Test.cs

public class Test
{
    [Fact]
    public async Task Single()
    {
        var db = new TestDbContext();
        await db.Orders.SingleAsync(x => x.OrderId == 1000);
    }
    
    [Fact]
    public async Task First()
    {
        var db = new TestDbContext();
        await db.Orders.FirstAsync(x => x.OrderId == 1000);
    }
}