4 Real Time Use of Partial Classes and Partial Method

Introduction




I have also created a video on partial classes and shown the real time use of it.

Fundamentals of partial classes

A partial class allows a single class to be divided into two separate physical files. During compile time these files get compiled into a single class.  For instance you can see in the following figure we have the customer class divided into two different files "customer1.cs" and "customer2.cs".

During compilation these files gets compiled into a single class internally. So when you create an object of the customer class you will be able to see methods lying in both the physical files. For instance you can see the "Add" method belongs to "customer1.cs" and the "Delete" method belongs to "customer2.cs" , but when the customer object is created we can see both "Add" and "Delete" methods.


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Fundamentals of partial methods

There is one more important concept in partial classes called partial methods. Partial methods helps us to define a method definition in one of the physical files and we can implement that method in the other physical files as shown in the following figure.

In the following figure you can see we have defined the "Validate" method in "Customer1.cs" and this validate method is implemented in "Customer2.cs". Please note the partial keywords attached to both of these methods.

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Use number 1: ASP.NET auto-generated code


The biggest use of partial classes is in technologies where there is code generation. The Microsoft team themselves use partial classes in ASP.NET, LINQ and EF code generation. For instance when we look at ASP.NET there are two parts, one is the auto-generated code of a page and the other is code you write.

The custom logic is written in the ".aspx.cs" file while the auto-generated logic is in the ".aspx.designer.cs" file as shown in the following figure:

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As a developer you would like the auto-generated code to do his work i.e. generate code when you drag and drop a button the ASP.NET designer., as in:

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The following is how the code snippet of the auto-generated code looks like:

public partial class WebForm1 {

     

        /// <summary>

        /// form1 control.

        /// </summary>

        /// <remarks>

        /// Auto-generated field.

        /// To modify move field declaration from designer file to code-behind file.

        /// </remarks>

        protected global::System.Web.UI.HtmlControls.HtmlForm form1;

  

        /// <summary>

        /// Button1 control.

        /// </summary>

        /// <remarks>

        /// Auto-generated field.

        /// To modify move field declaration from designer file to code-behind file.

        /// </remarks>

        protected global::System.Web.UI.WebControls.Button Button1;

       

        /// <summary>

        /// Label1 control.

        /// </summary>

        /// <remarks>

        /// Auto-generated field.

        /// To modify move field declaration from designer file to code-behind file.

        /// </remarks>

        protected global::System.Web.UI.WebControls.Label Label1;

    }

At the same time you would also like to customize the code in some other file so that the auto-generated part is not disturbed. For the same ASP.NET provides the ".aspx.cs" file which is a partial class wherein you can put your own custom logic.



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Use number 3:- Better maintenance by compacting large classes 

The other important use of partial classes is for better maintenance of the project. If you have large classes with many methods as shown in the following figure , it's a bit of a pain to maintain those classes.

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By using partial classes you can split them into physical files as shown in the following figure, thus improving your project and making it easier to maintain.

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Use number 4:- Multiple people working on the same class

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The last and final real time I see of partial classes is when we want two developers to work in the same class simultaneously. I agree this can be a very rare use as there are better options like using a version control software like TFS or Sub version, but in case you want something quick and local this option is not bad at all.

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