Setting parameter values of anonymous function in C#

Lets say i have the following code

private Func<T> _method;

public void SetExecutableMethod<T>(Func<T> methodParam)
{
    _method = methodParam;
}

public T ExecuteMethod(object[] parameterValues)
{
    //get the number of parameters _method has;
    var methodCallExpression = _method.Body as MethodCallExpression;
    var method = methodCallExpression.Method;
    ParameterInfo[] methodParams = method.GetParameters();

    //So i now have a list of parameters for the method call,
    //How can i update the parameter values for each of these?
    for (int i = 0; i < parameters.Count(); i++ )
    {
        methodParams[i] = ???''
    }

    return _method.Compile()();
}

public void InitAndTest()
{
    SetExecutableMethod( () => _service.SomeMethod1("param1 placeholder", "param2 placeholder") );

    T result1 = ExecuteMethod(new object[]{"Test1", "Test2"});
    T result2 = ExecuteMethod(new object[]{"Test3", "Test4"}););
}

In the above code, i want to set a private variable to some Func that points to an anonymoust function and never have to set it again. I then would like to be able to call ExecuteMethod(...) with different parameters. This method should update the parameter values of the variable _method and then invoke the method. I can read the number of parameters and their values fine, i just am not sure how to set the values for those parameter? Any thoughts on this?

Solution

This is not the way to do it. Right now, your _method field is a delegate of type Func<T>, and you expect that its body contains yet another method which is actually executed. That is a lot to expect from your callers. I would forget about this approach and look for something different.

One way would be to supply a method which takes an array of objects as its parameter (Func<object[], T>), and then invoke it directly with appropriate parameters (but never a method in its body). Even this is less common for a strongly typed language like C# since you lose all type safety (but then again, you do want to be pretty flexible with this framework you are designing).

The other way would be to get a MethodInfo instance, and then use its Invoke method. In a way, this might even express your intents better, because it will be obvious that the executable method is capable of virtually anything.

Next, you could use generics to get some type safety, and require that all your input parameters are wrapped inside a single parameter class. In that case, you might have a strongly typed Func<Tparam, Tresult> method, and your Execute method would accept a Tparam instance as its parameter. This would void the need for any reflection.

[Edit]

As I wrote, I would try to avoid reflection. Since you wrote you basically need a cache of method results, a simple approach might be something like:

Create a wrapper for your list of parameters so that you can compare them "by value". I added an example class, but you might even want to allow passing an IEqualityComparer explicitly, so that you don't have to override Equals for each partial parameter.

// implements `IEquatable` for a list of parameters
class Parameters : IEquatable<Parameters>
{
    private readonly object[] _parameters;
    public Parameters(object[] parms)
    {
        _parameters = parms;
    }

    #region IEquatable<Parameters> Members

    public bool Equals(Parameters other)
    {
        if (other == null)
            return false;

        if (_parameters.Length != other._parameters.Length)
            return false;

        // check each parameter to see if it's equal
        // ...     
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as Parameters);
    }

    public override int GetHashCode()
    { ... }

    #endregion
}

Create a cache for a single service. Using the wrapper class above, it should simply check if a cached result exists:

// contains cached results for a single service
class CachedCallInfo
{
    private readonly Func<object[], object> _method;
    private readonly Dictionary<Parameters, object> _cache
        = new Dictionary<Parameters, object>();

    public CachedCallInfo(Func<object[], object> method)
    {
        _method = method;
    }

    public T GetResult<T>(params object[] parameters)
    {
        // use out Parameters class to ensure comparison
        // by value
        var key = new Parameters(parameters);
        object result = null;

        // result exists?
        if (!_cache.TryGetValue(key, out result))
        {
            // do the actual service call
            result = _method(parameters);

            // add to cache
            _cache.Add(key, result);
        }
        return (T)result;
    }
}

Create the final class which will reference services by name:

public class ServiceCache
{
    private readonly Dictionary<string, CachedCallInfo> _services =
        new Dictionary<string, CachedCallInfo>();

    public void RegisterService(string name, Func<object[], object> method)
    {
        _services[name] = new CachedCallInfo(method);
    }

    // "params" keyword is used to simplify method calls
    public T GetResult<T>(string serviceName, params object[] parameters)
    {
        return _services[serviceName].GetResult<T>(parameters);
    }
}

Your cache setup will then look like this:

serviceCache.RegisterService("ServiceA", @params => DoSomething(@params));
serviceCache.RegisterService("ServiceB", @params => SomethingElse(@params));

And you would simply call it like this:

var result = serviceCache.GetResult("ServiceA", paramA, paramB, paramC);