Creating a lambda expression from a string

Question

Possible duplicate:
Is there an easy way to parse a string (lambda expression) in an action dellet?

I would like to store lambda expressions as strings in a configuration file and at runtime dynamically load these strings into lambda expressions in C #. My goal is to set up and introduce rules. Any utilities available to create lambda expressions from a string?

Are there any other easy solutions for the same purpose?

Answer 1

If you know the type of expressions ahead of time, you can compile them as part of the class, and then pull them out of the resulting assembly.

Here is an example that does (with expressions taking a string and returning a bool) and runs the resulting rules.

With the contents of c: \ temp \ rules.txt:

file => file.Length == 0 file => System.IO.Path.GetExtension(file) == ".txt" file => file == null 

Then the resulting result is as follows:

 Rules found in file: file => file.Length == 0, file => System.IO.Path.GetExtension(file) == ".txt", file => file == null, Checking rule file => (file.Length == 0) against input c:\temp\rules.txt: False Checking rule file => (GetExtension(file) == ".txt") against input c:\temp\rules.txt: True Checking rule file => (file == null) against input c:\temp\rules.txt: False 

A source:

 using System; using System.Xml.Linq; using System.Linq; using System.IO; using Microsoft.CSharp; using System.CodeDom.Compiler; using System.Reflection; using System.Linq.Expressions; class Program { private const string classTemplate = @" using System; using System.Linq.Expressions; public static class RulesConfiguration {{ private static Expression<Func<string, bool>>[] rules = new Expression<Func<string, bool>>[] {{ {0} }}; public static Expression<Func<string, bool>>[] Rules {{ get {{ return rules; }} }} }} "; static void Main(string[] args) { var filePath = @"c:\temp\rules.txt"; var fileContents = File.ReadAllLines(filePath); // add commas to the expressions so they can compile as part of the array var joined = String.Join("," + Environment.NewLine, fileContents); Console.WriteLine("Rules found in file: \n{0}", joined); var classSource = String.Format(classTemplate, joined); var assembly = CompileAssembly(classSource); var rules = GetExpressionsFromAssembly(assembly); foreach (var rule in rules) { var compiledToFunc = rule.Compile(); Console.WriteLine("Checking rule {0} against input {1}: {2}", rule, filePath, compiledToFunc(filePath)); } } static Expression<Func<string, bool>>[] GetExpressionsFromAssembly(Assembly assembly) { var type = assembly.GetTypes().Single(); var property = type.GetProperties().Single(); var propertyValue = property.GetValue(null, null); return propertyValue as Expression<Func<string, bool>>[]; } static Assembly CompileAssembly(string source) { var compilerParameters = new CompilerParameters() { GenerateExecutable = false, GenerateInMemory = true, ReferencedAssemblies = { "System.Core.dll" // needed for linq + expressions to compile }, }; var compileProvider = new CSharpCodeProvider(); var results = compileProvider.CompileAssemblyFromSource(compilerParameters, source); if (results.Errors.HasErrors) { Console.Error.WriteLine("{0} errors during compilation of rules", results.Errors.Count); foreach (CompilerError error in results.Errors) { Console.Error.WriteLine(error.ErrorText); } throw new InvalidOperationException("Broken rules configuration, please fix"); } var assembly = results.CompiledAssembly; return assembly; } } 

Answer 2

See Dynamic Linq . This is an old post, but always helpful.