object o = 123456U;
ulong l = (ulong) o; // fails

object o = 123456U;
ulong l = (ulong) (uint) o; // succeeds

void foo(object o)
{
switch (Type.GetTypeCode(o.GetType()))
   {
      case TypeCode.UInt32:
      case TypeCode.UInt64:
      ulong l = (ulong) o;
      RunUnsignedIntVersion(l);
      break;
      case TypeCode.Int32:
      case TypeCode.Int64:
      long n = (long) o;
      RunSignedVersion(n);
      break;
   }
}

foo(123456U);
foo(123456);

case TypeCode.Int32:
    RunSignedVersion((int) o);
    break;
case TypeCode.Int64:
    long n = (long) o;
    RunSignedVersion(n);
    break;

void foo(object o)
{
   switch (Type.GetTypeCode(o.GetType()))
   {
      case TypeCode.UInt32:
      case TypeCode.UInt64:
          ulong l = Convert.ToUInt64(o);
          RunUnsignedIntVersion(l);
          break;
      case TypeCode.Int32:
      case TypeCode.Int64:
          long n = Convert.ToInt64(o);
          RunSignedVersion(n);
          break;
   }
}

[CLSCompliant(false)]
    public static ulong ToUInt64(object value) {
        return value == null? 0: ((IConvertible)value).ToUInt64(null);
    }

    [CLSCompliant(false)]
    public static long ToInt64(object value) {
        return value == null? 0: ((IConvertible)value).ToInt64(null);
    }

case TypeCode.Int32:
    RunSignedVersion((int) o);
    break;
  case TypeCode.Int64:
    long n = (long) o;
    RunSignedVersion(n);
    break;

public static class ConversionExtensions
{
    public static ulong ToUInt64(this object value)
    {
        return ((IConvertible)value).ToUInt64();
    }
}

object o = 123456U;
ulong l = o.ToUInt64();