How can I evaluate C# code dynamically?

Using the Roslyn scripting API. More samples in the Roslyn wiki.

// add NuGet package 'Microsoft.CodeAnalysis.Scripting'
using Microsoft.CodeAnalysis.CSharp.Scripting;

await CSharpScript.EvaluateAsync("System.Math.Pow(2, 4)") // returns 16

You can also run any piece of code:

var script = await CSharpScript.RunAsync(@"
                class MyClass
                { 
                    public void Print() => System.Console.WriteLine(1);
                }")

And reference the code that was generated in previous runs:

await script.ContinueWithAsync("new MyClass().Print();");

Not really. You can use reflection to achieve what you want, but it won't be nearly as simple as in Javascript. For example, if you wanted to set the private field of an object to something, you could use this function:

protected static void SetField(object o, string fieldName, object value)
{
   FieldInfo field = o.GetType().GetField(fieldName, BindingFlags.Instance | BindingFlags.NonPublic);
   field.SetValue(o, value);
}

This is an eval function under c#. I used it to convert anonymous functions (Lambda Expressions) from a string. Source: http://www.codeproject.com/KB/cs/evalcscode.aspx

public static object Eval(string sCSCode) {

  CSharpCodeProvider c = new CSharpCodeProvider();
  ICodeCompiler icc = c.CreateCompiler();
  CompilerParameters cp = new CompilerParameters();

  cp.ReferencedAssemblies.Add("system.dll");
  cp.ReferencedAssemblies.Add("system.xml.dll");
  cp.ReferencedAssemblies.Add("system.data.dll");
  cp.ReferencedAssemblies.Add("system.windows.forms.dll");
  cp.ReferencedAssemblies.Add("system.drawing.dll");

  cp.CompilerOptions = "/t:library";
  cp.GenerateInMemory = true;

  StringBuilder sb = new StringBuilder("");
  sb.Append("using System;\n" );
  sb.Append("using System.Xml;\n");
  sb.Append("using System.Data;\n");
  sb.Append("using System.Data.SqlClient;\n");
  sb.Append("using System.Windows.Forms;\n");
  sb.Append("using System.Drawing;\n");

  sb.Append("namespace CSCodeEvaler{ \n");
  sb.Append("public class CSCodeEvaler{ \n");
  sb.Append("public object EvalCode(){\n");
  sb.Append("return "+sCSCode+"; \n");
  sb.Append("} \n");
  sb.Append("} \n");
  sb.Append("}\n");

  CompilerResults cr = icc.CompileAssemblyFromSource(cp, sb.ToString());
  if( cr.Errors.Count > 0 ){
      MessageBox.Show("ERROR: " + cr.Errors[0].ErrorText, 
         "Error evaluating cs code", MessageBoxButtons.OK, 
         MessageBoxIcon.Error );
      return null;
  }

  System.Reflection.Assembly a = cr.CompiledAssembly;
  object o = a.CreateInstance("CSCodeEvaler.CSCodeEvaler");

  Type t = o.GetType();
  MethodInfo mi = t.GetMethod("EvalCode");

  object s = mi.Invoke(o, null);
  return s;

}

I have written an open source project, Dynamic Expresso, that can convert text expression written using a C# syntax into delegates (or expression tree). Expressions are parsed and transformed into Expression Trees without using compilation or reflection.

You can write something like:

var interpreter = new Interpreter();
var result = interpreter.Eval("8 / 2 + 2");

or

var interpreter = new Interpreter()
                      .SetVariable("service", new ServiceExample());

string expression = "x > 4 ? service.SomeMethod() : service.AnotherMethod()";

Lambda parsedExpression = interpreter.Parse(expression, 
                          new Parameter("x", typeof(int)));

parsedExpression.Invoke(5);

My work is based on Scott Gu article http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx .

All of that would definitely work. Personally, for that particular problem, I would probably take a little different approach. Maybe something like this:

class MyClass {
  public Point point1, point2, point3;

  private Point[] points;

  public MyClass() {
    //...
    this.points = new Point[] {point1, point2, point3};
  }

  public void DoSomethingWith(int i) {
    Point target = this.points[i+1];
    // do stuff to target
  }
}

When using patterns like this, you have to be careful that your data is stored by reference and not by value. In other words, don't do this with primitives. You have to use their big bloated class counterparts.

