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Create lambda action delegate dynamically

Can I create an Action delegate with a lambda expression like this () => Method(args) dynamically when I have the target, MethodInfo and arguments to pass in?
Say I had a method that took an int argument and wanted to assign an Action delegate to call it with the argument 1 like so Action action = () => Method(1), but I want to do it dynamically. You could make a delegate and assign the action to dynamically invoke it, or assign it to invoke the method, but the performance is terrible.
public class DelegateData
{
public object target;
public MethodInfo method;
public object[] args;
}

Figured it out. It's quite easy with expressions, creating the delegate this way is of course incredibly slow (almost 1 second for 10.000 iterations), but afterward, the delegate is only twice as slow as it would be when created naturally. lambda.Compile is the culprit here, but I see no way around it.
var arguments = args.Select(a => Expression.Constant(a)).ToArray();
var lambda = Expression.Lambda(Expression.Call(Expression.Constant(target), method, arguments));
var result = (Action)lambda.Compile();

Get custom attribute of method executed in action

Here is my example metod
[TestStep("Do something")]
private void DoSomething()
{
}
Each method which looks like the one above is executed in a way which requires logging the method parameters:
private void LogStep(Action action)
{
string stepName = "[" + action.Method.Name + "] ";
var descr = Attribute.GetCustomAttribute(action.Method, typeof(TestStepAttribute)) as TestStepAttribute;
if (descr == null)
{
this.TestLog.AddWarningMessage(
(action.Method.DeclaringType == null ? string.Empty : action.Method.DeclaringType.FullName + ".") + action.Method.Name
+ ": missing description");
return;
}
stepName += descr.Description;
this.TestLog.EndGroup();
this.TestLog.BeginGroup(stepName);
}
And here I come with a problem. Executing LogStep like
LogStep(DoSomething)
works perfectly, but when I execute it using lambda expression
LogStep(() => DoSomething())
It tells me that there are no attributes of type TestStepAttribute in that Action.
At first glance it seems to be similar to How do I get the custom attributes of a method from Action<T>? but in my case I neither can change type of Action to Expression<Action> nor I know the method name.
Any suggestion'd be helpful.

When you execute it using a lambda expression, the lambda expression itself is the method. It happens that in its body there is a method call, but there could be something else there (like new object()). The only way to get access to the attributes of this inner method is to pass the lambda expression as Expression and to analyze the expression.
In order to handle both cases you will need two overloads of LogStep. However, you cannot have LogStep(Action) and LogStep(Expression<Action>) as overloads at the same time as the calls would be ambiguous. But it would work if one of them was LogStep(Delegate).
private void LogStep(Delegate action)
{
var attr = (TestStepAttribute)Attribute
.GetCustomAttribute(action.Method, typeof(TestStepAttribute));
Console.WriteLine("LogStep(Delegate action): " + attr?.Description);
}
private void LogStep(Expression<Action> actionExpr)
{
string descr = null;
var methodCall = actionExpr.Body as MethodCallExpression;
if (methodCall != null) {
var attribs = methodCall.Method.GetCustomAttributes(typeof(TestStepAttribute), true);
if (attribs.Length > 0) {
descr = ((TestStepAttribute)attribs[0]).Description;
}
}
Console.WriteLine("LogStep(Expression<Action> actionExpr): " + descr);
}
Tests:
LogStep(new Action(DoSomething)); // new Action() Is required here. Calls first overlaod.
LogStep(() => DoSomething()); // Calls second overload.
LogStep(() => new object()); // Calls second overload.
Note that you can compile and execute lambda expressions, in case you need to execute the method.

works perfectly, but when I execute it using lambda expression
LogStep(() => DoSomething()) It tells me that there are no attributes
of type TestStepAttribute in that Action.
Well of course won't find any attributes because you are passing a lambda expression which is basically a method and in that method you pass your method DoSomething() and the check is done on the lambda expression.

