multi entity plugin with interface

CalculateTotalAmountEarlyBoundPlugin.cs

public void Execute(IServiceProvider serviceProvider)
{
    var context = serviceProvider.GetService(typeof(IPluginExecutionContext)) as IPluginExecutionContext;
    var serviceFactory = serviceProvider.GetService(typeof(IOrganizationServiceFactory)) as IOrganizationServiceFactory;
    var service = serviceFactory.CreateOrganizationService(context.UserId);

    var earlyBound = (context.InputParameters["Target"] as Entity).Mock<IEntityWithTotalSum>();
    earlyBound.new_totalsum = earlyBound.new_netamount + earlyBound.new_margin;          
}