C# Boxing Question

Michael Williams

I have a question regarding boxing in c#. Currently, I have the need for a map like the following: Dictionary Where field value looks something like this (i'm leaving out obvious code): struct FieldValue { public object DataValue {...} public UnitType Units{...} } It has come to my attention that the majority of values I will be storing in this map will be value types, and I will be performing quite a few adds per execution of this program, so I would like to avoid some of the boxing penalty of storing value types in the DataValue parameter of the above object. Initially I thought, since I know all values will be either a number, a string, or a list of either type, that I would simply create 4 different fields, one for each type and store an enumeration that indicated which field was being used. I don't really like this solution as it wastes a significant amount of memory if I'm storing a double with every object instance, even if I'm not using it. Another idea I had was to do something like the following (all value types will be stored in doubles): class DoubleReference { public double DoubleValue{...} } That way the DoubleReference object could be instantiated beforehand and I think, if I understand correctly how things work, that I would be able to avoid at least an unboxing operation by storing DoubleReference rather than a regular double in the map of generic object types. My question is, does doing this make any sense? Will it gain me anything at all, or am I wasting my time here? Thanks in advance for any help. Mike

3 Replies

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Alan19y ago
My feeling, Mike, is that the possible cures may be worse than the disease ;)

You're going to need extra code to sort out the type of the data and extra memory to accomodate the value type / reference type dichotomy, as opposed to just assigning values to the object field willy-nilly and accepting the performance penalty of boxing if the data is a value type.

I'd be inclined to run with it as it stands and see whether performance is acceptable. Only if it isn't, would I consider your alternative solutions.
Michael Williams19y ago
Actually (i'm sorry for the poor formatting of the last post) I'm already using a Dictionary: Dictionary < string, FieldValue > with FieldValue is similar to the following: struct FieldValue { public object DataValue{...} public UnitType Units{...} } So the boxing on insert isn't a real problem, it's the boxing / unboxing on setting the object member of FieldValue. I would use generics for everything but because of the nature of what I'm doing, the Dictionary needs to hold FieldValue objects that correspond to different data types. Mike
Alan19y ago
If you're using .NET 2.0 or 3.0, then you could use the generic Dictionary class which is strongly typed and doesn't involve any boxing of structs at all:

http://msdn2.microsoft.com/en-us/library/xfhwa508.aspx