Conversion.cs

using System;
using System.ComponentModel;

public static class Conversion {
	public static T TryAs<T>(this string x, T otherwise = default(T))
	{
		var converter = TypeDescriptor.GetConverter(typeof(T));
		if (converter != null && x != null && converter.CanConvertFrom(x.GetType()))
		{
			try { return (T)converter.ConvertFrom(x); }
			catch{}
		}
		return otherwise;
	}
}

Program.cs

using System;
using Conversion;

public class Program
{
	public enum FooBar { Unknown, Foo, Bar }
	public static void Main()
	{
		Console.WriteLine("42".TryAs<int>() == 42);
		Console.WriteLine("42x".TryAs<int>() == default(int));
		Console.WriteLine("42x".TryAs<int>(16) == 16);
		Console.WriteLine("42x".TryAs<int?>() == null);
		Console.WriteLine("42x".TryAs<int?>(16) == 16);
		Console.WriteLine("foo".TryAs<FooBar>  () == FooBar.Foo);
		Console.WriteLine("xfoo".TryAs<FooBar> () == default(FooBar));
		Console.WriteLine("xfoo".TryAs<FooBar?>() == null);

		string nullThing = null;
		Console.WriteLine(nullThing.TryAs<int?>() == null);
		Console.WriteLine(nullThing.TryAs<int>(42) == 42);
	}
}