Here is a demo project in C# that should get you started.  If I find the time, I’ll post details on how this was put together (but I think most developers will benefit more from running and stepping through the source code anyway).

Hope this helps someone.  Happy Programming!

[System.Data.Linq.Mapping.DatabaseAttribute(Name="MyData")]
public partial class MyDataDataContext : System.Data.Linq.DataContext
{
    // blah ... blah ...
}

public partial class MyDataDataContext
{
    // My additional methods, constructors, etc.
}

As I’ve shown above, this allows you to extend a class without modifying the original code. The additional class definitions just live somewhere else, typically in another file, and the compiler integrates them as needed. That’s great for keeping your code out of the machine generated file, but you still need to be aware of what the machine generated code is doing before you extend it! One thing to check is the default (sqlmetal generated) constructor when you are planning to overload it with your own. In some cases, the default constructor will initialize dependencies, so if you write an overload, make sure it calls the default constructor too:

public partial class MyDataDataContext
{
    public MyDataContext() : this() // We must call the designer generated constructor because it contains initialization code.
    {
        // blah ... blah ...
    }
}

Notice how I used the constructor initialization keyword : this().
From the C# Specification Section 10.10.1

Don't forget to call the default constructor!

Happy Coding!