Using QueryExpression get the Link Entity details in D365

 public static void Main()

{

    EntityCollection salesOrderCollection = this.GetAccounts(customerId, salesOrderCreationDate);

}

public EntityCollection GetAccounts(string customerId, DateTime AccountBaseCreationDate)

{

    QueryExpression ContactItemQuery = new QueryExpression(AccountBase.LogicalName);

    // Add Condition to Customer.

    ContactItemQuery.Criteria.AddCondition(AccountBase.Id, ConditionOperator.Equal, customerId);

    FilterExpression mainFilter = new FilterExpression(LogicalOperator.And);

    mainFilter.AddCondition(AccountBase.CreatedOn, ConditionOperator.GreaterThan, AccountBaseCreationDate);

    FilterExpression statusCodeFilter = new FilterExpression(LogicalOperator.Or);

    statusCodeFilter.AddCondition(AccountBase.Status, ConditionOperator.Equal, (int)AccountBase.AccountStatus.Submitted);

    statusCodeFilter.AddCondition(AccountBase.Status, ConditionOperator.Equal, (int)AccountBase.AccountStatus.Fulfilled);

    statusCodeFilter.AddCondition(AccountBase.Status, ConditionOperator.Equal, (int)AccountBase.AccountStatus.Invoiced);

    mainFilter.AddFilter(statusCodeFilter);

    ContactItemQuery.Criteria.AddFilter(mainFilter)

            // Add Link to Price List Entity.

            LinkEntity Contact = new LinkEntity(

                AccountBase.LogicalName,

                ContactBase.LogicalName,

               AccountBase.Contact,

                ContactBase.Id,

                JoinOperator.Inner);

    // Add Link to ContactItems Entity.

    LinkEntity ContactItems = new LinkEntity(

       ContactBase.LogicalName,

       ContactItemsBase.LogicalName,

       ContactBase.Id,

       ContactItemsBase.Contact,

       JoinOperator.Inner);

    ContactItems.EntityAlias = "pi";

    ContactItems.Columns.AddColumns(ContactItemsBase.Product, ContactItemsBase.Unit);

    Contact.LinkEntities.Add(ContactItems);

    // Add Link to Unit Entity.

    LinkEntity unit = new LinkEntity(

       ContactItemsBase.LogicalName,

       UnitBase.LogicalName,

       ContactItemsBase.Unit,

       UnitBase.Id,

       JoinOperator.Inner);

    unit.LinkCriteria.AddCondition(UnitBase.UnitType, ConditionOperator.Equal, (int)UnitBase.UnitTypeValue.Case);

    ContactItems.LinkEntities.Add(unit);

    ContactItemQuery.LinkEntities.Add(Contact);

    // Execute Query.

    return this.organizationService.RetrieveMultiple(ContactItemQuery);

}