Multi-Context InMemory Database

The same name is enough. If your instances of DbContext do not 'see' the same in memory DB, it seems they use ones with different names. Make sure your DbContext is created once for the same name.

EF Core 2.0 even re-uses in memory databases with the same name:

In-memory databases must be named

The global unnamed in-memory database has been removed and instead all in-memory databases must be named. For example:

optionsBuilder.UseInMemoryDatabase("MyDatabase"); 

This creates/uses a database with the name “MyDatabase”. If UseInMemoryDatabase is called again with the same name, then the same in-memory database will be used, allowing it to be shared by multiple context instances.

This is possible nowadays, but indeed passing just the name is not enough if you use different context types. I'm using .net core 2.2 and had the exact same issue. My code now is now like this:

I create a InMemoryDatabaseRoot object like this in class level

//using Microsoft.EntityFrameworkCore.Storage;
private static readonly InMemoryDatabaseRoot InMemoryDatabaseRoot = new InMemoryDatabaseRoot();

When I add the db contextes I pass the root instance

services.AddDbContext<MyContext>(options =>
{
    options.UseInMemoryDatabase("MyContext", InMemoryDatabaseRoot);
    options.UseInternalServiceProvider(serviceProvider);
 });

 services.AddDbContext<MySecondContext>(options =>
 {
    options.UseInMemoryDatabase("MyContext", InMemoryDatabaseRoot);
    options.UseInternalServiceProvider(serviceProvider);
  });

I found it in a discussion here: https://github.com/aspnet/EntityFrameworkCore/issues/9613#issuecomment-430722420

Beside the same database name, the model must also be the same. This means that in case of an own implementation of IModelCacheKeyFactory, it's Create-Method must return "equal" objects.