难以生成多个文件的缺陷，微软似乎也不着急改进这一点。通过搜索，从InfoQ找到一篇文章《用T4生成多个文件》，链接到一篇文章，Damien Guard的扩展可以方便的生成多个文件。原文是英文，能看懂，然而如果翻译则斟酌字词太辛苦。
首先，保存以下代码为一个模板文件(例如保存文件名为Manager.ttinclude)：
<#@ assembly="" name="System.Core" #="">
<#@ assembly="" name="System.Data.Linq" #="">
<#@ assembly="" name="EnvDTE" #="">
<#@ assembly="" name="System.Xml" #="">
<#@ assembly="" name="System.Xml.Linq" #="">
<#@ import="" namespace="System" #="">
<#@ import="" namespace="System.CodeDom" #="">
<#@ import="" namespace="System.CodeDom.Compiler" #="">
<#@ import="" namespace="System.Collections.Generic" #="">
<#@ import="" namespace="System.Data.Linq" #="">
<#@ import="" namespace="System.Data.Linq.Mapping" #="">
<#@ import="" namespace="System.IO" #="">
<#@ import="" namespace="System.Linq" #="">
<#@ import="" namespace="System.Reflection" #="">
<#@ import="" namespace="System.Text" #="">
<#@ import="" namespace="System.Xml.Linq" #="">
<#@ import="" namespace="Microsoft.VisualStudio.TextTemplating" #="">
<#+ manager="" class="" records="" the="" various="" blocks="" so="" it="" can="" split="" them="" up="" class="" manager="" {="" private="" class="" block="" {="" public="" string="" name;="" public="" int="" start,="" length;="" }="" private="" block="" currentblock;="" private=""> files = new List();
private Block footer = new Block();
private Block header = new Block();
private ITextTemplatingEngineHost host;
private StringBuilder template;
protected List generatedFileNames = new List();
public static Manager Create(ITextTemplatingEngineHost host, StringBuilder template) {
return (host is IServiceProvider) ? new VSManager(host, template) : new Manager(host, template);
}
public void StartNewFile(String name) {
if (name == null)
throw new ArgumentNullException("name");
CurrentBlock = new Block { Name = name };
}
public void StartFooter() {
CurrentBlock = footer;
}
public void StartHeader() {
CurrentBlock = header;
}
public void EndBlock() {
if (CurrentBlock == null)
return;
CurrentBlock.Length = template.Length - CurrentBlock.Start;
if (CurrentBlock != header && CurrentBlock != footer)
files.Add(CurrentBlock);
currentBlock = null;
}
public virtual void Process(bool split) {
if (split) {
EndBlock();
String headerText = template.ToString(header.Start, header.Length);
String footerText = template.ToString(footer.Start, footer.Length);
String outputPath = Path.GetDirectoryName(host.TemplateFile);
files.Reverse();
foreach(Block block in files) {
String fileName = Path.Combine(outputPath, block.Name);
String content = headerText + template.ToString(block.Start, block.Length) + footerText;
generatedFileNames.Add(fileName);
CreateFile(fileName, content);
template.Remove(block.Start, block.Length);
}
}
}
protected virtual void CreateFile(String fileName, String content) {
if (IsFileContentDifferent(fileName, content))
File.WriteAllText(fileName, content);
}
public virtual String GetCustomToolNamespace(String fileName) {
return null;
}
public virtual String DefaultProjectNamespace {
get { return null; }
}
protected bool IsFileContentDifferent(String fileName, String newContent) {
return !(File.Exists(fileName) && File.ReadAllText(fileName) == newContent);
}
private Manager(ITextTemplatingEngineHost host, StringBuilder template) {
this.host = host;
this.template = template;
}
private Block CurrentBlock {
get { return currentBlock; }
set {
if (CurrentBlock != null)
EndBlock();
if (value != null)
value.Start = template.Length;
