NReco LambdaParser

Runtime parser for string expressions (formulas, method calls, properties/fields/arrays accessors). LambdaParser builds dynamic LINQ expression tree and compiles it to the lambda delegate. Types are resolved at run-time like in dynamic languages.

NuGet	Windows x64	Linux

• can be used in any .NET app: net45 (legacy .NET Framework apps), netstandard1.3 (.NET Core apps), netstandard2.0 (all modern .NET apps).
• any number of expression arguments (values can be provided as dictionary or by callback delegate)
• supports arithmetic operations (+, -, *, /, %), comparisons (==, !=, >, <, >=, <=), conditionals including (ternary) operator ( boolVal ? whenTrue : whenFalse )
• access object properties, call methods and indexers, invoke delegates
• dynamic typed variables: performs automatic type conversions to match method signature or arithmetic operations
• create arrays with C#-like or JSON syntax: new []{ 1, 2, 3} or new [1, 2, 3] or just [1, 2, 3]
• create dictionaries (key-value map) with C#-like or JSON syntax: new dictionary{ {"a", 1}, {"b", 2} } or new {"a":1} or {"a":1, "b":2}
• local variables that may go before main expression: var a = 5; var b = contextVar/total*100; (disabled by default, to enable use LambdaParser.AllowVars property)

Nuget package: NReco.LambdaParser

var lambdaParser = new NReco.Linq.LambdaParser();

var varContext = new Dictionary<string,object>();
varContext["pi"] = 3.14M;
varContext["one"] = 1M;
varContext["two"] = 2M;
varContext["test"] = "test";
Console.WriteLine( lambdaParser.Eval("pi>one && 0<one ? (1+8)/3+1*two : 0", varContext) ); // --> 5
Console.WriteLine( lambdaParser.Eval("test.ToUpper()", varContext) ); // --> TEST

(see unit tests for more expression examples)

Custom values comparison

By default LambdaParser uses ValueComparer for values comparison. You can provide your own implementation or configure its option to get desired behaviour:

• ValueComparer.NullComparison determines how comparison with null is handled. 2 options:
  • MinValue: null is treated as minimal possible value for any type - like .NET IComparer
  • Sql: null is not comparable with any type, including another null - like in SQL
• ValueComparer.SuppressErrors allows to avoid convert exception. If error appears during comparison exception is not thrown and this means that values are not comparable (= any condition leads to false).

var valComparer = new ValueComparer() { NullComparison = ValueComparer.NullComparisonMode.Sql };
var lambdaParser = new LambdaParser(valComparer); 

Caching Expressions

The UseCache property determines whether the LambdaParser should cache parsed expressions. By default, UseCache is set to true, meaning expressions are cached to improve performance for repeated evaluations of the same expression.

Therefore, using a singleton instance of LambdaParser is recommended, rather than creating a new instance each time.

You can disable caching by setting UseCache to false if you want to save memory, especially when evaluating a large number of unique expressions.

var lambdaParser = new LambdaParser();
lambdaParser.UseCache = false;

Who is using this?

NReco.LambdaParser is in production use at SeekTable.com and PivotData microservice (used for user-defined calculated cube members: formulas, custom formatting).

License

Copyright 2016-2025 Vitaliy Fedorchenko and contributors

Distributed under the MIT license