C#の匿名型やプロパティ、イテレーターなんかの言語機能は、コンパイラが内部的にフィールド、プロパティ、メソッド、クラスもろもろを生成します。

そのコンパイラが生成したフィールド、プロパティ、メソッド、クラスには、CompilerGeneratedという属性が付与されます。

SharpLabで、どういう風に付与されるか見てみましょう。

こんなC#のコードがあったら、

using System.Collections.Generic;
using System.Linq;

public class Person {
    public string FirstName { get; }
    public string LastName { get; }

	public Person(string firstName, string lastName)
        => (FirstName, LastName) = (firstName, lastName);
}

public class Program {
    public static void Main() {
		var array = new int[] {3, 1, 4, 1, 5, 9, 2};
        
        var result = array.Select(it => new { squared = it * it, cubed = it * it * it});
    }
    
    public static IEnumerable<int> Yield() {
        yield return 0;
        yield return 1;
        yield return 2;
    }
}

実際は、CompilerGeneratedが付与されたこんな感じになります。

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Permissions;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.Default | DebuggableAttribute.DebuggingModes.DisableOptimizations | DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints | DebuggableAttribute.DebuggingModes.EnableEditAndContinue)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
[CompilerGenerated]
[DebuggerDisplay("\\{ squared = {squared}, cubed = {cubed} }", Type = "<Anonymous Type>")]
internal sealed class <>f__AnonymousType0<<squared>j__TPar, <cubed>j__TPar>
{
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private readonly <squared>j__TPar <squared>i__Field;

    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private readonly <cubed>j__TPar <cubed>i__Field;

    public <squared>j__TPar squared => <squared>i__Field;

    public <cubed>j__TPar cubed => <cubed>i__Field;

    [DebuggerHidden]
    public <>f__AnonymousType0(<squared>j__TPar squared, <cubed>j__TPar cubed)
    {
        <squared>i__Field = squared;
        <cubed>i__Field = cubed;
    }

    [DebuggerHidden]
    public override bool Equals(object value)
    {
        <>f__AnonymousType0<<squared>j__TPar, <cubed>j__TPar> anon = value as <>f__AnonymousType0<<squared>j__TPar, <cubed>j__TPar>;
        return anon != null && EqualityComparer<<squared>j__TPar>.Default.Equals(<squared>i__Field, anon.<squared>i__Field) && EqualityComparer<<cubed>j__TPar>.Default.Equals(<cubed>i__Field, anon.<cubed>i__Field);
    }

    [DebuggerHidden]
    public override int GetHashCode()
    {
        return (405075540 * -1521134295 + EqualityComparer<<squared>j__TPar>.Default.GetHashCode(<squared>i__Field)) * -1521134295 + EqualityComparer<<cubed>j__TPar>.Default.GetHashCode(<cubed>i__Field);
    }

    [DebuggerHidden]
    public override string ToString()
    {
        object[] obj = new object[2];
        <squared>j__TPar val = <squared>i__Field;
        ref <squared>j__TPar reference = ref val;
        <squared>j__TPar val2 = default(<squared>j__TPar);
        object obj2;
        if (val2 == null)
        {
            val2 = reference;
            reference = ref val2;
            if (val2 == null)
            {
                obj2 = null;
                goto IL_0046;
            }
        }
        obj2 = reference.ToString();
        goto IL_0046;
        IL_0046:
        obj[0] = obj2;
        <cubed>j__TPar val3 = <cubed>i__Field;
        ref <cubed>j__TPar reference2 = ref val3;
        <cubed>j__TPar val4 = default(<cubed>j__TPar);
        object obj3;
        if (val4 == null)
        {
            val4 = reference2;
            reference2 = ref val4;
            if (val4 == null)
            {
                obj3 = null;
                goto IL_0081;
            }
        }
        obj3 = reference2.ToString();
        goto IL_0081;
        IL_0081:
        obj[1] = obj3;
        return string.Format(null, "{{ squared = {0}, cubed = {1} }}", obj);
    }
}
public class Person
{
    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private readonly string <FirstName>k__BackingField;

