I know that I can use ToList() in my Entity Framework queries, in which case all rows are loaded into memory. But, otherwise, when are those rows loaded?

If I have a query that returns many rows, how can I ensure they are not all loaded into memory? (Like a data reader.)

Does someone know where this information is documented more clearly?

CodePudding user response：

Have a look at the Buffering and streaming topic.

Buffering refers to loading all your query results into memory, whereas streaming means that EF hands the application a single result each time, never containing the entire resultset in memory. In principle, the memory requirements of a streaming query are fixed - they are the same whether the query returns 1 row or 1000; a buffering query, on the other hand, requires more memory the more rows are returned. For queries that result large resultsets, this can be an important performance factor.

Whether a query buffers or streams depends on how it is evaluated.

Avoid using ToList or ToArray if you intend to use another LINQ operator on the result - this will needlessly buffer all results into memory. Use AsEnumerable instead.

You might also want to read the Tracking, no-tracking and identity resolution topic.

The below shows an example of a streaming approach.

foreach (var blog in context.Posts
    .AsNoTracking() // Or .AsNoTrackingWithIdentityResolution
    .Where(p => p.Title.StartsWith("A"))
    )
{
    // ...
}