Compiling Code At Runtime with Lamar – Part 1

This code was originally written and proven out in the related Marten and described in a post titled Using Roslyn for Runtime Code Generation in Marten. This code was ripped out of Marten itself, but it’s happily running now in Lamar a couple years later.

As some of you know, I’ve been working on a new library called Lamar that I mean to be the next generation replacement for the venerable, but creaky StructureMap library. Besides the IoC Container support though, Lamar also provides some tooling and a model to generate and compile code at runtime, then ultimately load and use the newly generated types.

If all you want to do is take some C# code and compile that in memory to a new, in memory assembly, you can use
the Lamar.Compilation.AssemblyGenerator class.

Let’s say that you have a simple interface in your system like this:

    public interface IOperation
    {
        int Calculate(int one, int two);
    }

Next, let’s use AssemblyGenerator to compile code with a custom implementation of IOperation that we’ve generated
in code:

        [Fact]
        public void generate_code_on_the_fly()
        {
            var generator = new AssemblyGenerator();

            // This is necessary for the compilation to succeed
            // It's exactly the equivalent of adding references
            // to your project
            generator.ReferenceAssembly(typeof(Console).Assembly);
            generator.ReferenceAssembly(typeof(IOperation).Assembly);

            // Compile and generate a new .Net Assembly object
            // in memory
            var assembly = generator.Generate(@"
using Lamar.Testing.Samples;

namespace Generated
{
    public class AddOperator : IOperation
    {
        public int Calculate(int one, int two)
        {
            return one + two;
        }
    }
}
");

            // Find the new type we generated up above
            var type = assembly.GetExportedTypes().Single();
            
            // Use Activator.CreateInstance() to build an object
            // instance of our new class, and cast it to the 
            // IOperation interface
            var operation = (IOperation)Activator.CreateInstance(type);

            // Use our new type
            var result = operation.Calculate(1, 2);
         
            result.ShouldBe(3);
        }

There’s only a couple things going on in the code above:

1. I added an assembly reference for the .Net assembly that holds the IOperation interface
2. I passed a string to the ​GenerateCode() method, which successfully compiles my code and hands me back a .Net Assembly object
3. Load the newly generated type from the new Assembly
4. Use the new IOperation

If you’re not perfectly keen on doing brute force string manipulation to generate your code, you can
also use Lamar’s built in ISourceWriter to generate some of the code for you with
all its code generation utilities:

[Fact]
public void generate_code_on_the_fly_using_source_writer()
{
    var generator = new AssemblyGenerator();

    // This is necessary for the compilation to succeed
    // It's exactly the equivalent of adding references
    // to your project
    generator.ReferenceAssembly(typeof(Console).Assembly);
    generator.ReferenceAssembly(typeof(IOperation).Assembly);


    var assembly = generator.Generate(x =>
    {
        x.Namespace("Generated");
        x.StartClass("AddOperator", typeof(IOperation));
        
        x.Write("BLOCK:public int Calculate(int one, int two)");
        x.Write("return one + two;");
        x.FinishBlock();  // Finish the method
        
        x.FinishBlock();  // Finish the class
        x.FinishBlock();  // Finish the namespace
    });

    var type = assembly.GetExportedTypes().Single();
    var operation = (IOperation)Activator.CreateInstance(type);

    var result = operation.Calculate(1, 2);

    result.ShouldBe(3);
}

In Part 2, I’ll talk about the “frames” model that’s heavily used within Jasper (shown in this post).