I realized that's not exactly the question, but the question has been pretty well answered and I thought maybe an alternative approach might help.

I don't now if you absolutely want to execute C# statements, but you can already execute Javascript statements in C# 2.0. The open-source library Jint is able to do it. It's a Javascript interpreter for .NET. Pass a Javascript program and it will run inside your application. You can even pass C# object as arguments and do automation on it.

Also if you just want to evaluate expression on your properties, give a try to NCalc.

You can use reflection to get the property and invoke it. Something like this:

object result = theObject.GetType().GetProperty("Property" + i).GetValue(theObject, null);

That is, assuming the object that has the property is called "theObject" :)

You also could implement a Webbrowser, then load a html-file wich contains javascript.

Then u go for the document.InvokeScript Method on this browser. The return Value of the eval function can be catched and converted into everything you need.

I did this in several Projects and it works perfectly.

Hope it helps

Uses reflection to parse and evaluate a data-binding expression against an object at run time.

DataBinder.Eval Method

I have written a package, SharpByte.Dynamic, to simplify the task of compiling and executing code dynamically. The code can be invoked on any context object using extension methods as detailed further here.

For example,

someObject.Evaluate<int>("6 / {{{0}}}", 3))

returns 3;

someObject.Evaluate("this.ToString()"))

returns the context object's string representation;

someObject.Execute(@
"Console.WriteLine(""Hello, world!"");
Console.WriteLine(""This demonstrates running a simple script"");
");

runs those statements as a script, etc.

Executables can be gotten easily using a factory method, as seen in the example here--all you need is the source code and list of any expected named parameters (tokens are embedded using triple-bracket notation, such as {{{0}}}, to avoid collisions with string.Format() as well as Handlebars-like syntaxes):

IExecutable executable = ExecutableFactory.Default.GetExecutable(executableType, sourceCode, parameterNames, addedNamespaces);

Each executable object (script or expression) is thread-safe, can be stored and reused, supports logging from within a script, stores timing information and last exception if encountered, etc. There is also a Copy() method compiled on each to allow creating cheap copies, i.e. using an executable object compiled from a script or expression as a template for creating others.

Overhead of executing an already-compiled script or statement is relatively low, at well under a microsecond on modest hardware, and already-compiled scripts and expressions are cached for reuse.

You could do it with a prototype function:

void something(int i, string P1) {
    something(i, P1, String.Empty);
}

void something(int i, string P1, string P2) {
    something(i, P1, P2, String.Empty);
}

void something(int i, string P1, string P2, string P3) {
    something(i, P1, P2, P3, String.Empty);
}

and so on...

I was trying to get a value of a structure (class) member by it's name. The structure was not dynamic. All answers didn't work until I finally got it:

public static object GetPropertyValue(object instance, string memberName)
{
    return instance.GetType().GetField(memberName).GetValue(instance);
}

This method will return the value of the member by it's name. It works on regular structure (class).

You might check the Heleonix.Reflection library. It provides methods to get/set/invoke members dynamically, including nested members, or if a member is clearly defined, you can create a getter/setter (lambda compiled into a delegate) which is faster than reflection:

var success = Reflector.Set(instance, null, $"Property{i}", value);

Or if number of properties is not endless, you can generate setters and chache them (setters are faster since they are compiled delegates):

var setter = Reflector.CreateSetter<object, object>($"Property{i}", typeof(type which contains "Property"+i));
setter(instance, value);

Setters can be of type Action<object, object> but instances can be different at runtime, so you can create lists of setters.

Unfortunately, C# doesn't have any native facilities for doing exactly what you are asking.

However, my C# eval program does allow for evaluating C# code. It provides for evaluating C# code at runtime and supports many C# statements. In fact, this code is usable within any .NET project, however, it is limited to using C# syntax. Have a look at my website, http://csharp-eval.com, for additional details.