Question

I had a part of code that takes in lambda expressions at runtime, which I can then compile and invoke.

Something thing;

Expression<Action<Something>> expression = (c => c.DoWork());
Delegate del = expression.Compile();
del.DynamicInvoke(thing);

In order to save execution time, I stored those compiled delegates in a cache, a Dictionary<String, Delegate> which the key is the lambda expression string.

cache.Add("(Something)c => c.DoWork()", del);

For exact same calls, it worked fine. However I realized that I could receive equivalent lambdas, such as “d => d.DoWork()”, which I should actually use the same delegate for, and I wasn’t.

This got me wondering if there was a clean way (read “not using String.Replace”, I already did that as a temporary fix) to replace the elements in a lambda expression, like maybe replacing them by arg0 so that both

(c => c.DoWork()) and (d => d.DoWork())

are transformed and compared as (arg0 => arg0.DoWork()) by using something fuctionnally similar to injecting a Expression.Parameter(Type, Name) in a lambda.

Is that possible ? (Answers can include C#4.0)

Answer 1

I used strings, since it was the easisest way for me. You can’t manually change the name of the parameter expression (it has the “Name” property, but it is read-only), so you must construct a new expression from pieces. What I did is created a “nameless” parameter (actually, it gets an autogenerated name in this case, which is “Param_0”) and then created a new expression almost the same as the old one, but using the new parameter.

public static void Main()
{
    String thing = "test";

    Expression<Action<String>> expression = c => c.ToUpper();
    Delegate del = expression.Compile();
    del.DynamicInvoke(thing);

    Dictionary<String, Delegate> cache = new Dictionary<String, Delegate>();
    cache.Add(GenerateKey(expression), del);

    Expression<Action<String>> expression1 = d => d.ToUpper();
    var test = cache.ContainsKey(GenerateKey(expression1));
    Console.WriteLine(test);

}

public static string GenerateKey(Expression<Action<String>> expr)
{
    ParameterExpression newParam = Expression.Parameter(expr.Parameters[0].Type);
    Expression newExprBody = Expression.Call(newParam, ((MethodCallExpression)expr.Body).Method);
    Expression<Action<String>> newExpr = Expression.Lambda<Action<String>>(newExprBody, newParam);
    return newExpr.ToString();
}