Parse string to C# lambda Func

Question

Is there a way to convert string representation of lambda to a lambda Func?

Func<Product, bool> func = Parse<Product, bool>("product => product.Name.Length > 0");

I tried Dynamic LINQ but it doesn't work as expected - for example it doesn't expect lambda syntax =>.

Summary of answers:

• writing my own C# compiler - very funny
• firing up external compiler (like csc.exe) - very slow
• using DLINQ - as I said I don't see how it can parse lambda expressions

Why do I need this: because there's no way to pass lambdas to custom attributes like

[Secure(role => role.CanDoThis && role.AllowedCount > 5)]

So as a workaround I'd like to pass lambda as string: "role => role.CanDoThis && role.AllowedCount > 5". But seems like I'll have to use DLINQ like this: "CanDoThis && AllowedCount > 5" - since that's the syntax it understands. But my question was about true lambdas, I've already used DLINQ at the time of asking.

Answer 1

Have a look at Expression Trees.

Answer 2

You could parse the string and build up a lambda expression using the Expression class, essentially duplicating the function of the compiler.

Answer 3

You might be able to do something with CSharpCodeProvider (wrap the expression with some more code to create a valid class and compile it into an assembly, then load the assembly).

I believe that is how LINQPad does it.

Answer 4

I guess you have to resort to the CSharpCodeProvider. However, dealing with all possible local variable references might not be trivial. And how would you tell the CSharpCodeProvider about the type of the lambda parameter? I would probably create a template class looking like this:

class ExpressionContainer {
    public Expression<Func<Product, bool>> TheExpression;
    public string Length;

    public ExpressionContainer() {
        TheExpression = <user expression text>;
    }
}

Then do something like this:

string source = <Code from above>;
Assembly a;
using (CSharpCodeProvider provider = new CSharpCodeProvider(...) {
    List<string> assemblies = new List<string>();
    foreach (Assembly x in AppDomain.CurrentDomain.GetAssemblies()) {
        try {
            assemblies.Add(x.Location);
        }
        catch (NotSupportedException) {
            // Dynamic assemblies will throw, and in .net 3.5 there seems to be no way of finding out whether the assembly is dynamic before trying.
        }
    }

    CompilerResults r = provider.CompileAssemblyFromSource(new CompilerParameters(assemblies.ToArray()) { GenerateExecutable = false, GenerateInMemory = true }, source);
    if (r.Errors.HasErrors)
        throw new Exception("Errors compiling expression: " + string.Join(Environment.NewLine, r.Errors.OfType<CompilerError>().Select(e => e.ErrorText).ToArray()));
    a = r.CompiledAssembly;
}
object o = a.CreateInstance("ExpressionContainer");
var result = ( Expression<Func<Product, bool>>)o.GetType().GetProperty("TheExpression").GetValue(o);

Note, however, that for long-running applications, you should create all these in-memory assemblies in a separate appdomain since they can't be freed until the appdomain they reside in is unloaded.