Lambda expression is just another method. When you look at action.Method, that's the method you get (and action.Target will contain a closure, if any).
In the end, all you have is:
void SomeAnonymousMethod()
{
DoSomething();
}
To get to the method actually being called, you'd have to decompile the anonymous method first. And of course, you're probably using the lambda syntax to pass arguments while still working with a paremeter-less action, where it gets even crazier:
class SomeClosure
{
string argument1;
int argument2;
void AnonymousMethod()
{
var data = GetSomeData(argument2);
DoSomething(data, argument1);
}
}
How do you even tell that DoSomething is the method you need metadata of?
There isn't a way around this using lambda expressions. Fortunately, it doesn't seem you actually need that anyway, since you're never calling the argument. Instead of using Action, just use Delegate, and you can pass whatever method you need directly:
void DoSomething(string something, string otherThing)
{
... // Not important
}
void LogStep(Delegate someDelegate)
{
... // Exactly as before
}
LogStep((Action<string, string>)DoSomething);
You do have to manually cast at the invocation, sadly, otherwise the compiler gives you an error; you can keep the same signature for the LogStep method itself, though. Alternatively, you could use a simple T4 template to create multiple overloads of the LogStep method so that you can avoid the explicit cast in your hand-written code.

Updating Expression Parameter

Here's my code:
public static Response<TResult> Create<TResult>(Expression<Func<Response<TResult>>> method)
{
var objectMember = Expression.Convert(((MethodCallExpression)method.Body).Arguments[0], typeof(TResult));
var getterLambda = Expression.Lambda<Func<TResult>>(objectMember);
var getter = getterLambda.Compile();
TResult myObject = getter();
// Do something to 'myObject'
// Call original method with updated 'myObject' as the parameter
}
As you can see from the code snippet above, I have a generic method that accepts a method as a parameter.
Orginally, I was calling method.Compile()() which worked fine. However, I now need to update the parameter ("myObject") first before calling the method. So, I've added the lines above to get the parameter.
I then "Do something to 'myObject'". Finally, I want to call the original passed method, but replace the original parameter with the new, updated parameter.
What am I missing?
UPDATE
So, I've, technically, accomplished my need by the following code:
Response<TResult> result = (Response<TResult>)((MethodCallExpression)method.Body).Method.Invoke(parentType, new object[] { myObject });
But this is using a lot of reflection and casting. Is there a better way to do this?
Thanks,
Joshua

Pass Expression Tree as Method parameter

I have a function:
public void Execute(Expression<Action> expression)
{
var time = expression.Compile().Benchmark();
var msg = string.Format("{0} took {1} to complete",
ReflectionHelper.GetComponentCallDetails().ToString(),
time.ToString(#"hh\:mm\:ss\.ff"));
Logger.Info(msg);
}
The delegate that needs to be called is something like:
channels = GetAllChannelsImpl(maxResults);
I am relatively new to Expression Trees and cannot figure out a way to pass the Action delegate to the method.
I was able to do the same functionality using
public void Execute(Action action)
{
var time = action.Benchmark();
var msg = string.Format("{0} took {1} to complete",
ReflectionHelper.GetComponentCallDetails().ToString(),
time.ToString(#"hh\:mm\:ss\.ff"));
Logger.Info(msg);
}
and calling as
Execute(() =>
{
channels = GetAllChannelsImpl(maxResults);
});
But I wanted to use the Expression Tree based approach in order to eliminate the need to use the overhead of reflection to figure out the details of the method invoked to log it.
Can anyone suggest the right way to pass the expression tree for the above action delegate as a method parameter.

A lambda expression by itself does not have a type. The actual type it takes is inferred by the compiler depending on what you are trying to assign or cast to. With that said, any calls to your Execute() method using simple lambdas will be ambiguous since your lambda would be compatible as an Action or Expression<Action>. You would have to disambiguate this by explicitly casting to the type you expect.
// assign to a variable
Expression<Action> action1 = () => ...;
Execute(action1);
// cast
Execute((Expression<Action>)(() => ...));
// use the constructor
Execute(new Expression<Action>(() => ...));
It would be better IMHO to remove the ambiguous overload and rename one of the methods. I'd recommend renaming the expression overload to ExecuteExpression().

How to make a function private to a method?

