Convert.ChangeType doesn't handle nullables

10 Comments

• Here's my ugly generic ChangeType method (from my ORMapper):
  
  
  
  static public object ChangeType(object value, Type type) {
  
  if (value == null && type.IsGenericType) return Activator.CreateInstance(type);
  
  if (value == null) return null;
  
  if (type == value.GetType()) return value;
  
  if (type.IsEnum) {
  
  if (value is string)
  
  return Enum.Parse (type, value as string);
  
  else
  
  return Enum.ToObject(type, value);
  
  }
  
  if (!type.IsInterface && type.IsGenericType) {
  
  Type innerType = type.GetGenericArguments()[0];
  
  object innerValue = QueryHelper.ChangeType(value, innerType);
  
  return Activator.CreateInstance(type, new object[] { innerValue });
  
  }
  
  if (value is string && type == typeof(Guid)) return new Guid(value as string);
  
  if (value is string && type == typeof(Version)) return new Version(value as string);
  
  if (!(value is IConvertible)) return value;
  
  return Convert.ChangeType(value, type);
  
  }
  
  
  
  It may have more than you need, and there are bound to be even more special cases like Guid and Version, but it works so far.
  
  

• By the way, the call to QueryHelper.ChangeType is just a recursive call on the exact same method, so change the QueryHelper class name to your own class name.
  

• Thanks was looking into this issue..
  
  How about this as an option?
  
  public static class Converter {
  public static T ChangeType(object value) {
  TypeConverter tc = TypeDescriptor.GetConverter(typeof(T));
  return (T)tc.ConvertFrom(value);
  }
  
  public static void RegisterTypeConverter() where TC : TypeConverter {
  TypeDescriptor.AddAttributes(typeof(T), new TypeConverterAttribute(typeof(TC)));
  }
  }
  
  This allows you to create new TypeDescriptors and add them at runtime, for any missing converters e.g. Version
  
  public class VersionConverter : TypeConverter
  {
  public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value) {
  string strvalue = value as string;
  if (strvalue != null) {
  return new Version(strvalue);
  }
  else {
  return new Version();
  }
  }
  }
  
  Test code...
  
  static void Main(string[] args) {
  
  Converter.RegisterTypeConverter();
  
  int? i = Converter.ChangeType("123");
  DateTime dt = Converter.ChangeType("20:33");
  char c = Converter.ChangeType("x");
  Guid g = Converter.ChangeType("{32F92EEB-A703-4eb7-A9F8-62E09F87D03F}");
  Version v = Converter.ChangeType("1.2.3.4");
  DateTime? k = Converter.ChangeType(null);
  }
  
  
  
  

• Excellent solution Rich. Thanks.
  

• Thanks Rich, that rocks!
  

• Very elegant solution Rich, love it. Thanks.
  

• I slightly modified your http://aspalliance.com/852 code. I canno tput code there due to server error.
  
  I modified your code slightly, to make it generic. Thanks!
  
  public static T ChangeType(object value) {
  Type conversionType = typeof(T);
  if (conversionType.IsGenericType &&
  conversionType.GetGenericTypeDefinition().Equals(typeof(Nullable))) {
  if (value == null) { return default(T); }
  
  conversionType = Nullable.GetUnderlyingType(conversionType); ;
  }
  
  return (T)Convert.ChangeType(value, conversionType);
  }
  

• Great work. especially the generic method.
  
  Thankx.
  

• public static T ChangeType(object value)
  {
  Type conversionType = typeof(T);
  if (conversionType.IsGenericType && conversionType.GetGenericTypeDefinition().Equals(typeof(Nullable)))
  {
  if (value == null)
  {
  return default(T);
  }
  else
  { NullableConverter nullableConverter = new NullableConverter(conversionType);
  
  conversionType = nullableConverter.UnderlyingType;
  }
  }
  
  return (T)Convert.ChangeType(value, conversionType);
  }
  

• Does this work if the input value is not null but instead an empty string("")? I don't think conversion works from empty string to an underlying type like integer or double, maybe have to do string.isnullorempty(cstr(value)) and return null for that. Just a thought when dealing with user inputs...
  
  Really though, Microsoft needs to just fix this.