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T4, Generating interface automatically based on provided classes

With new techniques and patterns interface plays a key role in application architecture. Interface makes application extendable like defining file upload interface and implementing based on file system, Azure Blob storage, Amazon S3. At starting we might be implementing based on Azure Blob but later we might move to Windows based file system and so on.

Ideally we create interface based on need and start implementing actual default implementation class. Many a times at starting of implementation there is one to one mapping between Interface and Class. Like from above example File upload interface and the initial or default class implementation that we design and with time it will get extended.

In this article, we will try to create interface based on default class implementation. This is not at all recommended in Test Driven Design (TDD) where we test the application before actual code implementation but I feel sometimes and in some situations it is okay do that and test straight after creation. Like couple of  months back, I had written an article on Dependency Injection, Repository Pattern and other related patterns ( where we had created repository interface and then created actual classes. If we have lot of database models or frequent additions in functions then we need to do copy paste of function implementation on classes based on addition in interface.

In this way we can develop application little faster where application demands frequent additions of function. This example is based on upon the repository classes from above link. The idea is to generate interfaces automatically based on repository classes modification. This technique can be found with various tools like Re-sharper, Telerik JustCode etc to refactor codes by extracting interface from class but over here it is through T4.

Resources used 
I have took help of two libraries to read project files and to create multiple files.
MultiOutput.ttinclude ( : This is to generate multiple files through single T4 file.
VisualStudioAutomationHelper.ttinclude ( : This will help us in reading files from different project.
NOTE: Please change path of these files based on path in your project.
 <#@ template debug="true" hostSpecific="true" #>  
 <#@ output extension=".cs" #>  
 <#@ Assembly name="EnvDTE" #>  
 <#@ Assembly name="EnvDTE80" #>  
 <#@ Assembly name="System.ComponentModel.DataAnnotations" #>  
 <#@ import namespace="EnvDTE" #>  
 <#@ import namespace="EnvDTE80" #>  
 <#@ import namespace="System" #>  
 <#@ import namespace="System.Linq" #>  
 <#@ import namespace="System.Xml" #>  
 <#@ import namespace="System.Collections" #>  
 <#@ import namespace="System.Collections.Generic" #>  
 <#@ import namespace="System.ComponentModel.DataAnnotations" #>  
 <#@ import namespace="System.Text.RegularExpressions" #>  
 <#@ include file="T4Plugin/VisualStudioAutomationHelper.ttinclude" #>  
 <#@ include file="T4Plugin/" #>  
   var dbProjectNamespace = "MyProject.DB";  
   var modelProjectNamespace = "MyProject.Model";  
   var repositoryNamespace = dbProjectNamespace + ".Repository";  
   var domainModelNamespace = modelProjectNamespace + ".DomainModel";  
   var repositoryInterfaceNamespace = "MyProject.Interface.Repository";  
   // Get a reference to the current project.  
   var dbProject = VisualStudioHelper.GetProject(dbProjectNamespace);  
   // Database model project  
   var modelProject = VisualStudioHelper.GetProject(modelProjectNamespace);  
   // Get all class items from the code model  
   var allRepoClasses = VisualStudioHelper.  
     GetAllCodeElementsOfType(dbProject.CodeModel.CodeElements, EnvDTE.vsCMElement.vsCMElementClass, false)  
     .Where(model => model.FullName.StartsWith(repositoryNamespace));  
   var allModelClasses = VisualStudioHelper.GetAllCodeElementsOfType(modelProject.CodeModel.CodeElements,   
             EnvDTE.vsCMElement.vsCMElementClass, false);  
   // Iterate all database models   
   foreach(CodeClass2 modelClass in allModelClasses  
     .OfType<CodeClass>().Where(clas => clas.FullName.StartsWith(domainModelNamespace) &&   
     ).OrderBy(clas => clas.FullName))  
     var fileName = "I" + modelClass.Name + "Repository.Generated.cs";  
