In other words, how to use T4 templates without ANY runtime dependencies? Yes, it is possible, and quite simple and elegant actually.

In a desktop project, just open the Add New Item dialog, and search for "text template":

From the two available templates, the one that gives you a zero-dependency runtime-usable template is the first one: Preprocessed Text Template.

Once unfolded, you get the .tt file, but also a dependent .cs file automatically generated. Note the Custom Tool associated with the file:

If you open up the .cs file, you will see that it doesn't contain the rendered "Hello World!!!" I added in the .tt, but rather a full class named after the template file itself:

namespace ConsoleApplication1
{
    using System;
    
    #line 1 "C:\Temp\ConsoleApplication1\ConsoleApplication1\PreTextTemplate1.tt"
    [System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.VisualStudio.TextTemplating", "10.0.0.0")]
    public partial class PreTextTemplate1 : PreTextTemplate1Base
    {
        public virtual string TransformText()
        {
            this.GenerationEnvironment = null;
            this.Write("Hello World!!!");
            return this.GenerationEnvironment.ToString();
        }
    }
    
    #region Base class
    ...
    #endregion
}
...

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