C＃可查询 <T> 采取方法并不限制EF6实体的结果

Question

I'm using linq and EntityFramework 6 to limit the number of records returned from database.

My test table, AuditLogs , has a total of 20 records.

If I write:

var auditLogsList = context.AuditLogs.Take(10); // Type is IQueryable<AuditLog>
var listValues = auditLogsList.ToList();
var count = listValues.Count; // Correct output = 10

I get listValues.Count equals to 10, which is the expected output.

But, I don't know why, if I call method Take in the next line :

var auditLogsList = context.AuditLogs; // Type is IQueryable<AuditLog>
auditLogsList.Take(10);
var listValues = auditLogsList.ToList();
var count = listValues.Count; // Wrong output = 20

I get listValues.Count equals to 20, which is the total number of records of the table and it's not what I expect.

I need to use it the second way.

Any ideas if it's a bug or am I missing something here?

Note: I'm using .NET Framework 4.6.1 and Entity Framework 6

Answer 1

This line:

auditLogsList.Take(10);

Just creates an enumerator over 10 items - but you don't assign this anywhere, so it's thrown away.

Later on you do this:

var listValues = auditLogsList.ToList();

This is working with the whole list - hence why you get 20.

If you just want to take 10 items (as a list), you can change the above line to:

var listValues = auditLogsList.Take(10).ToList();

Answer 2

Actually you are using Take wrong. As MSDN link says that Take will Returns a specified number of contiguous elements from the start of a sequence. So you need to set it to a variable.

var auditLogsList = context.AuditLogs; 
auditLogsList = auditLogsList.Take(10);
var listValues = auditLogsList.ToList();
var count = listValues.Count; // output = 10

Hope helps,