What does it mean: plug-in's Execute method should be written to be stateless?

Question

In the "Write a plug-in" MSDN Documentation it says: The plug-in's Execute method should be written to be stateless because the constructor is not called for every invocation of the plug-in. Also, multiple system threads could execute the plug-in at the same time.

I am wondering what does it mean exactly to have the Execute method stateless?

Answer 1

A stateless method is a method that does not affect, nor depend on, the global state, or the state of its' defining object, when executed.

In your case, it must:

• not depend on the state of the plugin object when executed
• not change the state of the plugin while executing

Here's an example where a method is not stateless;

class StatefulSum
{
     private int a;
     private int b;

     public void SetA(int value) {
         a = value;
     }

     public void SetB(int value) {
         b = value;
     }

     public int ComputeSum() {             
         return a + b;
     }
}

And this is a more subtle example of a method that is not stateless:

class SubtleStatefulSum
{      
     private int partialSum;

     // Looks like it's stateless but it's not and in a 
     //  concurrent environment this method is a recipe for disaster
     public int ComputeSum(int a, int b)
     {           
         partialSum = 0;
         partialSum = partialSum + a;
         partialSum = partialSum + b;
         return partialSum;
     }
}

This is a basic example of a method that is stateless.

class BasicStateless
{      
     public int ComputeSum(int a, int b)
     {             
         return a + b;
     }
}

Of course, the parameters of the computation could be obtained at run time using a more complex mechanism such as the case of the Dynamics CRM plugin, via the IServiceProvider parameter.

You could conceivably have a stateless method like this:

class Stateless2
{
     public int ComputeSum(IServiceProvider provider)
     {
         var numService = (INumberService)provider.GetService(typeof(INumbersService));

         int a = numService.GetNumberA();
         int b = numService.GetNumberB();

         return a + b;
     }
}

Where the IServiceProvider instance knows how to retrieve an object that implements an INumberService interface which in turn know how to retrieve the numbers A and B. This is a combination of Dependency Injection and Inversion of Control.