Mocking a DBContext for Testing

The easiest means to mock a data context for testing, is to use an in-memory database.

An in-memory database is known by its string-name within a process. Each data context accessing the database instance must use its name, via options.

A few things are needed to mockup a dbcontext for testing:

• Entity class or classes

• Data Context class with DbSet for each entity class

• String name of the database

• Data Context builder options for an in-memory database

• Creating a Context Instance

1. Entity Class
   At least one entity class. This can be any simple entity class with an Id property, such as the following:

public class TestClass
{
    public int Id { get; set; }

    public string Val1 { get; set; }

    public int Val2 { get; set; }
}

2. Data Context
The data context class requires a DbSet for each test class. This can be as simple as the following:

public class TestDbContext : DbContext
{
    public TestDbContext([NotNullAttribute] DbContextOptions options) : base(options)
    // public cDBDContext_Base() : base()
    {
    }
    public TestDbContext() : base()
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        //base.OnModelCreating(modelBuilder);
    }

    public override int SaveChanges()
    {
        return base.SaveChanges();
    }

    public DbSet<TestClass> TestData { get; set; }
}

3. Database Name
This is the string-name of the database.
Any context using this name will reach the same in-memory database.

4. Data Context Builder Options
The following is a simple builder options for creating a data context to an in-memory database of a given name:

        var options = new DbContextOptionsBuilder<GenericRepositoryEF_Tests.TestDbContext>()
            .UseInMemoryDatabase(databaseName: stringdatabasename)
            .Options;

5. Creating a data context instance
The following is a method to create a data context instance and populate it with test data:

      string dbname = "testdatabase";
      
      // Create an options instance to give us a connection to the named in-memory database.
      var options = new DbContextOptionsBuilder<GenericRepositoryEF_Tests.TestDbContext>()
           .UseInMemoryDatabase(databaseName: dbname)
           .Options;

        // Insert seed data into the database using one instance of the context...
        using (var context = new GenericRepositoryEF_Tests.TestDbContext(options))
        {
            // Populate the list...
            for(int x = 0; x < 1000; x++)
            {
                GenericRepositoryEF_Tests.TestClass tc = new TestClass();
                tc.Val1 = x.ToString();
                tc.Val2 = x;

                context.TestData.Add(tc);
            }

            context.SaveChanges();
        }

6. Any subsequent context instance with the same database string name, will have access to the seeded data above.
Meaning, a second data context, created with the same options, can access the seed data, generated by the above code:

        // Create an options instance to give us a connection to the named in-memory database.
        var options1 = new DbContextOptionsBuilder<GenericRepositoryEF_Tests.TestDbContext>()
             .UseInMemoryDatabase(databaseName: "testdatabase")
             .Options;

        // Use a clean instance of the context to access data...
        using (var context = new GenericRepositoryEF_Tests.TestDbContext(options1))
        {
            // Now, do a simple query for some portion of the list...
            var query = from b in context.TestData
                        //where b.Val2 > 1000
                        select b;

            var res = query.ToList();
        }