I'm working on a method that needs to repeat a small operation at different spots, but the code to be repeated should be private to the method. The obvious solution is a nested function. Whatever I try however, the C# compiler barfs at me.
Something roughly equal to this Perl snippet:
my $method = sub {
$helper_func = sub { code to encapsulate };
# more code
&$helper( called whenever needed );
# more code
}
is what I am talking about, and what I'm trying to accomplish in C#.
No other method in the class should be able to access the helper function in this context. The most logical means of writing this construct in C#, as it appears to me would be something like this:
var helper = (/* parameter names */) => { /* code to encapsulate */ };
And actually make the compiler earn its keep.
Since such an assignment is forbidden, as is the equivalent using the older delegate(){} syntax in place of the lambda, and so is declaring a delegate type within a method—what csc actually allows me to write however, is this:
private delegate /* return type */ Helper(/* parameters */);
private /* return type */ method(/* parameters */) {
Helper helper = (/* parameter names */) => {
/* code to encapsulate */
};
// more code
helper( /* called whenever needed */ );
// more code
}
Which is all fine and dandy for not copy and pasting a chunk of code around and editing the parameters by hand but it leaks a private delegate type to the rest of the class rather than keeping it private to the method. Which defeats the purpose in the first place. Using goto statements and local variables for parameters would provide better encapsulation of "helper" in this context without sacrificing code reuse. If I wanted to simulate function calls by passing parameters through registers, I think would rather use an assembler. I haven't found an acceptable way of refactoring the code to avoid the problem altogether either.
So, is it even possible to force this Common Object Oriented Language to obey?

You actually can do this in C#.
Func<T1, T2, ..., TReturn> myFunc = (a, b, ...) =>
{
//code that return type TReturn
};
If you need an anonymous method of return type void use Action instead of Func:
Action<T1, T2, ...> myAction = (a, b, ...) =>
{
//code that doesn't return anything
};

If you are in C# 3.5 or higher you can take advantage of the lambdas and convenience delegate declarations Func<> and Action<>. So for instance
void DoSomething()
{
Func<int,int> addOne = (ii) => ii +1;
var two = addOne(1);
}
The reason you can't do
var addOne = (ii) => ii +1;
is because of Homoiconicity, the lambda can be interpreted as two different constructs, a delegate and an expression tree. Thus the need to be explicit in declaration.

If you explicitly type it, it will work, i.e.
Action<paramType1, paramType2> helperAction = (/* parameter names */) => { /* code to encapsulate */ };
Func<paramType1, paramType2, returnType> helperFunction = (/* parameter names */) => { /* code to encapsulate */ };
The reason var doesn't work is that a lambda expression can evaluate to multiple types (I believe either a delegate or expression tree, but don't quote me on that) and the compiler in this situation is unable to infer which was meant.

I recommend looking at the Action<T> and Func<TResult> delegates and their overloads. You can do something like this
static void Main(string[] args)
{
SomeMethod();
}
private static void SomeMethod()
{
Action<int> action = (num) => Console.WriteLine(num);
Enumerable.Range(1,10).ToList().ForEach(action);
Console.ReadKey();
}
Here SomeMethod is private and has a local Action<int> delgate that takes an int and does something to it.
I think the issue that you came across is that you can't use implicit typing (i.e. use var) when assigning a lambda expression to a variable.

You can't use the var keyword with lambdas or delegates because they both require additional context information (delegates require a return type, and lambdas require a return type and parameter types). For instance, the (params) => { code } syntax requires to be able to infer the parameter types and return types to work: you do this by explicitly giving it a type.
The generic System.Action delegate type (returns void) could do a good job at what you're trying:
Action<ArgumentType1, ArgumentType2, ...> myDelegate = (params) => { code };
Otherwise, there's also the System.Func, which has a return type, that must be passed as the last generic argument.

It depends on what your definition of hiding is.
The func/action solution (like the one Scott suggests)
void DoSomething()
{
Func<int,int> addOne = (ii) => ii +1;
var two = addOne(1);
}
Feals like hidding the method definition when writing regular C# code BUT is when looking at the IL equivalent of
//This is pseudo code but comes close at the important parts
public class Class1
{
//The actual type is different from this
private static Func<int, int> myMethod = AnonymousFunction;
public void f()
{
myMethod(0);
}
private static int AnonymousFunction(int i)
{
return 1;
}
}
So if you really want to get to the method from outside of the one "hidding" it you can do this with reflection The actual name generated for the field storing the delegate is illegal in C# bul valid in CLR context but that's the only thing that stand in the way of using the delegate as a regular delegate stored in a field (that is if you figue out the name :) )

It's quite simple actually. As the Method seems to have another responsibility than your current Class (why else would you hide this method) move your method into it's own Class and the part you want to have private into a private method in the new class.