In many cases you can call the same method either on an Entity Framework entity set or as an extension method on an in-memory collection. The results are normally the same but in some cases may be different.
For example, the .NET Framework implementation of the Contains method returns all rows when you pass an empty string to it, but the Entity Framework provider for SQL Server Compact 4.0 returns zero rows for empty strings. Therefore the code in the example (putting the Where statement inside an if statement) makes sure that you get the same results for all versions of SQL Server. Also, the .NET Framework implementation of the Contains method performs a case-sensitive comparison by default, but Entity Framework SQL Server providers perform case-insensitive comparisons by default. Therefore, calling the ToUpper method to make the test explicitly case-insensitive ensures that results do not change when you change the code later to use a repository, which will return an IEnumerable collection instead of an IQueryable object. (When you call the Contains method on an IEnumerable collection, you get the .NET Framework implementation; when you call it on an IQueryable object, you get the database provider implementation.)
Null handling may also be different for different database providers or when you use an IQueryable object compared to when you use an IEnumerable collection. For example, in some scenarios a Where condition such as table.Column != 0 may not return columns that have null as the value. By default, EF generates additional SQL operators to make equality between null values work in the database like it works in memory, but you can set the UseDatabaseNullSemantics flag in EF6 or call the UseRelationalNulls method in EF Core to configure this behavior.
In many cases you can call the same method either on an Entity Framework entity set or as an extension method on an in-memory collection. The results are normally the same but in some cases may be different.
For example, the .NET Framework implementation of the Contains method returns all rows when you pass an empty string to it, but the Entity Framework provider for SQL Server Compact 4.0 returns zero rows for empty strings. Therefore the code in the example (putting the Where statement inside an if statement) makes sure that you get the same results for all versions of SQL Server. Also, the .NET Framework implementation of the Contains method performs a case-sensitive comparison by default, but Entity Framework SQL Server providers perform case-insensitive comparisons by default. Therefore, calling the ToUpper method to make the test explicitly case-insensitive ensures that results do not change when you change the code later to use a repository, which will return an IEnumerable collection instead of an IQueryable object. (When you call the Contains method on an IEnumerable collection, you get the .NET Framework implementation; when you call it on an IQueryable object, you get the database provider implementation.)
Null handling may also be different for different database providers or when you use an IQueryable object compared to when you use an IEnumerable collection. For example, in some scenarios a Where condition such as table.Column != 0 may not return columns that have null as the value. By default, EF generates additional SQL operators to make equality between null values work in the database like it works in memory, but you can set the UseDatabaseNullSemantics flag in EF6 or call the UseRelationalNulls method in EF Core to configure this behavior.