Does “foreach” cause repeated Linq execution?

Question

I\'ve been working for the first time with the Entity Framework in .NET, and have been writing LINQ queries in order to get information from my model. I would like to progr

Answer 1

Using LINQ even without entities what you will get is that deferred execution is in effect. It is only by forcing an iteration that the actual linq expression is evaluated. In that sense each time you use the linq expression it is going to be evaluated.

Now with entities this is still the same, but there is just more functionality at work here. When the entity framework sees the expression for the first time, it looks if he has executed this query already. If not, it will go to the database and fetch the data, setup its internal memory model and return the data to you. If the entity framework sees it already fetched the data beforehand, it is not going to go to the database and use the memory model that it setup earlier to return data to you.

This can make your life easier, but it can also be a pain. For instance if you request all records from a table by using a linq expression. The entity framework will load all data from the table. If later on you evaluate the same linq expression, even if in the time being records were deleted or added, you will get the same result.

The entity framework is a complicated thing. There are of course ways to make it reexecute the query, taking into account the changes it has in its own memory model and the like.

I suggest reading "programming entity framework" of Julia Lerman. It addresses lots of issues like the one you having right now.