Partial Classes in C#

Question

Are there are good uses of Partial Classes outside the webforms/winforms generated code scenarios? Or is this feature basically to support that?

Answer 1

I use it in a data access layer. The generated classes like the mapper and queries a partial. If I need to add a mapper method for example to do a fancy load that's not generated I add it to the custom class.

At the end the programmer that uses the data layer in the business layer only sees one class with all the functionality he or she needs. And if the data source changes the generic parts can easily be generated without overwriting custom stuff.

Answer 2

I worked on a project a couple years ago where we had a typed DataSet class that had a ton of code in it: Methods in the DataTables, methods in the TableAdapters, declarations of TableAdapter instances, you name it. It was a massive central point of the project that everyone had to work on often, and there was a lot of source-control contention over the partial class code file.

So I split the code file into fix or six partial class files, grouped by function, so that we could work on smaller pieces and not have to lock the whole file every time we had to change some little thing.

(Of course, we could also have solved the problem by not using an exclusively-locking source-control system, but that's another issue.)

Answer 3

Another possible use for partial classes would be to take advantage of partial methods to make methods selectively disappear using conditional compilation - this would be great for debug-mode diagnostic code or specialized unit testing scenarios.

You can declare a partial method kind of like an abstract method, then in the other partial class, when you type the keyword "partial" you can take advantage of the Intellisense to create the implementation of that method.

If you surround one part with conditional build statements, then you can easily cut off the debug-only or testing code. In the example below, in DEBUG mode, the LogSomethingDebugOnly method is called, but in the release build, it's like the method doesn't exist at all - a good way to keep diagnostic code away from the production code without a bunch of branching or multiple conditional compilation blocks.

// Main Part
public partial class Class1
{
    private partial void LogSomethingDebugOnly();

    public void SomeMethod()
    {
        LogSomethingDebugOnly();
        // do the real work
    }
}

// Debug Part - probably in a different file
public partial class Class1
{

    #if DEBUG

    private partial void LogSomethingDebugOnly()
    {
        // Do the logging or diagnostic work
    }

    #endif
}

Answer 4

I just found a use for partial classes. I have a [DataContract] class that I use to pass data to the client. I wanted the client to be able to display the class in a specific way (text output). so I created a partial class and overrode the ToString method.

Answer 5

Correction, as Matt pointed out, both sides of the partial need to be in the same assembly. my bad.

Answer 6

EDIT: DSL Tools for Visual Studio uses partial classes.

Thus, it's a feature that many automatic generated code uses. Instead of using #region the automatic generated code goes to one file and the user code (also called custom code) goes to another and even in different directories so that the developer does not get confused with so many meaningless files.

It's good to have this choice which you can combine - but not forced to use -with inheritance

Also, it can be handy to separate the logic of some classes among several directories. Of course, for machines, it's the same, but it enhances the user readability experience.