Sample Custom Workflow Code Dynamics 365 CE or Dataverse

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

namespace ModernSchoolServerSideCode
{
    public class getoutstandingamount : CodeActivity
    {
        [Input("Student Record")]
        [ReferenceTarget("modern_student")]
        public InArgument<EntityReference> TargetStudent { get; set; }

        protected override void Execute(CodeActivityContext context)
        {
            IWorkflowContext workflowContext = context.GetExtension<IWorkflowContext>();
            IOrganizationServiceFactory serviceFactory = context.GetExtension<IOrganizationServiceFactory>();

            // Use the context service to create an instance of IOrganizationService.             
            IOrganizationService service = serviceFactory.CreateOrganizationService(workflowContext.InitiatingUserId);

            EntityReference currentStudentRecord = TargetStudent.Get(context);

            var fetchXML = @"<fetch version='1.0' mapping='logical' no-lock='false' distinct='true'>
                                <entity name='modern_studentpayment'>
                                    <attribute name='modern_studentpaymentid'/>
                                    <attribute name='modern_name'/>
                                    <attribute name='createdon'/>
                                    <attribute name='modern_fees'/>
                                    <filter type='and'>
                                        <condition attribute='statecode' operator='eq' value='0'/>
                                        <condition attribute='modern_student' operator='eq' value='{0}'  uitype='modern_student'/>
                                    </filter>
                                </entity>
                                </fetch>";
            fetchXML = string.Format(fetchXML, currentStudentRecord.Id);

            EntityCollection ec =  service.RetrieveMultiple(new FetchExpression(fetchXML));

            decimal totaloutstandingamiunt = 0;

            for (int i = 0; i < ec.Entities.Count; i++)
            {
                totaloutstandingamiunt = totaloutstandingamiunt + ((Money)ec.Entities[i].Attributes["modern_fees"]).Value;
            }

            OutAmount.Set(context, totaloutstandingamiunt);

        }

        [Output("OutstandingAmount")]
        public OutArgument<decimal> OutAmount { get; set; }
    }
}