Generates valid, readable C# from an Expression Tree.
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Latest commit ec65631 Jul 10, 2018

README.md

ExpressionToCode

ExpressionToCode generates valid, readable C# from an Expression Tree. (nuget: ExpressionToCodeLib)

An example:

  ExpressionToCode.ToCode(
    () => new[] { 1.0, 2.01, 3.5 }.SequenceEqual(new[] { 1.0, 2.01, 3.5 })
  )
== "() => new[] { 1.0, 2.01, 3.5 }.SequenceEqual(new[] { 1.0, 2.01, 3.5 })"

ExpressionToCode also provides something like Groovy's Power Assert which includes the code of the failing assertion's expression and the values of its subexpressions. This functionality is particularly useful in a unit testing framework such as NUnit or xUnit.NET. When you execute the following (failing) assertion:

PAssert.That(()=>Enumerable.Range(0,1000).ToDictionary(i=>"n"+i)["n3"].ToString()==(3.5).ToString());

The assertion fails with the following message:

assertion failed

Enumerable.Range(0, 1000).ToDictionary(i => "n" + i)["n3"].ToString(CultureInfo.InvariantCulture) == 3.5.ToString(CultureInfo.InvariantCulture)
   →   false (caused assertion failure)

Enumerable.Range(0, 1000).ToDictionary(i => "n" + i)["n3"].ToString(CultureInfo.InvariantCulture)
     →   "3"

Enumerable.Range(0, 1000).ToDictionary(i => "n" + i)["n3"]
     →   3

Enumerable.Range(0, 1000).ToDictionary(i => "n" + i)
     →   new Dictionary<string, int> {
              ["n0"] = 0,
              ["n1"] = 1,
              ["n2"] = 2,
              ["n3"] = 3,
              ["n4"] = 4,
              ["n5"] = 5,
              ["n6"] = 6,
              ["n7"] = 7,
              ["n8"] = 8,
              ["n9"] = 9,
              ["n10"] = 10,
              ["n11"] = 11,
              ["n12"] = 12,
              ["n13"] = 13,
              ["n14"] = 14,
              ["n15"] = 15,
              ["n16"] = 16,
              ["n17"] = 17,
              ["n18"] = 18,
              ["n19"] = 19,
              ["n20"] = 20,
              ["n21"] = 21,
              ["n22"] = 22,
              ["n23"] = 23,
              ["n24"] = 24,
              ["n25"] = 25,
              ["n26"] = 26,
              ["n27"] = 27,
              ["n28"] = 28,
              ["n29"] = 29,
              ...
          }

Enumerable.Range(0, 1000)
     →   { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, ... }

3.5.ToString(CultureInfo.InvariantCulture)
     →   "3.5"

ExpressionToCode's output is configurable in various ways. For expressions with small values, a values-on-stalks rendering might instead be used:

var a = 2;
var b = 5;
ExpressionToCodeConfiguration.DefaultAssertionConfiguration.WithAnnotator(CodeAnnotators.ValuesOnStalksCodeAnnotator)
    .Assert(() => Math.Max(a, b) > new[] { 3, 8, 13, 4 }.Average() );
Math.Max(a, b) > new[] { 3, 8, 13, 4 }.Average()  :  assertion failed
      │  │  │       │                     │
      │  │  │       │                     7.0
      │  │  │       new[] { 3, 8, 13, 4 }
      │  │  5
      │  2
      5

Note that the default configuration for asserts (i.e. PAssert.That) limits the length of sequences and strings; the default configuration of code-generation does not.

ExpressionToCode was inspired by Power Asssert.NET. It differs from PowerAssert.NET by supporting a larger portion of the lambda syntax and that the generated C# is more frequently valid; the aim is to generate valid C# for all expression trees created from lambda's. Currently supported:

Expression tree support

  • Supports static field and property access
  • Supports more operators, e.g. logical and bitwise negation
  • Recognizes C# indexer use (e.g. dict["mykey"]==3), in addition to special cases for array indexers and string indexers
  • Adds parentheses where required by operator precedence and associativity (e.g. () => x - (a - b) + x * (a + b) is correctly regenerated)
  • Generates valid numeric and other constant literals including escapes and suffixes where required (e.g. 1m + (decimal)Math.Sqrt(1.41))
  • Supports C# syntactic sugar for object initializers, object member initializers, list initializers, extension methods, anonymous types (issues #12, #3), etc
  • Uses the same spacing rules Visual Studio does by default
  • Supports nested Lambdas
  • Expands generic type instances and nullable types into normal C# (e.g. Func<int, bool> and int?)
  • Recognizes references to this and omits the keyword where possible (#5)
  • Recognizes closed-over variables and prints something plausible, rather than the crypic compiler-generated names.
  • Omits most implicit casts (e.g. object.Equals(3, 4) instead of object.Equals((object)3, (object)4)) - user defined implicit cast operators are not elided (#4)
  • Detects when type parameters to methods are superfluous (#13).

Not implemented (yet?):

  • Use LINQ query syntax where possible - issue #6.
  • Explicitly cast otherwise inferable lambda when required due to ambiguous overloads - issue #14.
  • Warn when == differs from .Equals or .SequenceEquals, as Power Assert.NET does (issue #2).
  • See all open issues.

ExpressionToCode API

All classes live in the ExpressionToCodeLib namespace.

These are:

  • PAssert for making assertions in NUnit tests and elsewhere.
    • PAssert.That and PAssert.IsTrue are identical; both test the provided boolean expression and print a readable error message on failure
  • ExpressionToCode Renders a System.Linq.Expressions.Expression object to source code.
    • ExpressionToCode.ToCode (several overloads) simply renders the expression as source code.
    • ExpressionToCode.AnnotatedToCode (several overloads) renders the expression as source code, then annotates all subexpressions which are computable with their value using the stalk-like rendering as shown on the Project Home page.

Two public helper classes exist:

  • PAssertFailedException thrown on assertion failure.
  • ObjectToCode Renders .NET objects to code; a helper class.
    • ObjectToCode.PlainObjectToCode renders simple objects that can be parsed by the C# compiler. This includes strings, chars, decimals, floats, doubles, all the integer types, booleans, enums, nulls, and default struct values.
    • ObjectToCode.ComplexObjectToPseudoCode renders as best it can anything thrown at it; but the resultant rendering is not necessarily compilable. This is used to display the values of subexpressions.

A complete listing of the public api is here

Supported platforms


Requires .NET 4.5.2 or .net standard 1.6 (previous versions support older platforms)


If you have any questions, you can contact me via github or mail eamon at nerbonne dot org.

See the documentation above, then download from or import using NuGet, or just checkout the source (license: Apache 2.0 or the MIT license, at your option)!