C# – LinqToSQL and auditing changed fields

clinqlinq-to-sqlnetor-designer

Here's another one of these LinqToSQL questions where I'm sure I must have missed the boat somewhere, because the behavior of the O/R Designer is very puzzling to me…

I have a base class for my LinqToSQL tables, which I called LinqedTable. I've successfully used reflection to get hold of all the properties of the descendant classes and do other standard stuff.

Now I want to have some automatic auditing of my tables, so that whenever a LinqedTable record is inserted or deleted, or a field value changes, I will insert a record into an audit table, detailing the change type, the field name, and its value pre- and post-save.

I thought I would be able to do it using the PropertyChanging event, keeping track of all the changed properties before a save, then clearing the collection of changes after each SubmitChanges() call. But – the generated code from the O/R designer, for some bizarre reason, doesn't give you the property name in the PropertyChanging event – it sends an empty string! (WHY?!) It does send the property name in the PropertyChanged event, but that's already too late for me to get the original value.

I thought to grab all the original values of all properties using the OnLoaded() partial method – but that is private by definition, and I need access to that method in the base class. Even if I used reflection to get hold of that method, that would mean I would have to implement the other half of the partial method for every one of my tables, which kinda defeats the purpose of having inheritance!

I also can't find any suitable method in the DataContext to use or override.

So what would you recommend to get this audit functionality working?

Best Answer

You can use GetChangeSet on the DataContext to retrieve a list of updates, inserts and deletes that have occurred on all tables within a context. You can use ITable.GetOriginalEntityState to retrieve the original values of a changed entity. However, when you retrieve the original values of a deleted or updated record, the associations will not be available so you will have to rely on foreign key values only in that area if you need to process related entities. You can Use ITable.GetModifiedMembers to help retrieve only values that have changed.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

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C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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