Extending the code-first strongly typed pipelines to show how they might be loaded from config

BasePipelineTypes.cs

using System;

namespace StronglyTypedPipelines
{

    /// <summary>
    /// A base interface required so that reflection code can create a Step from its type name,
    /// without needing to understand its generic parameters first.
    /// </summary>
    public interface IPipelineStep
    {
    }

    /// <summary>
    /// Base type for individual pipeline steps.
    /// Descendants of this type map an input value to an output value.
    /// The input and output types can differ.
    /// </summary>
    public interface IPipelineStep<INPUT, OUTPUT> : IPipelineStep
    {
        OUTPUT Process(INPUT input);
    }

    /// <summary>
    /// An extension method for combining PipelineSteps together into a data flow.
    /// </summary>
    public static class PipelineStepExtensions
    {
        public static OUTPUT Step<INPUT, OUTPUT>(this INPUT input, IPipelineStep<INPUT, OUTPUT> step)
        {
            return step.Process(input);
        }
    }

    /// <summary>
    /// The base type for a complete pipeline.
    /// Descendant types can use their constructor to compile a set of PipelineSteps together
    /// the PipelineStepExtensions.Step() method, and assign this to the PipelineSteps property here.
    /// The initial and final types of the set of steps must match the input and output types of this class,
    /// but the intermediate types can vary.
    /// </summary>
    public abstract class Pipeline<INPUT, OUTPUT> : IPipelineStep<INPUT, OUTPUT>
    {
        public Func<INPUT, OUTPUT> PipelineSteps { get; protected set; }

        public OUTPUT Process(INPUT input)
        {
            return PipelineSteps(input);
        }
    }

}

ConcreteExamplePipeline.cs

namespace StronglyTypedPipelines
{
    public class BasicPipeline : Pipeline<int, float>
    {
        public BasicPipeline()
        {
            PipelineSteps = input => input
                .Step(new DoubleStep())
                .Step(new ToStringStep())
                .Step(new DuplicateStep())
                .Step(new ToFloatStep());
        }
    }

    public class InnerPipeline : Pipeline<int, int>
    {
        public InnerPipeline()
        {
            PipelineSteps = input => input
                .Step(new ThirdStep())
                .Step(new RoundStep());
        }
    }

    public class NestedPipeline : Pipeline<int, string>
    {
        public NestedPipeline()
        {
            PipelineSteps = input => input
            .Step(new DoubleStep())
            .Step(new InnerPipeline())
            .Step(new ToStringStep());
        }
    }

    public class BranchingPipeline : Pipeline<int, string>
    {
        public BranchingPipeline()
        {
            PipelineSteps = input => input
            .Step(new OptionalStep<int, int>(
                    f => f > 50,
                    new InnerPipeline()
                ))
            .Step(new ChoiceStep<int, int>(
                    f => f > 100,
                    new HalfStep(),
                    new DoubleStep()
                ))
            .Step(new ToStringStep());
        }
    }
}

ConcreteExampleSteps.cs

namespace StronglyTypedPipelines
{
    public class DoubleStep : IPipelineStep<int, int>
    {
        public int Process(int input)
        {
            return input * 2;
        }
    }

    public class HalfStep : IPipelineStep<int,int>
    {
        public int Process(int input)
        {
            return input / 2;
        }
    }

    public class ThirdStep : IPipelineStep<int, float>
    {
        public float Process(int input)
        {
            return input / 3f;
        }
    }

    public class RoundStep : IPipelineStep<float, int>
    {
        public int Process(float input)
        {
            return (int)input;
        }
    }

    public class ToStringStep : IPipelineStep<int, string>
    {
        public string Process(int input)
        {
            return input.ToString();
        }
    }

    public class DuplicateStep : IPipelineStep<string, string>
    {
        public string Process(string input)
        {
            return input + "." + input;
        }
    }

    public class ToFloatStep : IPipelineStep<string, float>
    {
        public float Process(string input)
        {
            return float.Parse(input);
        }
    }
}

