Question

I would like to implement conversion between two library classes by Convert.ChangeType in C#. I can change neither of the two types. For example converting between Guid and byte[].

Guid g = new Guid(); object o1 = g; byte[] b = (byte[]) Convert.ChangeType(o1, typeof(byte[])); // throws exception 

I am aware that Guid provides a ToByteArray() method, but I would like to have that called when Guid is converted to byte[]. The reason behind this is that the conversion also takes place in library code (AseDataAdapter) which I can not modify. So is it possible to define a conversion rule between two types without modifying the sourcecode of either of the two classes?

I was experimenting with TypeConverter, but doesn’t seem to work either:

Guid g = new Guid(); TypeConverter tc = TypeDescriptor.GetConverter(typeof(Guid)); byte[] b2 = (byte[])tc.ConvertTo(g, typeof(byte[])); // throws exception 

The variable tc gets set to System.ComponentModel.GuidConverter which doesn’t support conversions to byte[]. Can I have two TypeConverters for the same class? Even if I could, wouldn’t I need to prepend an attribute to the source code of the class to assign a TypeConverter?

Thanks

Answer 1

You can change the registered TypeConverter for something using TypeDescriptor.AddAttributes; this isn’t quite the same as Convert.ChangeType, but it may suffice:

using System; using System.ComponentModel; static class Program {     static void Main()     {         TypeDescriptor.AddAttributes(typeof(Guid), new TypeConverterAttribute(             typeof(MyGuidConverter)));          Guid guid = Guid.NewGuid();         TypeConverter conv = TypeDescriptor.GetConverter(guid);         byte[] data = (byte[])conv.ConvertTo(guid, typeof(byte[]));         Guid newGuid = (Guid)conv.ConvertFrom(data);     } }  class MyGuidConverter : GuidConverter {     public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)     {         return sourceType == typeof(byte[]) || base.CanConvertFrom(context, sourceType);     }     public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)     {         return destinationType == typeof(byte[]) || base.CanConvertTo(context, destinationType);     }     public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)     {         if (value != null && value is byte[])         {             return new Guid((byte[])value);         }         return base.ConvertFrom(context, culture, value);     }     public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)     {         if (destinationType == typeof(byte[]))         {             return ((Guid)value).ToByteArray();         }         return base.ConvertTo(context, culture, value, destinationType);     } }