✅ Nested from clause in query syntax

Faker · 2025-03-12T20:54:37.810Z

```C# var largeCitiesList = ( from country in countries from city in country.Cities where city.Population > 10000 select city ).ToList(); // or split the expression IEnumerable largeCitiesQuery = from country in countries from city in country.Cities where city.Population > 10000 select city; var largeCitiesList2 = largeCitiesQuery.ToList(); ``` Hello guys, can someone explain the use of the nested `from` clause in the code above please. Why do we use nested from? Couldn't we just use the dot notation, like country.Cities? Why would it matter here?

A

ACiDCA7•3/12/25, 9:01 PM

you have a list of countries... every country has a list of cities.. you want every city that has a population > 10000

what you have written or copied is called a linq query. its specifically for querying datastructures (and db cough)

J

jcotton42•3/12/25, 9:01 PM

It's "flattening" the cities into one sequence.

J

jcotton42•3/12/25, 9:01 PM

So you end up with one list of cities, instead of a list of lists of cities.

J

jcotton42•3/12/25, 9:03 PM

In method syntax this would be

var largeCities = countries
  .SelectMany(country => country.Cities)
  .Where(city => city.population > 10000)
  .ToList();

var largeCities = countries
  .SelectMany(country => country.Cities)
  .Where(city => city.population > 10000)
  .ToList();

var largeCities = countries
  .SelectMany(country => country.Cities)
  .Where(city => city.population > 10000)
  .ToList();

var largeCities = countries
  .SelectMany(country => country.Cities)
  .Where(city => city.population > 10000)
  .ToList();

A

Angius•3/12/25, 9:04 PM

Equivalent in regular LINQ would be

var largeCitiesList = countries
    .SelectMany(c => c.Cities)
    .Where(c => c.Population > 10_000)
    .ToList();

var largeCitiesList = countries
    .SelectMany(c => c.Cities)
    .Where(c => c.Population > 10_000)
    .ToList();

var largeCitiesList = countries
    .SelectMany(c => c.Cities)
    .Where(c => c.Population > 10_000)
    .ToList();

var largeCitiesList = countries
    .SelectMany(c => c.Cities)
    .Where(c => c.Population > 10_000)
    .ToList();

A

Angius•3/12/25, 9:04 PM

Ninja'd

F

FakerOP•3/12/25, 9:15 PM

yep I see

F

FakerOP•3/12/25, 9:18 PM

hmm I was trying to know how the structure would be with a single from clause, say we want to select only countries, so we have something like

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

Does the list become something this:

List<Country>
[
    { 
        Name = "USA",
        Cities = 
        [
            { Name = "New York", Population = 8419600 },
            { Name = "Los Angeles", Population = 3980400 },
            { Name = "Smalltown", Population = 8000 }
        ]
    },
    { 
        Name = "Canada",
        Cities = 
        [
            { Name = "Toronto", Population = 2930000 },
            { Name = "Vancouver", Population = 631000 }
        ]
    }
]

List<Country>
[
    { 
        Name = "USA",
        Cities = 
        [
            { Name = "New York", Population = 8419600 },
            { Name = "Los Angeles", Population = 3980400 },
            { Name = "Smalltown", Population = 8000 }
        ]
    },
    { 
        Name = "Canada",
        Cities = 
        [
            { Name = "Toronto", Population = 2930000 },
            { Name = "Vancouver", Population = 631000 }
        ]
    }
]

List<Country>
[
    { 
        Name = "USA",
        Cities = 
        [
            { Name = "New York", Population = 8419600 },
            { Name = "Los Angeles", Population = 3980400 },
            { Name = "Smalltown", Population = 8000 }
        ]
    },
    { 
        Name = "Canada",
        Cities = 
        [
            { Name = "Toronto", Population = 2930000 },
            { Name = "Vancouver", Population = 631000 }
        ]
    }
]

List<Country>
[
    { 
        Name = "USA",
        Cities = 
        [
            { Name = "New York", Population = 8419600 },
            { Name = "Los Angeles", Population = 3980400 },
            { Name = "Smalltown", Population = 8000 }
        ]
    },
    { 
        Name = "Canada",
        Cities = 
        [
            { Name = "Toronto", Population = 2930000 },
            { Name = "Vancouver", Population = 631000 }
        ]
    }
]

I'm a bit confused because of the curly braces... I know we use them for object initializer but here we want to represent them in the list, is it actually how it appears internally ?

J

jcotton42•3/12/25, 9:27 PM

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

var query = (from country in countries
select country).ToList();

is the same as

var query = countries.ToList();

var query = countries.ToList();

var query = countries.ToList();

var query = countries.ToList();

FFaker hmm I was trying to know how the structure would be with a single from clause, s...