ConfigBasedPipeline.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Xml.Linq;

namespace StronglyTypedPipelines
{

    public abstract class ConfigBasedPipeline<INPUT, OUTPUT> : Pipeline<INPUT, OUTPUT>
    {
        public ConfigBasedPipeline(XElement pipelineXml)
        {
            if (pipelineXml == null)
            {
                throw new ArgumentNullException(nameof(pipelineXml));
            }

            var pipelineSteps = parsePipelineSteps(pipelineXml);
            validatePipelineSteps(pipelineSteps);

            PipelineSteps = input => processPipelineSteps(pipelineSteps, input);
        }

        private OUTPUT processPipelineSteps(IList<IPipelineStep> pipelineSteps, INPUT input)
        {
            object output = input;

            foreach (IPipelineStep step in pipelineSteps)
            {
                MethodInfo mi = step.GetType().GetMethod("Process", BindingFlags.Public | BindingFlags.Instance);
                output = mi.Invoke(step, new[] { output });
            }

            return (OUTPUT)output;
        }

        private IList<IPipelineStep> parsePipelineSteps(XElement pipelineXml)
        {
            var pipeline = new List<IPipelineStep>();

            foreach (var xStep in pipelineXml.Elements("step"))
            {
                string typeName = xStep.Attribute("type").Value;

                var type = Type.GetType(typeName);
                var ctr = type.GetConstructor(Type.EmptyTypes);
                var obj = (IPipelineStep)ctr.Invoke(Type.EmptyTypes);

                pipeline.Add(obj);
            }

            return pipeline;
        }

        private void validatePipelineSteps(IList<IPipelineStep> pipelineSteps)
        {
            int stepNumber = 0;

            Type pipelineBaseInterface = this.GetType().GetInterface("IPipelineStep`2");
            Type currentInputType = pipelineBaseInterface.GenericTypeArguments[0];
            Type outputType = pipelineBaseInterface.GenericTypeArguments[1];
            foreach (var step in pipelineSteps)
            {
                stepNumber += 1;

                Type stepBaseInterface = step.GetType().GetInterface("IPipelineStep`2");
                Type stepInType = stepBaseInterface.GenericTypeArguments[0];
                Type stepOutType = stepBaseInterface.GenericTypeArguments[1];

                if (currentInputType != stepInType)
                {
                    string msg = "Step #{0} {1} input type {2} does not match current type {3}.";
                    throw new InvalidOperationException(string.Format(msg, stepNumber, step.GetType().Name, stepInType.Name, currentInputType.Name));
                }
                currentInputType = stepOutType;
            }
            if (currentInputType != outputType)
            {
                string msg = "Final step #{0} {1} output type {2} does not equal output of pipeline {3}.";
                throw new InvalidOperationException(string.Format(msg, stepNumber, pipelineSteps.Last().GetType().Name, currentInputType.Name, outputType.Name));
            }
        }

    }

    public class ExampleConfigBasedPipeline : ConfigBasedPipeline<int, string>
    {
        public ExampleConfigBasedPipeline(XElement pipelineXml) : base(pipelineXml)
        {
        }
    }

}

ConfigBasedPipeline.xml

<?xml version="1.0" encoding="utf-8" ?> 
<pipeline name="example">
  <step type="StronglyTypedPipelines.DoubleStep, StronglyTypedPipelines" />
  <!-- Will error: <step type="StronglyTypedPipelines.ThirdStep, StronglyTypedPipelines"/> -->
  <step type="StronglyTypedPipelines.ToStringStep, StronglyTypedPipelines" />
  <step type="StronglyTypedPipelines.DuplicateStep, StronglyTypedPipelines" />
  <!-- Will error: <step type="StronglyTypedPipelines.ToFloatStep, StronglyTypedPipelines"/> -->
</pipeline>

