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What is the difference between IQueryable and IEnumerable in C#?
Think about three separate questions: where the data comes from, how the query is represented, and when it runs. Mixing those together leads to the mistaken idea that every IQueryable<T> query is SQL or that every IEnumerable<T> query runs immediately.
| Aspect | IEnumerable<T> / LINQ to Objects |
Provider-backed IQueryable<T> |
|---|---|---|
| Query representation | Operators generally use delegates to process values locally. | Operators build expression trees for a query provider to interpret. Microsoft’s standard query operators overview describes the distinction. |
| Where work happens | In the application, over the sequence being processed. | Provider-dependent. EF Core can send supported operations to a relational database; other providers may translate or execute differently. See IQueryable<T> and Expression Trees. |
| What can run | Ordinary C# delegate code can run locally. | Only expressions the provider can interpret and translate can run remotely. Translation depends on the provider and its supported operations. |
| When work starts | Deferred sequence operators run when enumerated; scalar operators and materializers run immediately. | Building the query usually does not send it to the database. Consuming results triggers execution. |
| Memory and transfer | Local processing uses values available to the application. | Server-side filtering can reduce returned data; client-side work may require fetching more. ToList buffers results, while AsEnumerable does not create a list. |
The C# compiler also chooses operators based on the source’s static type and available overloads. The same LINQ-looking syntax can therefore represent local delegate calls or provider expression trees.
How does an EF Core query cross the line from C# to SQL?
Suppose context.Blogs is an EF Core database-backed source:
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var query = context.Blogs
.Where(b => b.Rating > minimum)
.Select(b => new { b.BlogId, b.Url });
var blogs = await query.ToListAsync();The calls before ToListAsync build a query representation. When the results are consumed, EF Core attempts to translate supported parts—such as the rating filter and selected columns—into a database query. It then materializes the returned rows as a list. The precise translation depends on the EF Core provider and version.
Microsoft’s EF Core documentation puts the general approach this way: “As a general rule, Entity Framework Core attempts to evaluate a query on the server as much as possible.” The important qualification is that not every C# expression can be translated.
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What if a method cannot be translated?
In EF Core 3.0 and later, client evaluation is allowed in the top-level projection, such as the final Select. EF Core can retrieve the data needed for that projection and run the unsupported projection work in the application. An untranslatable expression in other parts of the query, such as a Where filter, normally causes a runtime exception rather than silently downloading rows to evaluate the filter locally. This boundary is described in Microsoft’s Client vs. Server Evaluation guidance, updated 2025-09-06. Earlier EF Core versions allowed broader client evaluation and could issue warnings instead.
If local processing is intentional, an explicit AsEnumerable or materialization call can switch subsequent operators to client-side work. Be deliberate: a local filter may require retrieving more rows than a database filter would, and ToList holds the resulting collection in memory.
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Does AsQueryable make a query run in SQL?
No. Calling AsQueryable() on an in-memory sequence does not attach a database connection or create a database provider. When the source does not already implement IQueryable<T>, the method wraps it so that execution uses the corresponding Enumerable operators. For example:
List<Blog> blogs = GetBlogs();
var query = blogs.AsQueryable()
.Where(b => b.Rating > minimum);That filter still runs over the list in the application. AsQueryable can preserve a provider-backed source when one is already present, but it cannot turn ordinary in-memory data into a remote query. See Microsoft’s Queryable.AsQueryable Method.
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When does an EF Core query execute?
Creating a query and executing it are usually separate events. EF Core builds its query representation as operators are composed; it sends a database query when the result is consumed. Common triggers include enumeration, ToList, ToArray, First, Single, and Count, along with their applicable async counterparts. Microsoft’s How Queries Work page notes that some details in its pipeline description are out of date, so the useful point here is the broad execution trigger: consuming results.
Does IEnumerable also use deferred execution?
Yes. Deferred execution is not unique to IQueryable<T>. With LINQ to Objects, operators such as Where and Select commonly describe work that happens when the sequence is enumerated. Immediate scalar operators such as Count, Max, Average, and First execute when called. ToList and ToArray also execute the sequence and cache its results. These behaviors are covered in Microsoft’s Introduction to LINQ Queries.
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A deferred query may run again if enumerated again. If the underlying source changes between enumerations, the results can change too. By contrast, a materialized list or array represents the values captured when it was created.
Which should you use?
- Use a provider-backed
IQueryable<T>while composing database filters, ordering, and projections that your provider can translate. Let the database do supported work when that reduces data transferred or avoids unnecessary local processing. - Use
IEnumerable<T>for local sequence processing, or after deliberately switching from a provider query to client-side operators. - Materialize deliberately.
ToListandToArrayexecute and buffer results;AsEnumerablechanges the operators that follow without making a buffered copy.
Neither interface is inherently faster. Actual performance depends on the amount of data returned, what the provider can translate, database indexes, round trips, and any client-side work after retrieval. LINQ syntax is not a promise that every expression becomes SQL; the provider’s translation capabilities set that boundary. Microsoft’s broader LINQ overview introduces LINQ’s different sources and execution contexts.
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