Question

How would one best go about allowing the input of a c# program to control method invocation? For example:

Assume we have a delegate type:

delegate void WriteMe(); 

And a couple of methods:

void PrintInt() { Console.WriteLine(10); }
void PrintString() { Console.WriteLine("Hello world."); }

And allowing the input to select the invocation order:

public static WriteMe ProcessInvocationInput(int[] val) {
    WriteMe d = null; 
    foreach (int i in val) {
        switch (i) {
            case 1: d += PrintInt; break; 
            case 2: d += PrintString; break; 
        } 
    }
}

And the code that calls it all:

static void Main(string args[]) {
    int[] values = {1, 2, 3}; // Obviously this array could be filled 
                              // from actual input (args, file, wherever)
    WriteMe d = ProcessInvocationInput(values); 

    d(); 
}

The reason I’m posting this question is because it seems rather complex to implement what seems like a simple idea. I know another way to accomplish this behavior is with the reflection API, but that would be even more convoluted.

Answer 1

That really depends on the scope you’re trying to cover. For simple cases you could use a switch (I’d suggest and enum to make it clear):

enum InputCommands
{
    PrintHelloWorld = 1,
    ExecuteFixedBinary = 2,
    ...
}

switch((InputCommands)inputInt)
{
    case InputCommands.PrintHelloWorld: ...
    case InputCommands.ExecuteFixedBinary: ...
}

But if you’re writing a shell, than you need something more robust, like some sort of IExecutableCommand interface implemented by various classes.

interface IExecutableCommand
{
    void Execute(string arg);
}

You will have to implement some parser to handle multiple invocation requests and/or handle more complex arguments.

If you want to use Reflection, be sure to validate your input! That could be done by only executing methods with a custom attribute on them.

class ExecutableMethodAttribute : Attribute { }

[ExecutableMethod]
void Foo() 
{ 
   ...
}

Filtering out methods with this attribute is easy enough:

someAssembly.GetTypes()
  .SelectMany(t => t.GetMethods())
  .Where(mi => mi.GetCustomAttributes(typeof(ExecutableMethodAttribute), true).Any())