In C# 3.5, How do you pass which method to call on an object as a parameter

Question

I have two methods in C# 3.5 that are identical bar one function call, in the snippet below, see clientController.GetClientUsername vs clientController.GetClientGraphicalUsername

    private static bool TryGetLogonUserIdByUsername(IGetClientUsername clientController, string sClientId, out int? logonUserId)
    {
        string username;
        if (clientController.GetClientUsername(sClientId, out username))
        {
     // ... snip common code ...
        }

        return false;
    }

    private static bool TryGetLogonUserIdByGraphicalUsername(IGetClientUsername clientController, string sClientId, out int? logonUserId)
    {
        string username;
        if (clientController.GetClientGraphicalUsername(sClientId, out username))
        {
     // ... snip common code ...
        }

        return false;
    }

Is there a way (delegates, lamda's ?) that I can pass in which method on clientController I want to call?

Thanks!

Answer 1

You can pass a MethodInfo, which can be looked up statically. However, I agree that a redesign may be called for.

private static readonly MethodInfo getRegularLogin = typeof(IGetClientUsername).GetMethod("GetClientUsername");
private static bool TryGetLogonUserIdByUsername(IGetClientUsername clientController, string sClientId, out int? logonUserId)
{

    string username;
    return TryGetLoginReflective(getRegularLogin, clientController, sClientId, out username, out logonUserId);
}

private static readonly MethodInfo getGraphicalLogin = typeof(IGetClientUsername).GetMethod("GetClientGraphicalUsername");
private static bool TryGetLogonUserIdByGraphicalUsername(IGetClientUsername clientController, string sClientId, out int? logonUserId)
{
    string username;
    return TryGetLoginReflective(getGraphicalLogin,  clientController, sClientId, out username, out logonUserId);
}

private static bool TryGetLoginReflective(MethodInfo method, IGetClientUsername clientController, string sClientId, out string username, out int? logonUserId)
{
    object[] args = new object[]{sClientId, null};
    if((bool)method.Invoke(clientController, args))
    {
         // ... snip common code ...
    }
    logonUserId = ...;

    username = (string)args[1];
    return false;
}

Answer 2

While you can pass a delegate as a parameter, I suggest going with a different route. Encapsulate the body of the if statement which involves common code in another function and call that one in both functions.

Visual Studio has a "Refactor -> Extract Method" feature in the context menu. You can just fill in one of the bodies, select the body and use that feature to extract a method out of it automatically.

Answer 3

The type of a function is written as Func < inParam1, inParam2, ..., returnParam>. I'm not sure offhand if "out" parameters are passed properly in "Func" types, but you should be able to make your function like

void TryGetLogon(Func<IGetClientUsername, string, out int?, bool> f) { 
   // ...
   f(x, y, z, a);
}
// ...
TryGetLogon(TryGetLogonUserIdByGraphicalUsername);

Answer 4

How about simply passing in a boolean flag?

private static bool TryGetLogonUserIdByUsername(
    IGetClientUsername clientController,
    string sClientId, out int? logonUserId, bool graphical)
{
    string username;
    bool gotClient = false;

    if (graphical)
    {
        gotClient = clientController.GetClientGraphicalUsername(
            sClientId, out username);
    }
    else
    {
        gotClient = clientController.GetClientUsername(
            sClientId, out username);
    }

    if (gotClient)
    {
               // ... snip common code ...
    }

    return false;
}

Answer 5

Sure. Just define a delegate like so:

public delegate bool GetUsername(string clientID, out string username);

And then pass it into your function and call it:

private static bool TryGetLogonUserId(IGetClientUsername clientController, string sClientId, out int? logonUserId, GetUsername func)
{
    string username;
    if (func.Invoke(sClientId, out username))
    {
       // ... snip common code ...
    }
    return false;
}

To call the function with the delegate, you'll do this:

TryGetLogonUserId(/* first params... */, clientController.GetClientUsername);