     // Replace model namespace with repository namespace  
     var nameSpace = modelClass.Namespace.Name.Replace(domainModelNamespace, repositoryInterfaceNamespace);  
 namespace <#= nameSpace #>  
   // <auto-generated>  
   //   This code was generated from a template and will be re-created if deleted  
   //       with default values if executed.  
   // </auto-generated>  
   using <#= modelClass.Namespace.Name #>;  
   using System;  
   /// <summary>  
   /// Interface to interact with <see cref="<#= modelClass.FullName#>"/> domain model.  
   /// </summary>  
   public partial interface I<#= modelClass.Name#>Repository  
        : IRepository<<#= modelClass.Name#>>  
     // Repository classes  
     foreach(var repoClass in allRepoClasses.OfType<CodeClass2>().  
       Where(model => model.Name == modelClass.Name + "Repository"))  
       foreach(var partialClas in repoClass.PartialClasses.OfType<CodeClass2>())  
         var allFunctions = VisualStudioHelper.GetAllCodeElementsOfType(partialClas.Members, EnvDTE.vsCMElement.vsCMElementFunction, false);  
         foreach(var func in allFunctions.OfType<CodeFunction2>()  
           .Where(fun => fun.Name != modelClass.Name + "Repository"))  
           string strDoc=String.Empty;  
           if(!string.IsNullOrEmpty(func.DocComment) &&   
             func.Name != modelClass.Name + "Repository"){  
             var lines = func.DocComment.Split('\n');  
             for(int idx = 1; idx < (lines.Length-1); idx++) {  
                 #>   /// <#= lines[idx] #> <#  
     <#= GenerateFunctionStub(func) #>  
   private string GenerateFunctionStub(CodeFunction2 func)  
     var parametrs = new StringBuilder();  
     foreach (var item in func.Parameters.OfType<CodeParameter2>()) {  
       if ((parametrs.Length > 0)) {  
         parametrs.Append(", ");  
       // TODO: Implement other parameter kind  
       switch (item.ParameterKind)  
         case vsCMParameterKind.vsCMParameterKindOut:  
           parametrs.Append("out ");  
         case vsCMParameterKind.vsCMParameterKindRef:  
           parametrs.Append("ref ");  
       parametrs.AppendFormat("{0} {1}",  
           (item.Type.AsFullName.StartsWith("System") ?  
           "global::" + (item.Type.AsFullName.StartsWith("System.Nullable<System")?  
                   item.Type.AsFullName.Replace("System.Nullable<System", "System.Nullable<global::System") :  
           : item.Type.AsFullName)  
           , item.FullName);  
     // Build up the line from the function  
     var funcBody = new StringBuilder();  
     funcBody.AppendFormat("{0}{1} {2}({3})",   
       (func.Type.AsFullName.StartsWith("System") ? "global::" +  
       func.Type.AsFullName.Replace("<System","<global::System").Replace(",System", ",global::System")  
       ,string.IsNullOrEmpty(func.Type.AsFullName) ? "void" : ""  
       , func.Name, parametrs);  
     if (func.FunctionKind == EnvDTE.vsCMFunction.vsCMFunctionConstant) {  
       funcBody.Append(" const");  
     return funcBody.ToString();  
   public string GetXmlComment(string xmlDocComment, int tabAddition)  
       return string.Empty;  
     var comment = new StringBuilder();  
     string appender = string.Empty;  
     for(int tabCtr = 0; tabCtr < tabAddition; tabCtr++){  
       appender += "\t";  
     var lines = xmlDocComment.Split('\n');  
     for(int ctrLine = 1; ctrLine < (lines.Length-1); ctrLine++){  
       comment.AppendFormat("{0} {1}", appender, lines[ctrLine]);  
     return comment.ToString();  

var dbProjectNamespace = "MyProject.DB": Database layer project name.
var modelProjectNamespace = "MyProject.Model": Database model project name.
var repositoryNamespace = dbProjectNamespace + ".Repository": Repository namespace from where actual class files would be read to generate interface.
var domainModelNamespace = modelProjectNamespace + ".DomainModel": The database model classes for generating interfaces. Based on these files interfaces are generated for model.
   var repositoryInterfaceNamespace = "MyProject.Interface.Repository": The actual interface repository namespace.

This T4 can generate interfaces, function with written XML comments via repository classes.

Similarly in this way we can created repository classes automatically, unit of work properties and many more.

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