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88
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what is
the difference between returning iqueryable vs ienumerable.
IQueryable<Customer>
custs = from c in
db.Customers
where c.City == "<City>" select c;
IEnumerable<Customer> custs = from c in db.Customers
where c.City == "<City>" select c;
Will
both be deferred execution? When should one be preferred over the other?
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7 Answers
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up vote 319 down vote accepted
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Yes,
both will give you deferred execution.
The
difference is that IQueryable<T> is the interface that allows
LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your
query on an IQueryable<T>, that query will be executed in the database,
if possible.
For the
IEnumerable<T> case, it will be LINQ-to-object, meaning that all
objects matching the original query will have to be loaded into memory from
the database.
In code:
IQueryable<Customer>
custs = ...;
// Later on... var goldCustomers = custs.Where(c => c.IsGold);
That
code will execute SQL to only select gold customers. The following code, on
the other hand, will execute the original query in the database, then
filtering out the non-gold customers in the memory:
IEnumerable<Customer>
custs = ...;
// Later on... var goldCustomers = custs.Where(c => c.IsGold);
This is
quite an important difference, and working on IQueryable<T> can in many
cases save you from returning too many rows from the database. Another prime
example is doing paging: If you use Take and Skip on IQueryable, you will
only get the number of rows requested; doing that on an IEnumerable<T>
will cause all of your rows to be loaded in memory.
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