Functional C#: Debugging Method Chains

One of the most common questions I get in regard to the Map and Tee extension methods I presented in my recent Pluralsight course is “That’s great…but how do I debug these chains?” I get it – debugging a lengthy method chain can seem like a monumental task upon first glance but I assure you, it really isn’t all that difficult or even much different from what you’re accustomed to with more traditional, imperative C# code.

I’ve found that when debugging method chains I typically already have a good idea where the problem is. Spoiler: It’s in the code I wrote. That means that I can almost always automatically rule out any chained in framework or other third-party library methods as the source of the problem. It also means that setting a breakpoint within a chained lambda expression or method is often an adequate first step in isolating the problem. This is especially useful when working with pure, deterministic methods because you can then write a test case around the method in question and already have the breakpoint right where you need it.

In some situations though, you want to follow computation through the chain but constantly stepping through the extension methods can be both tedious and distracting, especially when the chained method is outside of your control and won’t be stepped into anyway. Fortunately this is easily resolved with a single attribute.

The System.Diagnostics.DebuggerNonUserCodeAttribute class is intended specifically for this purpose. As MSDN states, this attribute instructs the debugger to step through rather than into the decorated type or member. You can either apply this attribute to individual methods or to the extension class to prevent the methods from disrupting your debugging experience. For my projects I opted to simply suppress all of the extension methods by decorating the class like this:

public static class FunctionalExtensions
    public static TResult Map<TSource, TResult>(
        this TSource @this,
        Func<TSource, TResult> map) => map(@this);

    // -- Snipped --

With the attribute applied, you can simply set a breakpoint on the chain and step into it as you normally would. Then, instead of having to walk through each of the extension methods you’ll simply be taken right into the chained methods or lambda expressions.