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Flee (Supports Net6.0, Net5.0, Netstandard2.1, Netstandard2.0)

Fast Lightweight Expression Evaluator. Convert this project to c#.

Project Description

Flee is an expression parser and evaluator for the .NET framework. It allows you to compute the value of string expressions such as sqrt(a^2 + b^2) at runtime. It uses a custom compiler, strongly-typed expression language, and lightweight codegen to compile expressions directly to IL. This means that expression evaluation is extremely fast and efficient.


  • Fast and efficient expression evaluation
  • Small, lightweight library
  • Compiles expressions to IL using a custom compiler, lightweight codegen, and the DynamicMethod class
  • Expressions (and the IL generated for them) are garbage-collected when no longer used
  • Does not create any dynamic assemblies that stay in memory
  • Backed by a comprehensive suite of unit tests
  • Culture-sensitive decimal point
  • Fine-grained control of what types an expression can use
  • Supports all arithmetic operations including the power (^) operator
  • Supports string, char, boolean, and floating-point literals
  • Supports 32/64 bit, signed/unsigned, and hex integer literals
  • Features a true conditional operator
  • Supports short-circuited logical operations
  • Supports arithmetic, comparison, implicit, and explicit overloaded operators
  • Variables of any type can be dynamically defined and used in expressions
  • CalculationEngine: Reference other expressions in an expression and recalculate in natural order
  • Expressions can index arrays and collections, access fields and properties, and call functions on various types
  • Generated IL can be saved to an assembly and viewed with a disassembler

Installing Flee

You should install Flee with NuGet:

Install-Package Flee

Or via the .NET Core command line interface:

dotnet add package Flee

NuGet Packages

Name NuGet
Flee Flee

More information

  • Examples to learn how to create and evaluate expressions.


Flee is licensed under the LGPL. This means that as long as you dynamically link (ie: add a reference) to the officially released assemblies, you can use it in commercial and non-commercial applications.