If you're looking for a 1:1 match between how your javascript is executed in .NET and how it would be executed in a windows program such as Internet Explorer, there is a way to reuse the Windows script engines (not only Jscript but also VBScript or any other ActiveX Scripting language) that's described here on SO: parse and execute JS by C#

This is how your code could be implemented using this technique:

        var script = @"
                           function show( )
                           {
                                  return  parseInt('123asd'); //in js it's 123
                           }"; // Note I have removed the return as it's not needed here

        using (ScriptEngine engine = new ScriptEngine("jscript"))
        {
            ParsedScript parsed = engine.Parse(script);
            Console.WriteLine(parsed.CallMethod("show"));
        }
    }

This will output 123 as expected. Note I think the original code could be improved with the new dynamic C# keyword, so we could probably write Console.WriteLine(parsed.show()) directly.