J

jcotton42•3/12/25, 9:27 PM

Also, where are you getting that from?

Jjcotton42 ```cs var query = (from country in countries select country).ToList(); ``` is th...

F

FakerOP•3/13/25, 8:35 AM

oh ok

Jjcotton42 Also, where are you getting that from?

F

FakerOP•3/13/25, 8:35 AM

I asked chatGPT to show me how it would appear but I guess it was wrong :c

F

FakerOP•3/13/25, 9:00 AM

but hmm I have a question, consider this from Microsoft Docs:

C#
City[] cities = [
    new City("Tokyo", 37_833_000),
    new City("Delhi", 30_290_000),
    new City("Shanghai", 27_110_000),
    new City("São Paulo", 22_043_000)
];

//Query syntax
IEnumerable<City> queryMajorCities =
    from city in cities
    where city.Population > 30_000_000
    select city;

// Execute the query to produce the results
foreach (City city in queryMajorCities)
{
    Console.WriteLine(city);
}

// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

// Method-based syntax
IEnumerable<City> queryMajorCities2 = cities.Where(c => c.Population > 30_000_000);
// Execute the query to produce the results
foreach (City city in queryMajorCities2)
{
    Console.WriteLine(city);
}
// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

C#
City[] cities = [
    new City("Tokyo", 37_833_000),
    new City("Delhi", 30_290_000),
    new City("Shanghai", 27_110_000),
    new City("São Paulo", 22_043_000)
];

//Query syntax
IEnumerable<City> queryMajorCities =
    from city in cities
    where city.Population > 30_000_000
    select city;

// Execute the query to produce the results
foreach (City city in queryMajorCities)
{
    Console.WriteLine(city);
}

// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

// Method-based syntax
IEnumerable<City> queryMajorCities2 = cities.Where(c => c.Population > 30_000_000);
// Execute the query to produce the results
foreach (City city in queryMajorCities2)
{
    Console.WriteLine(city);
}
// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

C#
City[] cities = [
    new City("Tokyo", 37_833_000),
    new City("Delhi", 30_290_000),
    new City("Shanghai", 27_110_000),
    new City("São Paulo", 22_043_000)
];

//Query syntax
IEnumerable<City> queryMajorCities =
    from city in cities
    where city.Population > 30_000_000
    select city;

// Execute the query to produce the results
foreach (City city in queryMajorCities)
{
    Console.WriteLine(city);
}

// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

// Method-based syntax
IEnumerable<City> queryMajorCities2 = cities.Where(c => c.Population > 30_000_000);
// Execute the query to produce the results
foreach (City city in queryMajorCities2)
{
    Console.WriteLine(city);
}
// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

C#
City[] cities = [
    new City("Tokyo", 37_833_000),
    new City("Delhi", 30_290_000),
    new City("Shanghai", 27_110_000),
    new City("São Paulo", 22_043_000)
];

//Query syntax
IEnumerable<City> queryMajorCities =
    from city in cities
    where city.Population > 30_000_000
    select city;

// Execute the query to produce the results
foreach (City city in queryMajorCities)
{
    Console.WriteLine(city);
}

// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

// Method-based syntax
IEnumerable<City> queryMajorCities2 = cities.Where(c => c.Population > 30_000_000);
// Execute the query to produce the results
foreach (City city in queryMajorCities2)
{
    Console.WriteLine(city);
}
// Output:
// City { Name = Tokyo, Population = 37833000 }
// City { Name = Delhi, Population = 30290000 }

I tried it on my IDE and I obtained the output mentioned, which is correct but the thing is, why do we have the

{}

{}

{}

{} I'm confused about that

F

FakerOP•3/13/25, 9:03 AM

C#
record City(string Name, long Population);
record Country(string Name, double Area, long Population, List<City> Cities);
record Product(string Name, string Category);

C#
record City(string Name, long Population);
record Country(string Name, double Area, long Population, List<City> Cities);
record Product(string Name, string Category);

C#
record City(string Name, long Population);
record Country(string Name, double Area, long Population, List<City> Cities);
record Product(string Name, string Category);

C#
record City(string Name, long Population);
record Country(string Name, double Area, long Population, List<City> Cities);
record Product(string Name, string Category);

Oh ok that's because City was declared using

record

record

record

record ?

FFaker ```C# record City(string Name, long Population); record Country(string Name, dou...

J

jcotton42•3/13/25, 9:05 AM

Yes.

record

record

record

record supplies a

ToString

ToString

ToString

ToString that does that.

F

FakerOP•3/13/25, 9:05 AM

Yep I see, it's clearer now, thanks !!

✅ Nested from clause in query syntax

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