Is possible to cast a variable to a type stored in another variable?

Question

This is what I need to do:

object foo = GetFoo();
Type t = typeof(BarType);
(foo as t).FunctionThatExistsInBarType();

Can something like this be done?

Answer 1

You can use the Convert.ChangeType method.

object foo = GetFoo(); 
Type t = typeof(string);
string bar = (string)Convert.ChangeType(foo, t);

Answer 2

No, you cannot. C# does not implement duck typing.

You must implement an interface and cast to it.

(However there are attempts to do it. Look at Duck Typing Project for an example.)

Answer 3

Since dynamics were added to c#, I think we can do it in this way:

class Program {
    static void Main(string[] args) {
        List<int> c = new List<int>(); 
        double i = 10.0;
        Type intType = typeof(int);
        c.Add(CastHelper.Cast(i, intType)); // works, no exception!
    }
}

class CastHelper {
    public static dynamic Cast(object src, Type t) {
        var castMethod = typeof(CastHelper).GetMethod("CastGeneric").MakeGenericMethod(t);
        return castMethod.Invoke(null, new[] { src });
    }
    public static T CastGeneric<T>(object src) {
        return (T)Convert.ChangeType(src, typeof(T));
    }
}

Answer 4

Your original question was flawed in that you ask to treat a variable as a type which is not known at compile time but note that you have string defined on the left hand side when you declare your variable. C# as of 3.5 is statically typed.

Once dynamic is available you could do something like this:

dynamic foo = GetFoo();
foo.FunctionThatExistsInBarType(); 

For when you don't know what the type is but you know it will always support the instance method FunctionThatExistsInBarType();

for now you are forced to use reflection (or code gen which really amounts to much the same thing but more expensive up front and faster later).

// any of these can be determined at runtime
Type t = typeof(Bar);
string methodToCall = "FunctionThatExistsInBarType";
Type[] argumentTypes = new Type[0];
object[] arguments = new object[0];
object foo;
// invoke the method - 
// example ignores overloading and exception handling for brevity
// assumption: return type is void or you don't care about it
t.GetMethod(methodToCall, BindingFalgs.Public | BindingFlags.Instance)
    .Invoke(foo, arguments);

Answer 5

Provided you know all required types at compile-time, duck typingis (sort of) possible:

class BarFoo {}
class Foo {}
class Bar {}

class Program
{
    static void Main( )
    {
        var foo = new Foo( );
        var bar = new Bar( );
        var barfoo = new BarFoo( );

        Console.WriteLine(DoStuff(foo));
        Console.WriteLine(DoStuff(bar));
        Console.WriteLine(DoStuff(barfoo));

    }

    static string DoStuff(Foo foo) { return "DoStuff(Foo foo)"; }
    static string DoStuff(Bar bar) { return "DoStuff(Bar bar)"; }
    static string DoStuff(Base fb) { return "DoStuff(object fb)"; }
}

Output:

Dostuff(Foo foo)
Dostuff(Bar bar);
DoStuff(object fb);

If you end up implementing a lot of methods that basically do exactly the same, consider implementing an interface.