Entity Framework There is already an open DataReader Exception

Exception: There is already an open DataReader associated with this Command which must be closed first

When executing the following code.

foreach (var blog in blogs)
{
    blog.Name += " updated";
    blog.Url += " updated";
    context.SaveChanges();
}

It gives the following exception.

InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.

StackOverflow Related Questions

• There is already an open DataReader associated with this Command which must be closed first

Solution

This can happen if you execute a query while iterating over the results from another query.

Solution 1

• This can be easily solved by allowing MARS in your connection string.
• Add MultipleActiveResultSets=true to the provider part of your connection string.

<connectionStrings>
    <add name="BloggingContextDb" connectionString="Data Source=(localdb)\ProjectsV13;Initial Catalog=BloggingContextDb;
        MultipleActiveResultSets=true;" providerName="System.Data.SqlClient" />
</connectionStrings>

• Enabling MARS should only be done for a very small subset of problems/use-cases.
• In most cases, the error is caused by BAD CODE within the calling application.

Solution 2

Adding .ToList() to the collection using in foreach loop will likely solve the problem.

var blogs = blogs.ToList();
foreach (var blog in blogs)
{
    blog.Name += " updated";
    blog.Url += " updated";
    context.SaveChanges();
}

Try it online

Solution 3

Call SaveChanges() outside foreach loop

foreach (var blog in blogs)
{
    blog.Name += " updated";
    blog.Url += " updated";
}
context.SaveChanges();

Try it online