Cannot compare elements of type 'System.Collections.Generic.ICollection`1 Only primitive types, enumeration types and entity types are supported

Question

I have written this code

IQueryable<Site> sites = context.MainTable.Include("RelatedTable");  if (!string.IsNullOrEmpty(param1)) {     sites = sites.Where(s => s.RelatedTable != null && s.RelatedTable.Any(p => p.Name == param1.ToLower() && p.PolicyType == "primary")); }  foreach (string secondaryPolicy in secondaryPolicies) {     sites = sites.Where(s => s.RelatedTable != null && s.RelatedTable.Any(p => p.Name == secondaryPolicy.ToLower() && p.PolicyType == "secondary")); }  return sites.ToList(); 

However at the ToList line I am getting the exception

Cannot compare elements of type 'System.Collections.Generic.ICollection`1[[Project1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]'. Only primitive types, enumeration types and entity types are supported.

Answer 1

You can't compare a related table to null directly. Instead, compare against your foreign key member (assuming that PrimaryTable reference RelatedTable using a member called RelatedTableId.

sites.Where(s => s.RelatedTableId != null && s.RelatedTable.Any(     p => p.Name == param1.ToLower() && p.PolicyType == "primary")); 

You may even be able to get away with removing the null check completely. Since this query is run against the database, you won't get a NullReferenceException and it may work. You'll have to double check on that though.