C#•3y ago

✅ EF Core long running query causing concurrency issues

Hello. I have a .NET application that uses a DbContext with PostgreSQL. To test the performance of my app, I added a million rows to one of the tables.

I noticed that it takes a pretty long time to fetch all of these rows, and if I run the method to get all the rows again before the previous query finishes, it throws the following exception:

System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext.

System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext.

System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext.

System.InvalidOperationException: A second operation was started on this context instance before a previous operation completed. This is usually caused by different threads concurrently using the same instance of DbContext.

This is an example of how it is set up currently:

private MyDbContext _dbContext;

public DbService(MyDbContext dbContext)
{
  _dbContext = dbContext ?? throw new ArgumentNullException(nameof(dbContext));
}

// This method throws the excpetion if called again before the previous query is finished
public async Task<List<MyObj>> GetAllObjects()
{
  return await _dbContext.MyTable.AsNoTracking().ToListAsync();
}

private MyDbContext _dbContext;

public DbService(MyDbContext dbContext)
{
  _dbContext = dbContext ?? throw new ArgumentNullException(nameof(dbContext));
}

// This method throws the excpetion if called again before the previous query is finished
public async Task<List<MyObj>> GetAllObjects()
{
  return await _dbContext.MyTable.AsNoTracking().ToListAsync();
}

private MyDbContext _dbContext;

public DbService(MyDbContext dbContext)
{
  _dbContext = dbContext ?? throw new ArgumentNullException(nameof(dbContext));
}

// This method throws the excpetion if called again before the previous query is finished
public async Task<List<MyObj>> GetAllObjects()
{
  return await _dbContext.MyTable.AsNoTracking().ToListAsync();
}

private MyDbContext _dbContext;

public DbService(MyDbContext dbContext)
{
  _dbContext = dbContext ?? throw new ArgumentNullException(nameof(dbContext));
}

// This method throws the excpetion if called again before the previous query is finished
public async Task<List<MyObj>> GetAllObjects()
{
  return await _dbContext.MyTable.AsNoTracking().ToListAsync();
}

Saber•10/18/23, 11:18 PM

As the error says, multiple operations cannot be preformed on the same context at the same time. You need separate instances to be able to have multiple queries running at the same time

SpaxterOP•10/18/23, 11:19 PM

Right, but creating a new instance of the DbContext for every method call would be tough though wouldn't it?

Jimmacle•10/18/23, 11:21 PM

no, you can just inject IDbContextFactory<MyDbContext>IDbContextFactory<MyDbContext> and use that

Jimmacle•10/18/23, 11:21 PM

assuming you're using MSDI

SpaxterOP•10/18/23, 11:22 PM

Oh that's a thing? Wow, how did I not know that

SpaxterOP•10/18/23, 11:22 PM

Awesome, thanks!

Jimmacle•10/18/23, 11:22 PM

dbcontexts are supposed to be short lived, i almost always new up a fresh one for each operation or tightly related set of operations

SpaxterOP•10/18/23, 11:22 PM

Sounds about right, I was about to write my own version of a DbContextFactory, guess I'm glad I asked here first

✅ EF Core long running query causing concurrency issues

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