Peter Johnson's Blog

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.

The PumaCode.org Blog: using Convert.ChangeType() on nullable types in C#

Thanks was looking into this issue..

How about this as an option?

   public static class Converter {

       public static T ChangeType<T>(object value) {

           TypeConverter tc = TypeDescriptor.GetConverter(typeof(T));

           return (T)tc.ConvertFrom(value);

       }

       public static void RegisterTypeConverter<T, TC>() 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<Version, VersionConverter>();

          int? i = Converter.ChangeType<int?>("123");

          DateTime dt = Converter.ChangeType<DateTime>("20:33");

          char c = Converter.ChangeType<char>("x");

          Guid g = Converter.ChangeType<Guid>("{32F92EEB-A703-4eb7-A9F8-62E09F87D03F}");

          Version v = Converter.ChangeType<Version>("1.2.3.4");

          DateTime? k = Converter.ChangeType<DateTime?>(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<T>(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<T>(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.

What’s going on in SilverlightDS? It’s been a while since I’ve made any major release changes to SilverlightDS