How to access Pre and Post entity images in a Custom Workflow Activity

For workflow extensions entity images are available using the key values PreBusinessEntity and PostBusinessEntity respectively for the pre and post entity images.

using System.Activities;
using Microsoft.Xrm.Sdk;
using Microsoft.Xrm.Sdk.Workflow;

public class SampleCustomActivity : CodeActivity
{
    protected override void Execute(CodeActivityContext executionContext)
    {
        // Get the context service.
        IWorkflowContext context = executionContext.GetExtension<IWorkflowContext>();

        Entity preImage = context.PreEntityImages["PreBusinessEntity"];
        Entity preImage = context.PreEntityImages["PostBusinessEntity"];
    }
}

Note: PreBusinessEntity and PostBusinessEntity only works if the Workflow is a real-time workflow.

Reference: https://docs.microsoft.com/en-us/powerapps/developer/common-data-service/workflow/workflow-extensions#entity-images-for-workflow-extensions