Can I expand a string that contains C# literal expressions at runtime

Question

If I have a string that contains a c# string literal expression can I \"expand\" it at runtime

    public void TestEvaluateString()
    {
        string Dumm         


        

Answer 1

If you're just looking to do "simple" escape characters as defined on the Microsoft site, you can use this routine and save importing external libs:

public static class StringExtensions
{
    /* https://msdn.microsoft.com/en-us/library/aa691087(v=vs.71).aspx */
    private readonly static SortedDictionary<char, char> EscapeMap = new SortedDictionary<char, char>
    {
        { '\'', '\'' },
        { '"', '\"' },
        { '\\', '\\' },
        { '0', '\0' },
        { 'a', '\a' },
        { 'b', '\b' },
        { 'f', '\f' },
        { 'n', '\n' },
        { 'r', '\r' },
        { 't', '\t' },
        { 'v', '\v' },
    };

    public static string UnescapeSimple(this string escaped)
    {
        if (escaped == null)
            return escaped;

        var sb = new StringBuilder();

        bool inEscape = false;
        var s = 0;
        for (var i = 0; i < escaped.Length; i++)
        {
            if (!inEscape && escaped[i] ==  '\\')
            {
                inEscape = true;
                continue;
            }

            if (inEscape)
            {
                char mapChar;
                if (EscapeMap.TryGetValue(escaped[i], out mapChar))
                {
                    sb.Append(escaped.Substring(s, i - s - 1));
                    sb.Append(mapChar);

                    s = i + 1;
                }
                inEscape = false;
            }
        }

        sb.Append(escaped.Substring(s));

        return sb.ToString();
    }
}

Here's a unit test to prove it:

    [TestMethod]
    public void UnescapeSimpleTest()
    {
        var noEscapes = @"This is a test".UnescapeSimple();
        Assert.AreEqual("This is a test", noEscapes, nameof(noEscapes));

        var singleEscape = @"\n".UnescapeSimple();
        Assert.AreEqual("\n", singleEscape, nameof(singleEscape));

        var allEscape = @"\'\""\\\0\a\b\f\n\r\t\v".UnescapeSimple();
        Assert.AreEqual("\'\"\\\0\a\b\f\n\r\t\v", allEscape, nameof(allEscape));

        var textInEscapes = @"\tthis\n\ris\\a\ntest".UnescapeSimple();
        Assert.AreEqual("\tthis\n\ris\\a\ntest", textInEscapes, nameof(textInEscapes));

        var backslashNoEscapes = @"\,\h\qtest".UnescapeSimple();
        Assert.AreEqual(@"\,\h\qtest", backslashNoEscapes, nameof(backslashNoEscapes));

        var emptyStr = "".UnescapeSimple();
        Assert.AreEqual("", emptyStr, nameof(emptyStr));

        // Prove Enviroment.NewLine is "\r\n" and not "\n\r" (Windows PC)
        var newLine = @"\r\n".UnescapeSimple();
        Assert.AreEqual(Environment.NewLine, newLine, nameof(newLine));

        // Double check prior test (Windows PC)
        var newLineWrong = @"\n\r".UnescapeSimple();
        Assert.AreNotEqual(Environment.NewLine, newLineWrong, nameof(newLineWrong));
    }

Feel free to tweak the EscapeMap or rename the function UnescapeSimple (awkward I know).

Note that this solution doesn't handle Unicode escape characters or hex or octal, it just handles the simple single character ones.

Answer 2

Not sure if this is the simplest way, but by referencing the Microsoft.JScript namespace you can reparse it with the javascript eval function.

Here's a test for the code at the bottom

var evalToString = Evaluator.MyStr("test \\r\\n test");

This will turn the \r into a carriage return.

And the implementation

public class Evaluator
{
    public static object MyStr(string statement)
    {
        return _evaluatorType.InvokeMember(
                    "MyStr",
                    BindingFlags.InvokeMethod,
                    null,
                    _evaluator,
                    new object[] { statement }
                 );
    }

    static Evaluator()
    {
        ICodeCompiler compiler;
        compiler = new JScriptCodeProvider().CreateCompiler();

        CompilerParameters parameters;
        parameters = new CompilerParameters();
        parameters.GenerateInMemory = true;

        CompilerResults results;
        results = compiler.CompileAssemblyFromSource(parameters, _jscriptSource);

        Assembly assembly = results.CompiledAssembly;
        _evaluatorType = assembly.GetType("Evaluator.Evaluator");

        _evaluator = Activator.CreateInstance(_evaluatorType);
    }

    private static object _evaluator = null;
    private static Type _evaluatorType = null;
    private static readonly string _jscriptSource =

        @"package Evaluator
        {
           class Evaluator
           {
              public function MyStr(expr : String) : String 
              { 
                 var x;
                 eval(""x='""+expr+""';"");
                 return x;
              }
           }
        }";
}

Answer 3

Regex.Unescape would be my method of choice.

Answer 4

Similar to Mikael answer but using the CSharpCodeProvider:

    public static string ParseString(string txt)
    {
        var provider = new Microsoft.CSharp.CSharpCodeProvider();
        var prms = new System.CodeDom.Compiler.CompilerParameters();
        prms.GenerateExecutable = false;
        prms.GenerateInMemory = true;
        var results = provider.CompileAssemblyFromSource(prms, @"
namespace tmp
{
    public class tmpClass
    {
        public static string GetValue()
        {
             return " + "\"" + txt + "\"" + @";
        }
    }
}");
        System.Reflection.Assembly ass = results.CompiledAssembly;
        var method = ass.GetType("tmp.tmpClass").GetMethod("GetValue");
        return method.Invoke(null, null) as string;
    }

You might be better off using a dictionary of wildcards and just replacing them in the string.