Do you take care when casting IQueryable to IEnumerable?

When you cast IQueryable to IEnumerable and then query the data from there, Entity Framework must collect the data at the point you do the cast. This can result in very significant extra database load, and extra processing on the client side.

NOTE: Using .AsEnumerable() achieves the same effect.

Counting

// All examples below will result in a SQL query similar to:
// SELECT * FROM Sales

// The ToList generates a list of all records client side and then counts them.
int count1 = context.Sales
    .ToList()
    .Count();

// This implicitly treats the sales as an enumerable and enumerates all the items to count them.
IEnumerable<Sale> sales = context.Sales;
int count2 = sales.Count;	

// EF Core will evaluate everything before `.AsEnumerable()` and after that line, everything is in-memory.
int count3 = context.Sales
    .AsEnumerable()
    .Count();
    
// This is the most common source of `IEnumerable` casting which can cause significant performance issues.
public IEnumerable<Sale> GetSales() => context.Sales;

// The code on the first glance looks alright but in fact it fetches the entire table from SQL Server
// because it receives the query as `IEnumerable` before running `.Count()`.
int count4 = GetSales().Count();

// All of the examples below will result in SQL query:
// SELECT COUNT(*) FROM Sales
int count1 = context.Sales.Count();

IQueryable<Sale> query = _context.Sales;
int count2 = query.Count();

public IQueryable<Sale> GetSales() => context.Sales;
int count3 = GetSales().Count();

Where

// All of the examples below will result in a SQL query like:
// SELECT * FROM Sales

List<Sale> sales1 = context.Sales
    .AsEnumerable()
    .Where(x => x.Id == 5)
    .ToList();

private IEnumerable<Sale> Sales { get { return context.Sales; } }
List<Sale> sales2 = Sales
    .Where(x => x.Id == 5)
    .ToList();

// All Examples will result in a SQL query like:
// SELECT * FROM Sales WHERE Id = 5

List<Sale> sales1 = context.Sales
    .Where(x => x.Id == 5)
    .ToList();

private IQueryable<Sale> Sales { get { return context.Sales; } }
List<Sale> sales2 = Sales
    .Where(x => x.Id == 5)
    .ToList();