DecisionPipelineSteps.cs

using System;

namespace StronglyTypedPipelines
{
    public class OptionalStep<INPUT, OUTPUT> : IPipelineStep<INPUT, OUTPUT> where INPUT : OUTPUT
    {
        private IPipelineStep<INPUT, OUTPUT> _step;
        private Func<INPUT, bool> _choice;

        public OptionalStep(Func<INPUT, bool> choice, IPipelineStep<INPUT, OUTPUT> step)
        {
            _choice = choice;
            _step = step;
        }

        public OUTPUT Process(INPUT input)
        {
            if (_choice(input))
            {
                return _step.Process(input);
            }
            else
            {
                return input;
            }
        }
    }

    public class ChoiceStep<INPUT, OUTPUT> : IPipelineStep<INPUT, OUTPUT> where INPUT : OUTPUT
    {
        private IPipelineStep<INPUT, OUTPUT> _first;
        private IPipelineStep<INPUT, OUTPUT> _second;
        private Func<INPUT, bool> _choice;

        public ChoiceStep(Func<INPUT, bool> choice, IPipelineStep<INPUT, OUTPUT> first, IPipelineStep<INPUT, OUTPUT> second)
        {
            _choice = choice;
            _first = first;
            _second = second;
        }

        public OUTPUT Process(INPUT input)
        {
            if (_choice(input))
            {
                return _first.Process(input);
            }
            else
            {
                return _second.Process(input);
            }
        }
    }
}

ExampleProgram.cs

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Xml.Linq;

namespace StronglyTypedPipelines
{
    class Program
    {
        private static void BasicPipelineTest()
        {
            Console.WriteLine("Basic Pipeline Test");

            var input = 49;
            Console.WriteLine(string.Format("Input Value: {0} [{1}]", input, input.GetType().Name));

            var pipeline = new BasicPipeline();
            var output = pipeline.Process(input);

            Console.WriteLine(string.Format("Output Value: {0} [{1}]", output, output.GetType().Name));
            Console.WriteLine();
        }

        private static void NestedPipelineTest()
        {
            Console.WriteLine("Nested Pipeline Test");

            var input = 103;
            Console.WriteLine(string.Format("Input Value: {0} [{1}]", input, input.GetType().Name));

            var pipeline = new NestedPipeline();
            var output = pipeline.Process(input);

            Console.WriteLine(string.Format("Output Value: {0} [{1}]", output, output.GetType().Name));
            Console.WriteLine();
        }

        private static void BranchingPipelineTest()
        {
            Console.WriteLine("Branching Pipeline Test");

            foreach(int input in new int[] { 1, 10, 100, 1000 })
            {
                Console.WriteLine(string.Format("Input Value: {0} [{1}]", input, input.GetType().Name));

                var pipeline = new BranchingPipeline();
                var output = pipeline.Process(input);

                Console.WriteLine(string.Format("Output Value: {0} [{1}]", output, output.GetType().Name));
            }

            Console.WriteLine();
        }

        private static void ModifiablePipelineTest()
        {
            Console.WriteLine("Configured Pipeline Test");

            var input = 13;
            Console.WriteLine(string.Format("Input Value: {0} [{1}]", input, input.GetType().Name));

            XDocument xd = XDocument.Load("ConfigBasedPipeline.xml");
            //
            // Patching the configuration data would go here
            //
            XElement pipelineXml = xd.Document.Element("pipeline");


            try
            {
                var pipeline = new ExampleConfigBasedPipeline(pipelineXml);
                var output = pipeline.Process(input);

                Console.WriteLine(string.Format("Output Value: {0} [{1}]", output, output.GetType().Name));
            }
            catch(Exception ex)
            {
                Console.WriteLine("ERROR: " + ex.Message);
            }
            
            Console.WriteLine();
        }

        static void Main(string[] args)
        {
            BasicPipelineTest();
            NestedPipelineTest();
            BranchingPipelineTest();

            ModifiablePipelineTest();
        }
    }
}

Yeah totally agree the behaviour of async can be confusing. There are good books about this stuff though, if you want to learn a bit more - "C# In Depth" by Jon Skeet covers useful detail about the hows & whys of async. And it also covers lots of other helpful stuff for people new to the C# language overall. Maybe work will let you put a copy on expenses for learning?

Glad you're getting on top of all this though - happy have been able to help. Certainly not going to waste all this interesting material - what I've worked out will end up on my blog and I'll add new gists for the changed code as well.