// This can be a method on a generic class, it does not matter.
public void DoSomething<T>(T instance)
{
    if (instance == null) throw new ArgumentNullException("instance");

static void DoSomething<T>(T instance)
{
    if (EqualityComparer<T>.Default.Equals(instance, null))
        throw new ArgumentNullException("instance");

if (((object) instance) == null)
    throw new ArgumentNullException("instance");

static bool IsNullCast<T>(T instance)
{
    return ((object) instance == null);
}

private const int Iterations = 100000000;

static void Test(Action a)
{
    // Start the stopwatch.
    Stopwatch s = Stopwatch.StartNew();

    // Loop
    for (int i = 0; i < Iterations; ++i)
    {
        // Perform the action.
        a();
    }

    // Write the time.
    Console.WriteLine("Time: {0} ms", s.ElapsedMilliseconds);

    // Collect garbage to not interfere with other tests.
    GC.Collect();
}

Console.WriteLine("Value type");
Test(() => IsNullCast(1));
Console.WriteLine();

Console.WriteLine("Non-null nullable value type");
Test(() => IsNullCast((int?)1));
Console.WriteLine();

Console.WriteLine("Null nullable value type");
Test(() => IsNullCast((int?)null));
Console.WriteLine();

// The object.
var o = new object();

Console.WriteLine("Not null reference type.");
Test(() => IsNullCast(o));
Console.WriteLine();

// Set to null.
o = null;

Console.WriteLine("Not null reference type.");
Test(() => IsNullCast<object>(null));
Console.WriteLine();

Value type
Time: 1171 ms

Non-null nullable value type
Time: 18779 ms

Null nullable value type
Time: 9757 ms

Not null reference type.
Time: 812 ms

Null reference type.
Time: 849 ms

static class IsNullHelper<T>
{
    private static Predicate<T> CreatePredicate()
    {
        // If the default is not null, then
        // set to false.
        if (((object) default(T)) != null) return t => false;

        // Create the expression that checks and return.
        ParameterExpression p = Expression.Parameter(typeof (T), "t");

        // Compare to null.
        BinaryExpression equals = Expression.Equal(p,
            Expression.Constant(null, typeof(T)));

        // Create the lambda and return.
        return Expression.Lambda<Predicate<T>>(equals, p).Compile();
    }

    internal static readonly Predicate<T> IsNull = CreatePredicate();
}

Console.WriteLine("Value type");
Test(() => IsNullHelper<int>.IsNull(1));
Console.WriteLine();

Console.WriteLine("Non-null nullable value type");
Test(() => IsNullHelper<int?>.IsNull(1));
Console.WriteLine();

Console.WriteLine("Null nullable value type");
Test(() => IsNullHelper<int?>.IsNull(null));
Console.WriteLine();

// The object.
var o = new object();

Console.WriteLine("Not null reference type.");
Test(() => IsNullHelper<object>.IsNull(o));
Console.WriteLine();

Console.WriteLine("Null reference type.");
Test(() => IsNullHelper<object>.IsNull(null));
Console.WriteLine();

Value type
Time: 959 ms

Non-null nullable value type
Time: 1365 ms

Null nullable value type
Time: 788 ms

Not null reference type.
Time: 604 ms

Null reference type.
Time: 646 ms