Visual Studio T4 template that invokes an external compiler on a file in the project to generate a C# code file.

CompileModel.tt

<#@ template debug="false" hostspecific="true" language="C#" #>
<#@ assembly name="System.Core" #>
<#@ import namespace="System.IO" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Text" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="System.Diagnostics" #>
<#@ assembly name="EnvDTE" #>
<#@ import namespace="EnvDTE" #>
<#@ output extension=".cs" #>
<# 
    IServiceProvider serviceProvider = (IServiceProvider)this.Host;
    DTE dte = serviceProvider.GetService(typeof(DTE)) as DTE;

    var solutionRoot = new DirectoryInfo(dte.Solution.FullName).Parent.FullName;
    var compiler = Path.Combine(solutionRoot, @"Compiler\bin\Debug\Compiler.exe");

    var model = this.Host.ResolvePath("Model.dsl");

    // Start the child process.
    var p = new System.Diagnostics.Process();
            
    // Redirect the output stream of the child process.
    p.StartInfo.UseShellExecute = false;
    p.StartInfo.RedirectStandardOutput = true;
    p.StartInfo.FileName = compiler;
    p.StartInfo.Arguments = String.Format("\"{0}\"", model);
    p.Start();

    // Do not wait for the child process to exit before
    // reading to the end of its redirected stream.
    // p.WaitForExit();
    // Read the output stream first and then wait.
    string output = p.StandardOutput.ReadToEnd();
    p.WaitForExit();
#>
<#= output #>