How can I evaluate C# code dynamically?

Using the Roslyn scripting API (more samples here):

// 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();");

DISCLAIMER: This answer was written back in 2008. The landscape has changed drastically since then.

Look at the other answers on this page, especially the one detailing Microsoft.CodeAnalysis.CSharp.Scripting.

Rest of answer will be left as it was originally posted but is no longer accurate.

Unfortunately, C# isn't a dynamic language like that.

What you can do, however, is to create a C# source code file, full with class and everything, and run it through the CodeDom provider for C# and compile it into an assembly, and then execute it.

This forum post on MSDN contains an answer with some example code down the page somewhat:
create a anonymous method from a string?

I would hardly say this is a very good solution, but it is possible anyway.

What kind of code are you going to expect in that string? If it is a minor subset of valid code, for instance just math expressions, it might be that other alternatives exists.

Edit: Well, that teaches me to read the questions thoroughly first. Yes, reflection would be able to give you some help here.

If you split the string by the ; first, to get individual properties, you can use the following code to get a PropertyInfo object for a particular property for a class, and then use that object to manipulate a particular object.

String propName = "Text";
PropertyInfo pi = someObject.GetType().GetProperty(propName);
pi.SetValue(someObject, "New Value", new Object[0]);

Link: PropertyInfo.SetValue Method

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;

}