Edit and Continue Part 1 – Introduction

When discussing the Emit API in my last post, I mentioned that Roslyn gives users the ability to emit deltas between compilations. As far as I know this API is only used by Visual Studio’s Edit and Continue (EnC) feature. When you edit a running program the compiler is smart enough to only emit the changes you’ve made to the previous compilation. The CLR is then smart enough to load these changes and preserve the state of the running program.

I’ve created a (large) sample on how to use Roslyn and the CLR to modify a running process that is available on GitHub. Over the next week we’ll take a look at what it takes to use both Roslyn and the CLR to achieve this.

Part 1: Introduction
Part 2: EnC and Roslyn
Part 3: EnC and The CLR

I’ve had my eye on the Compilation.EmitDifference() API for almost a year now. I work on a Visual Studio extension called Alive that shows developers exactly what their source code does the moment they write it. This means that every time a user edits their code the extension re-compiles and re-emits the binary for their updated source code.

Re-emitting the compiled binary was a large bottleneck for us and created consistent GC pressure. When you emit a compilation you’re essentially dumping a big byte[] to memory. Worse still, if this byte[] contains over 85,000 elements then it goes straight to the large object heap. In our case these arrays weren’t long lived; the moment our users type we have to recompile and the previous binary becomes useless. Compilation.EmitDifference() allowed us to avoid emitting this giant array for every compilation and greatly reduce our extension’s memory footprint.

We can look at two approaches to consuming this API by comparing EnC and Alive. The primary difference between the two approaches is the preservation of state. EnC pauses execution of your program, lets you change it and resumes execution while retaining the previous program state. Alive has no need to preserve state between executions. It runs a given method and then waits for further instructions.

This difference means that EnC calculates the deltas between each compilation it creates, preserving state. Alive calculates deltas between the initial base compilation and the current state of the code.

How EnC builds deltas across compilations

EnC Deltas

How Alive builds deltas across compilations

EnC2

The above deltas are simplified for the sake of explanation. In reality they exist as pairs of IL/Metadata deltas. Deltas also aren’t generated at the statement level, when you edit a method the CLR actually replaces the entire method with your new code.

There are also restrictions on what constitutes a valid edit. For detailed rules I’ll defer to Mike Stall’s post on valid edits but it’s possibly outdated. (One valid edit he doesn’t mention is the addition of new top-level types to a program) Programs that use these APIs should have fallback plans for invalid edits. Visual Studio’s EnC simply displays an error saying that it cannot continue while invalid edits are present. Alive falls back to its old approach and re-emits the compilation in its entirety.

In part two we’ll take a look at what it takes to get Roslyn to generate deltas between two compilations.

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