Commented Issue: System.Web.Http.OData: HTTP PATCH with Delta should allow more than just basic objects for properties. [562]

This post focuses on the functionality of the TrySetPropertyValue method of the Delta class.

When doing an http PATCH with the (very cool) Delta class, one can only update properties that are basic types or classes. Inherited classes, Nullables and Generics aren't supported.

As far as I can tell, this is due to the type checking that happens in the TrySetPropertyValue(_name, _value) method of the Delta class. If the serialized _value object (in my case from Json) doesn't match exactly the type of the _name property, the method will exit and not set the property.

This is why it doesn't work with e.g. ICollection<T> or any derived classes.

Another type-related issue (this is maybe a bug and not a feature request) is that the Json parser converts integers to 64 bit ones, which won't match the 32 bit int properties declared on the entity and thus get ignored by this method.
Also, Nullable types return false for .IsClass and thus can't be set to null according to the logic in the method.

I would suggest changing the method as such to fix these issues (might require further changes in the deserializer to support collection properties):

public bool TrySetPropertyValue(string name, object value)
{
if (name == null)
{
throw Error.ArgumentNull("name");
}

if (!_propertiesThatExist.ContainsKey(name))
{
return false;
}

PropertyAccessor<TEntityType> cacheHit = _propertiesThatExist[name];
Type valueType = value != null ? value.GetType() : null;
// Check if the types differ and doesn't inherit
if (cacheHit.Property.PropertyType != valueType
&& !cacheHit.Property.PropertyType.IsAssignableFrom(valueType))
{
if (HasConversionOperator(valueType, cacheHit.Property.PropertyType))
{
// try to cast to the correct type
try
{
var sConvert = TypeDescriptor.GetConverter(valueType);
string temp = sConvert.ConvertToString(value);
var tConvert = TypeDescriptor.GetConverter(cacheHit.Property.PropertyType);
value = tConvert.ConvertFromString(temp);
}
catch
{
return false;
}
}
else if (!((cacheHit.Property.PropertyType.IsClass || Nullable.GetUnderlyingType(cacheHit.Property.PropertyType) != null)
&& value == null))
{
// If you cannot cast and we're not setting it null, don't set the property
return false;
}
}

//.Setter.Invoke(_entity, new object[] { value });
cacheHit.SetValue(_entity, value);
_changedProperties.Add(name);
return true;
}


... where HasConversionOperator is a function that checks if an object can be casted to another e.g. :
public static bool HasConversionOperator(Type from, Type to)
{
if (from == null)
return false;
Func<Expression, UnaryExpression> bodyFunction = body => Expression.Convert(body, to);
ParameterExpression inp = Expression.Parameter(from, "inp");
try
{
// If this succeeds then we can cast 'from' type to 'to' type using implicit coercion
Expression.Lambda(bodyFunction(inp), inp).Compile();
return true;
}
catch (InvalidOperationException)
{
return false;
}
}

I converted to string and then to the target type, because some conversions aren't supported by TypeConverter e.g. : Int64 to Nullable<int>

Comments: Fixed with http://aspnetwebstack.codeplex.com/SourceControl/changeset/4a1dd862fbce. Also, note that Delta<T> works with only settable properties which might be an issue for collection properties as they are in general get-only. I have opened issue 670 to track it.