currentBlock = value;
}
}
private class VSManager: Manager {
private EnvDTE.ProjectItem templateProjectItem;
private EnvDTE.DTE dte;
private Action checkOutAction;
private Action<>> projectSyncAction;
public override String DefaultProjectNamespace {
get {
return templateProjectItem.ContainingProject.Properties.Item("DefaultNamespace").Value.ToString();
}
}
public override String GetCustomToolNamespace(string fileName) {
return dte.Solution.FindProjectItem(fileName).Properties.Item("CustomToolNamespace").Value.ToString();
}
public override void Process(bool split) {
if (templateProjectItem.ProjectItems == null)
return;
base.Process(split);
projectSyncAction.EndInvoke(projectSyncAction.BeginInvoke(generatedFileNames, null, null));
}
protected override void CreateFile(String fileName, String content) {
if (IsFileContentDifferent(fileName, content)) {
CheckoutFileIfRequired(fileName);
File.WriteAllText(fileName, content);
}
}
internal VSManager(ITextTemplatingEngineHost host, StringBuilder template)
: base(host, template) {
var hostServiceProvider = (IServiceProvider) host;
if (hostServiceProvider == null)
throw new ArgumentNullException("Could not obtain IServiceProvider");
dte = (EnvDTE.DTE) hostServiceProvider.GetService(typeof(EnvDTE.DTE));
if (dte == null)
throw new ArgumentNullException("Could not obtain DTE from host");
templateProjectItem = dte.Solution.FindProjectItem(host.TemplateFile);
checkOutAction = (String fileName) => dte.SourceControl.CheckOutItem(fileName);
projectSyncAction = (IEnumerable keepFileNames) => ProjectSync(templateProjectItem, keepFileNames);
}
private static void ProjectSync(EnvDTE.ProjectItem templateProjectItem, IEnumerable keepFileNames) {
var keepFileNameSet = new HashSet(keepFileNames);
var projectFiles = new Dictionary();
var originalFilePrefix = Path.GetFileNameWithoutExtension(templateProjectItem.get_FileNames(0)) + ".";
foreach(EnvDTE.ProjectItem projectItem in templateProjectItem.ProjectItems)
projectFiles.Add(projectItem.get_FileNames(0), projectItem);
// Remove unused items from the project
foreach(var pair in projectFiles)
if (!keepFileNames.Contains(pair.Key) && !(Path.GetFileNameWithoutExtension(pair.Key) + ".").StartsWith(originalFilePrefix))
pair.Value.Delete();
// Add missing files to the project
foreach(String fileName in keepFileNameSet)
if (!projectFiles.ContainsKey(fileName))
templateProjectItem.ProjectItems.AddFromFile(fileName);
}
private void CheckoutFileIfRequired(String fileName) {
var sc = dte.SourceControl;
if (sc != null && sc.IsItemUnderSCC(fileName) && !sc.IsItemCheckedOut(fileName))
checkOutAction.EndInvoke(checkOutAction.BeginInvoke(fileName, null, null));
}
}
} #>
接着，在T4模板文件里引用这个模板，并声明一个Manager类实例：
<#@ template="" language="C#" hostspecific="True" #=""> <#@include file="Manager.ttinclude" #=""> <# var="" manager="Manager.Create(Host," generationenvironment);="" #="">
使用两行代码可使代码输出到单独文件，你要输出的代码可写在这两个语句中间，StartNewFile的参数就是输出的文件名：
<# manager.startnewfile("employee.generated.cs");="" #="">
<# manager.endblock();="" #="">
比如可以这样写：
<# manager.startnewfile("employee.generated.cs");="" #="">
public class Employee { }
<# manager.endblock();="" #="">
还可以为每个输出文件输出同样的头部或顶部，只需要相应的语句：
<# manager.startheader();="" #=""> // Code generated by a template using System; <# manager.endblock();="" #=""> <# manager.startfooter();="" #=""> // It's the end <# manager.endblock();="" #="">
最后使用这句来执行输出多个文件：
<# manager.process(true);="" #="">