T4 Example: Code Generation

This simple T4 example shows how to generate a C# enum class from an external resource, in this case a SQL database.

First of all, let’s create our database:

USE [master]
GO

CREATE DATABASE [T4Demo] 
GO

USE [T4Demo]
GO

CREATE TABLE [dbo].[AssetClass](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [Description] [varchar](50) NOT NULL,
 CONSTRAINT [PK_AssetType] PRIMARY KEY CLUSTERED ( [Id] ASC )
)
GO

INSERT [AssetClass] VALUES ('Equity');
INSERT [AssetClass] VALUES ('Fixed Income');
INSERT [AssetClass] VALUES ('Cash');
GO

Next we’ll create a C# console application and add a T4 text template file:

<#@ template debug="false" hostspecific="true" language="C#" #>
<#@ output extension=".cs" #>
<#@ assembly name="System.Data" #>
<#@ import namespace="System.Data.SqlClient" #>
<#@ import namespace="System.Text.RegularExpressions" #>
<#@ import namespace="System.IO" #>

public enum <#= Path.GetFileNameWithoutExtension(Host.TemplateFile) #>
{
<#
  if(Validate())
  {
    BuildEnum();
  }    
#>
}

<#+

private bool Validate()
{
  if(string.IsNullOrEmpty(Server)) { Error("No server was specified"); }
  if(string.IsNullOrEmpty(Database)) { Error("No database was specified"); }
  if(!IntegratedSecurity) 
  { 
    if(string.IsNullOrEmpty(User)) { Error("No user id was specified"); }
    if(string.IsNullOrEmpty(Pwd)) { Error("No password was specified"); }    
  }

  if(string.IsNullOrEmpty(Table)) { Error("No table was specified"); }
  if(string.IsNullOrEmpty(ValueColumn)) { Error("The value column was specified"); }
  if(string.IsNullOrEmpty(TextColumn)) { Error("The text column was specified"); }
    
  return !this.Errors.HasErrors;
}

private void BuildEnum()
{
  using (SqlConnection conn = new SqlConnection(BuildConnStr()))
  {
    SqlCommand cmd = conn.CreateCommand();
    cmd.CommandText = string.Format(
      "select {0}, {1} from {2}", 
      ValueColumn, 
      TextColumn, 
      Table);
    cmd.CommandType = System.Data.CommandType.Text;

    conn.Open();
    SqlDataReader rdr = cmd.ExecuteReader();

    PushIndent("\t");
    while (rdr.Read())
    {
      string enm = string.Format("{0} = {1},", 
    CreateValidIdentifier(rdr[TextColumn].ToString()),
        rdr[ValueColumn].ToString());
      WriteLine(enm);
    }

    WriteLine("Unknown = 0");
    PopIndent();
    rdr.Close();
    conn.Close();
  }
}

// Strips spaces and full stops
private string CreateValidIdentifier(string input)
{
  string pattern = @"[\.\[\]\s]";           
  Regex regex = new Regex(pattern, RegexOptions.None);
  return regex.Replace(input, "");
}

private string BuildConnStr()
{
  SqlConnectionStringBuilder csb = new SqlConnectionStringBuilder();
  csb.DataSource = Server;
  csb.InitialCatalog = Database;
  csb.IntegratedSecurity = IntegratedSecurity;
  
  if(!IntegratedSecurity) 
  {
    csb.UserID = User;
    csb.Password = Pwd;
  }

  return csb.ConnectionString;
}

private string Server = ".";
private string Database = "T4Demo";
private bool IntegratedSecurity = true;
private string User = "";
private string Pwd = "";
private string Table = "dbo.AssetClass";
private string ValueColumn = "Id";
private string TextColumn = "Description";

#>

Now save the file and you'll have your own up-to-date enum class driven from your database:

public enum AssetClass
{
    Equity = 1,
    FixedIncome = 2,
    Cash = 3,
    Unknown = 0
}

You can also set up Visual Studio to always regenerate the file on each build if you prefer.

Source code:  http://stevenhollidge.com/blog-source-code/T4CodeGenerationEnums.zip