Convert an Expression Tree to Source Code string

Question

I have a function that has the following signature...

public string DoJunk(Expression<Func<bool>> expression)

I'm trying to find a way to convert the "expression" parameter back to something resembling the original source code (or at least a c# representation of the original souce code). So, if someone calls the function like this...

DoJunk(() => (i + j) * 9 == Math.Round((double)j / (i - 3), 4))

...I'd like to be able to convert the expression to this...

(i + j) * 9 == Math.Round((double)j / (i - 3), 4)

Has anyone done this?

I'm experimenting with some ideas for a unit testing framework. My idea is that if you test against a lambda and the test fails, you could actually show the code that failed. For example, instead of getting "expected: 4 actual: 5", you could get a message more like "expected: user.Age == 4 actual: user.Age == 5" — herbrandson, Sep 10 '09 at 20:57
You ask for a C#-like string representation, but I've written [a library that outputs other representations](https://github.com/zspitz/ExpressionTreeToString): Visual Basic - like, factory methods needed to create a similar expression; object/collection initialization syntax. — Zev Spitz, Dec 26 '19 at 01:03

Steve Wilkes · Answer 1 · 2023-01-23T11:56:30.063

14

I've just happened across this; I've written a free, open-source library which provides an extension method to create a source-code-like string from an Expression:

using AgileObjects.ReadableExpressions;

var myExpression = CreateBigExpressionTree();
var expressionSource = myExpression.ToReadableString();

I've written a blog about it, the source is on GitHub, there's a NuGet package containing the extension method, and I've written a set of Debugger Visualizers for VS 2010 -> 2022 which are in the Visual Studio Marketplace.

edited Jan 23 '23 at 11:56

answered Apr 05 '16 at 00:23

Steve Wilkes

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Well, looking at the nuget download count, this is way more in demand than I expected. – Savo Pejović May 01 '20 at 06:24
I have [a mapper project](https://www.nuget.org/packages/AgileObjects.AgileMapper) which depends on it - that will have increased the download count. AutoMapper have recommended my library for examining their mapping execution plans, too! :) – Steve Wilkes May 01 '20 at 08:17
That explains it :) – Savo Pejović May 01 '20 at 09:11

score 9 · Answer 2 · edited Oct 24 '16 at 22:15

9

Here's an interesting article, with code, discussing the conversion of expression trees back into something that resembles (roughly) the original source:

Expression Trees-Lambdas to CodeDom Conversion

As a side-note, have you tried calling the expression's ToString method?

Expression<Func<int, int, bool>> expr =
    (i, j) => (i + j) * 9 == Math.Round((double)j / (i - 3), 4);

Console.WriteLine(expr.ToString());
// (i, j) => (Convert(((i + j) * 9)) = Round((Convert(j) / Convert((i - 3))), 4))

Console.WriteLine(expr.Body.ToString());
// (Convert(((i + j) * 9)) = Round((Convert(j) / Convert((i - 3))), 4))

edited Oct 24 '16 at 22:15

Robert Harvey

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answered Sep 10 '09 at 00:59

LukeH

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I have tried using ToString(), but it gives me something pretty nasty looking... (Convert(((value(LambdaToStringSpike.Program+<>c__DisplayClass0).i + value(LambdaToStringSpike.Program+<>c__DisplayClass0).j) * 9)) = Round((Convert(value(LambdaToStringSpike.Program+<>c__DisplayClass0).j) / Convert((value(LambdaToStringSpike.Program+<>c__DisplayClass0).i - 3))), 4)) – herbrandson Sep 10 '09 at 17:24
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Yep, I think the output you're seeing is what's generated behind-the-scenes by the C# compiler to handle the captured variables `i` and `j`. In my examples, `i` and `j` are locals, so the output is much closer to the original source code. – LukeH Sep 10 '09 at 20:28

Convert an Expression Tree to Source Code string

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