gistfile1.cs

        public double calculate(string expr)
        {
            // Try to replace things with Math

            string[] mathfunktions = this.mathfunktions().Select(x => x.Name).ToArray();
            foreach (var item in mathfunktions)
            {
                if (expr.ToLower().Contains(item.ToLower()))
                {
                    string exprlocal = expr;
                    int expstart = exprlocal.IndexOf(item.ToLower());
                    exprlocal = exprlocal.Substring(expstart, exprlocal.Length - expstart);
                    string exprexpression = item;
                    int expend = exprlocal.IndexOf(')');
                    if (expend != exprlocal.Length - 1)
                    {
                        exprlocal = exprlocal.Remove(expend + 1);
                    }
                    string replace = exprlocal;
                    exprlocal = exprlocal.Replace(exprexpression.ToLower(), string.Empty);
                    exprlocal = exprlocal.Substring(1, exprlocal.Length - 1);
                    exprlocal = exprlocal.Remove(exprlocal.Length - 1);

                    string insert = " + Math." + exprexpression + "(" + exprlocal + ") + ";
                    expr = expr.Replace(replace, insert);
                    if (expr.Length == insert.Length)
                    {
                        expr = expr.Replace(" + ", string.Empty);
                    }
                }
            }


            // Compile and so on....
            Microsoft.CSharp.CSharpCodeProvider cp = new Microsoft.CSharp.CSharpCodeProvider();
            System.CodeDom.Compiler.CompilerParameters cpar = new System.CodeDom.Compiler.CompilerParameters();
            cpar.GenerateInMemory = true;
            cpar.GenerateExecutable = false;
            cpar.ReferencedAssemblies.Add("system.dll");
            cpar.ReferencedAssemblies.Add(Application.ExecutablePath);
            string src = "using System;" +
             "class myclass:AdvCalc.calc" +
             "{" +
             "public myclass(){}" +
             "public override double calculate()" +
             "{" +
             "return " + expr + ";" +
             "}" +
             "}";

            System.CodeDom.Compiler.CompilerResults cr = cp.CompileAssemblyFromSource(cpar, src);
            
            calc myobj = null;

            if (cr.Errors.Count == 0 && cr.CompiledAssembly != null)
            {
                Type ObjType = cr.CompiledAssembly.GetType("myclass");
                try
                {
                    if (ObjType != null)
                    {
                        myobj = (calc)Activator.CreateInstance(ObjType);
                    }
                }
                catch
                {
                    // Error 0
                    MessageBox.Show("Could not calculate");
                    return 0;
                }

                // Get result
                return myobj.calculate();
            }
            else
            {
                // Error 0
                MessageBox.Show("Could not calculate");
                return 0;
            }   
        }

        public System.Reflection.MethodInfo[] mathfunktions()
        {
            return typeof(Math).GetMethods().Where(x => x.GetParameters().Length == 1 && (x.GetParameters()[0].ParameterType == typeof(double) || x.GetParameters()[0].ParameterType == typeof(int))).ToArray();
        }