Program.cs

using System.Globalization;

namespace QueryExpressions;

class Program
{
    static void Main()
    {
    }

    public static void QueryExpression()
    {
        IEnumerable<CultureInfo> commaCultures =
            from culture in CultureInfo.GetCultures(CultureTypes.AllCultures)
            where culture.NumberFormat.NumberDecimalSeparator == ","
            select culture;

        foreach (CultureInfo culture in commaCultures)
        {
            Console.WriteLine(culture.Name);
        }
    }

    public static void WithoutLinq()
    {
        CultureInfo[] allCultures = CultureInfo.GetCultures(CultureTypes.AllCultures);
        foreach (CultureInfo culture in allCultures)
        {
            if (culture.NumberFormat.NumberDecimalSeparator == ",")
            {
                Console.WriteLine(culture.Name);
            }
        }
    }

    public static void ExtractingOneProperty()
    {
        IEnumerable<string> commaCultures =
            from culture in CultureInfo.GetCultures(CultureTypes.AllCultures)
            where culture.NumberFormat.NumberDecimalSeparator == ","
            select culture.Name;

        foreach (string cultureName in commaCultures)
        {
            Console.WriteLine(cultureName);
        }
    }

    public static void EffectOfQueryExpression()
    {
        IEnumerable<string> commaCultures =
            CultureInfo.GetCultures(CultureTypes.AllCultures)
            .Where(culture => culture.NumberFormat.NumberDecimalSeparator == ",")
            .Select(culture => culture.Name);
    }

    public static void ExpansionOfTrivialSelect()
    {
        IEnumerable<CultureInfo> commaCultures =
            CultureInfo.GetCultures(CultureTypes.AllCultures)
            .Where(culture => culture.NumberFormat.NumberDecimalSeparator == ",");
    }

    public static void QueryWithLetClause()
    {
        IEnumerable<string> commaCultures =
            from culture in CultureInfo.GetCultures(CultureTypes.AllCultures)
            let numFormat = culture.NumberFormat
            where numFormat.NumberDecimalSeparator == ","
            select culture.Name;
    }

    public static void ExpansionOfMultiVariableQueryExpression()
    {
        IEnumerable<string> commaCultures =
            CultureInfo.GetCultures(CultureTypes.AllCultures)
            .Select(culture => new { culture, numFormat = culture.NumberFormat })
            .Where(vars => vars.numFormat.NumberDecimalSeparator == ",")
            .Select(vars => vars.culture.Name);
    }

    public static void AccidentalReevaluation()
    {
        IEnumerable<CultureInfo> commaCultures =
            from culture in CultureInfo.GetCultures(CultureTypes.AllCultures)
            where culture.NumberFormat.NumberDecimalSeparator == ","
            select culture;

        object[] numbers = [1, 100, 100.2, 10000.2];

        foreach (object number in numbers)
        {
            foreach (CultureInfo culture in commaCultures)
            {
                Console.WriteLine(string.Format(culture, "{0}: {1:c}",
                                  culture.Name, number));
            }
        }
    }

    // The next examples illustrate types defined by .NET. Since .NET defines
    // these, we do not want to define our own versions, hence the #if false
#if false
        public interface IQueryable : IEnumerable
        {
            Type ElementType { get; }
            Expression Expression { get; }
            IQueryProvider Provider { get; }
        }

        public interface IQueryable<out T> : IEnumerable<T>, IQueryable
        {
        }

        public interface IQueryProvider
        {
            IQueryable CreateQuery(Expression expression);
            IQueryable<TElement> CreateQuery<TElement>(Expression expression);
            object? Execute(Expression expression);
            TResult Execute<TResult>(Expression expression);
        }

        public static class Enumerable
        {
            public static IEnumerable<TSource> Where<TSource>(
                this IEnumerable<TSource> source,
                Func<TSource, bool> predicate)
            ...
        }

        public static class Queryable
        {
            public static IQueryable<TSource> Where<TSource>(
                this IQueryable<TSource> source,
                Expression<Func<TSource, bool>> predicate)
            ...
        }
#endif
}