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Posted 20 Apr 2002


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Runtime C# Expression Evaluator

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4.83/5 (71 votes)
20 Apr 2002Ms-PL1 min read
A C# class (and library if needed) to do runtime evaluations of C# expressions


This is a simple class library (or just .cs file if you wish) to allow for runtime compilation and evaluation of C# code blocks. There are both static methods of the Evaluator class that allow for simple use (but at a performance penalty) or you can use the object directly to create multiple evaluations:

Console.WriteLine("Test0: {0}", Evaluator.EvaluateToInteger("(30 + 4) * 2"));
Console.WriteLine("Test1: {0}", Evaluator.EvaluateToString("\"Hello \" + \"There\""));
Console.WriteLine("Test2: {0}", Evaluator.EvaluateToBool("30 == 40"));
Console.WriteLine("Test3: {0}", Evaluator.EvaluateToObject("new DataSet()"));

EvaluatorItem[] items = {
                          new EvaluatorItem(typeof(int), "(30 + 4) * 2", "GetNumber"),
                          new EvaluatorItem(typeof(string), "\"Hello \" + \"There\"", 
                          new EvaluatorItem(typeof(bool), "30 == 40", "GetBool"),
                          new EvaluatorItem(typeof(object), "new DataSet()", "GetDataSet")

Evaluator eval = new Evaluator(items);
Console.WriteLine("TestStatic0: {0}", eval.EvaluateInt("GetNumber"));
Console.WriteLine("TestStatic1: {0}", eval.EvaluateString("GetString"));
Console.WriteLine("TestStatic2: {0}", eval.EvaluateBool("GetBool"));
Console.WriteLine("TestStatic3: {0}", eval.Evaluate("GetDataSet"));

How does it work? I am using the CodeDOM to create a simple assembly with a single class in it. I simply transform each of the EvaluatorItems into a method of the class. When you call EvaluateInt() or Evaluate(), I use reflection to get a MethodInfo object and call the method.

The source code comes packaged in a Class Library with a Test program that you can use to get examples of use:

To compile the expressions, I am creating a new CSharpCodeProvider and setting Compiler attributes (like adding references, telling it to generate it in memory, etc.). Then I am building a dummy class that I can append my methods on. Lastly I compile it (check for errors) and save the object to use to call with the MethodInfo structure:

ICodeCompiler comp = (new CSharpCodeProvider().CreateCompiler());
CompilerParameters cp = new CompilerParameters();
cp.GenerateExecutable = false;
cp.GenerateInMemory = true;

StringBuilder code = new StringBuilder();
code.Append("using System; \n");
code.Append("using System.Data; \n");
code.Append("using System.Data.SqlClient; \n");
code.Append("using System.Data.OleDb; \n");
code.Append("using System.Xml; \n");
code.Append("namespace ADOGuy { \n");
code.Append("  public class _Evaluator { \n");
foreach(EvaluatorItem item in items)
  code.AppendFormat("    public {0} {1}() ", 
  code.Append("{ ");
  code.AppendFormat("      return ({0}); ", item.Expression);
code.Append("} }");

CompilerResults cr = comp.CompileAssemblyFromSource(cp, code.ToString());
if (cr.Errors.HasErrors)
  StringBuilder error = new StringBuilder();
  error.Append("Error Compiling Expression: ");
  foreach (CompilerError err in cr.Errors)
    error.AppendFormat("{0}\n", err.ErrorText);
  throw new Exception("Error Compiling Expression: " + error.ToString());
Assembly a = cr.CompiledAssembly;
_Compiled = a.CreateInstance("ADOGuy._Evaluator");

When I call the _Compiled object, I use a MethodInfo object to Invoke the call and return the result to the user:

public object Evaluate(string name)
  MethodInfo mi = _Compiled.GetType().GetMethod(name);
  return mi.Invoke(_Compiled, null);


This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)


About the Author

Shawn Wildermuth
United States United States
No Biography provided

Comments and Discussions

GeneralC# Parser Generator Pin
jhreich22-Jul-02 3:48
Memberjhreich22-Jul-02 3:48 
GeneralRe: C# Parser Generator Pin
Anonymous23-Jul-02 17:33
MemberAnonymous23-Jul-02 17:33 
GeneralRe: C# Parser Generator Pin
Jeff Varszegi25-Apr-04 20:29
professionalJeff Varszegi25-Apr-04 20:29 
GeneralRe: C# Parser Generator Pin
Keith Rule11-Nov-05 6:35
professionalKeith Rule11-Nov-05 6:35 
GeneralI like to know more... how to put the assembly into a SQL2000 Binary field... Pin
19-Jun-02 17:50
suss19-Jun-02 17:50 
GeneralTotally amazing Pin
NielsH24-May-02 9:29
MemberNielsH24-May-02 9:29 
GeneralGreat! Pin
Holger Persch22-Apr-02 20:02
MemberHolger Persch22-Apr-02 20:02 
Generalseriously cool! Pin
ector22-Apr-02 4:56
Memberector22-Apr-02 4:56 
I had no idea the managed environment was this useful...

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