Proxy Design Pattern

In this article we are going to discuss the Proxy design pattern. It is one among the 23 design patterns by Gof. As usual we can start with the Challenge and Solution style.

Challenge

You are working on automating Excel COM objects. The business logic has to think too much about instantiating COM, doing ground works before calling the actual functionalities etc.

The same code could get replaced by DCOM tomorrow. So the Excel access code is spread throughout your application. How to do a better design?

Definition

"Provide a surrogate or placeholder for another object to control access to it."

Implementation

We can solve the above problem by using a Proxy pattern. As the definition says, we will have to create a placeholder or wrapper around the original object to control access to it. In this way we can make the following advantages:

• Talk to COM or DCOM Excel object by changing configuration in one place
• Give application a simple interface to talk with Excel COM/DCOM
• Feel application think like it is talking to a local object

Application

Following is the application with data:

Old Code

Following is the old code where application is forced to think too much about the Excel COM object and method of assigning values to the cells.

//Start Excel and get Application object.
Microsoft.Office.Interop.Excel.Application oXL = new Microsoft.Office.Interop.Excel.Application();
oXL.Visible = true;

//Get a new workbook.
Microsoft.Office.Interop.Excel._Workbook oWB = (Microsoft.Office.Interop.Excel._Workbook)(oXL.Workbooks.Add(Missing.Value));
Microsoft.Office.Interop.Excel._Worksheet oSheet = (Microsoft.Office.Interop.Excel._Worksheet)oWB.ActiveSheet;

//Add table headers going cell by cell.
oSheet.Cells[1, 1] = "Name";
oSheet.Cells[1, 2] = "Address";
oSheet.Cells[1, 3] = "Salary";

//Format A1:D1 as bold, vertical alignment = center.
oSheet.get_Range("A1", "C1").Font.Bold = true;
oSheet.get_Range("A1", "C1").VerticalAlignment = Microsoft.Office.Interop.Excel.XlVAlign.xlVAlignCenter;

int i = 2;
foreach (Employee employee in _list)
{
    string[] values = new string[3];
    values[0] = employee.Name;
    values[1] = employee.Address;
    values[2] = employee.Salary.ToString();

    oSheet.get_Range("A" + i.ToString(), "C" + i.ToString()).Value = values;
    i++;
}

oXL.Visible = true;
oXL.UserControl = true;

The problem with above code is too much Excel logic is mixed with the application logic. Now we can see the new code with proxy pattern implemented.

New Code with Proxy Pattern

ExcelProxy proxy = new ExcelProxy();
proxy.Save(_list);

The code is only 2 lines and the Excel COM object creation, cell value assigning etc are taken care by the ExcelProxy class.

Application Execution

On executing the application we can see the results inside Microsoft Excel.

Other Examples of Proxy Pattern

We can have many real world examples which implement the Proxy pattern. When we add a WCF reference a Proxy is created. This class takes care of the connection details, serialization etc.

Summary

In this article we have seen the usage of Proxy design pattern along with an example. The source code attached contains the example we have discussed.