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How many concurrent statements does C# SqlConnection support?

Let's say I am working on Windows service running 10 threads. All threads use the same SqlConnection object but different SqlCommand object and perform operations like select, insert, update and delete on either different tables or same table but different data. Will it work? Will a single SqlConnection object be able to handle 10 simultaneous statements?

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    I think that the answer is simply one. AFAIK the SqlConnection class is not thread-safe. Commented May 6, 2020 at 16:17

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How many concurrent statements does C# SqlConnection support?

You can technically have multiple "in-flight" statements, but only one acutally executing.

A single SqlConnection maps to a single Connection and Session in SQL Server. In Sql Server a Session can only have a single request active at-a-time. If you enable MultipeActiveResultsets you can start a new query before the previous one is finished, but the statements are interleaved, never run in parallel.

MARS enables the interleaved execution of multiple requests within a single connection. That is, it allows a batch to run, and within its execution, it allows other requests to execute. Note, however, that MARS is defined in terms of interleaving, not in terms of parallel execution.

And

execution can only be switched at well defined points.

https://learn.microsoft.com/en-us/sql/relational-databases/native-client/features/using-multiple-active-result-sets-mars?view=sql-server-ver15

So you can't even guarantee that another statement will run whenever one becomes blocked. So if you want to run statements in parallel, you need to use multiple SqlConnections.

Note also that a single query might use a parallel execution plan, and have multiple tasks running in parallel.

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Either multiple connection or a single one using a queue/dequeue (depending on the kind of load it's expected to get)
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David Browne gave you the answer the ask, but there might be something else you need to know:

Let's say I am working on Windows service running 10 threads. All threads use the same SqlConnection object but different SqlCommand object and perform operations like select, insert, update and delete on either different tables or same table but different data.

This design just seems wrong on several fronts:

  1. You keep a disposeable resource around and open. My rule for Disposeable stuff is: "Create. Use. Dispose. All in the same piece of code, ideally using a using block." Keeping disposeable stuff around or even sharing it between threads is jsut not worth the danger of forgetting to close it.

There is no performance advantage: SqlConnection uses internall connection pooling without any side effects. And even if there is a relevant speed advantage, they would not be worth the dangers.

  1. You are using Mutltithreading with Database Access. Multithreading is one way to implement multitasking, but not one you should use until you need it. Multithreading is only usefull with CPU bound work. Otherweise you should generally be using async/await or similar appraoches. DB Operations are either disk or network bound.

There is one exception to this rule, and that is if your application is a Server. Servers are teh rare example of something being pleasingly parallel. So having a large Threadpool to process incomming requests in paralell is very common. It is rather rare that you write one of those, however. Mostly you just run your code in a existing server infrastructure that deals with that.

  1. If you do have heavy CPU work, chances are you are retreiving to much. It is a very common beginners mistake to retreive a lot, then do filtering in C# code. Do not do that. Do as much filtering and processing as possible in the Query. You will not be able to beat the speed of the DB-Server, and at best you tie up your network pointlessly.
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