    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private readonly string <LastName>k__BackingField;

    public string FirstName
    {
        [CompilerGenerated]
        get
        {
            return <FirstName>k__BackingField;
        }
    }

    public string LastName
    {
        [CompilerGenerated]
        get
        {
            return <LastName>k__BackingField;
        }
    }

    public Person(string firstName, string lastName)
    {
        <FirstName>k__BackingField = firstName;
        <LastName>k__BackingField = lastName;
    }
}
public class Program
{
    [Serializable]
    [CompilerGenerated]
    private sealed class <>c
    {
        public static readonly <>c <>9 = new <>c();

        public static Func<int, <>f__AnonymousType0<int, int>> <>9__0_0;

        internal <>f__AnonymousType0<int, int> <Main>b__0_0(int it)
        {
            return new <>f__AnonymousType0<int, int>(it * it, it * it * it);
        }
    }

    [CompilerGenerated]
    private sealed class <Yield>d__1 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IDisposable, IEnumerator
    {
        private int <>1__state;

        private int <>2__current;

        private int <>l__initialThreadId;

        int IEnumerator<int>.Current
        {
            [DebuggerHidden]
            get
            {
                return <>2__current;
            }
        }

        object IEnumerator.Current
        {
            [DebuggerHidden]
            get
            {
                return <>2__current;
            }
        }

        [DebuggerHidden]
        public <Yield>d__1(int <>1__state)
        {
            this.<>1__state = <>1__state;
            <>l__initialThreadId = Environment.CurrentManagedThreadId;
        }

        [DebuggerHidden]
        void IDisposable.Dispose()
        {
        }

        private bool MoveNext()
        {
            switch (<>1__state)
            {
                default:
                    return false;
                case 0:
                    <>1__state = -1;
                    <>2__current = 0;
                    <>1__state = 1;
                    return true;
                case 1:
                    <>1__state = -1;
                    <>2__current = 1;
                    <>1__state = 2;
                    return true;
                case 2:
                    <>1__state = -1;
                    <>2__current = 2;
                    <>1__state = 3;
                    return true;
                case 3:
                    <>1__state = -1;
                    return false;
            }
        }

        bool IEnumerator.MoveNext()
        {
            //ILSpy generated this explicit interface implementation from .override directive in MoveNext
            return this.MoveNext();
        }

        [DebuggerHidden]
        void IEnumerator.Reset()
        {
            throw new NotSupportedException();
        }

        [DebuggerHidden]
        IEnumerator<int> IEnumerable<int>.GetEnumerator()
        {
            if (<>1__state == -2 && <>l__initialThreadId == Environment.CurrentManagedThreadId)
            {
                <>1__state = 0;
                return this;
            }
            return new <Yield>d__1(0);
        }

        [DebuggerHidden]
        IEnumerator IEnumerable.GetEnumerator()
        {
            return System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator();
        }
    }

    public static void Main()
    {
        int[] obj = new int[7];
        RuntimeHelpers.InitializeArray(obj, (RuntimeFieldHandle)/*OpCode not supported: LdMemberToken*/);
        int[] source = obj;
        IEnumerable<<>f__AnonymousType0<int, int>> enumerable = source.Select(<>c.<>9__0_0 ?? (<>c.<>9__0_0 = <>c.<>9.<Main>b__0_0));
    }

    [IteratorStateMachine(typeof(<Yield>d__1))]
    public static IEnumerable<int> Yield()
    {
        return new <Yield>d__1(-2);
    }
}
[CompilerGenerated]
internal sealed class <PrivateImplementationDetails>
{
    [StructLayout(LayoutKind.Explicit, Pack = 1, Size = 28)]
    private struct __StaticArrayInitTypeSize=28
    {
    }

    internal static readonly __StaticArrayInitTypeSize=28 572ED8A201DB898C54AEE5E176701DF001029BE8/* Not supported: data(03 00 00 00 01 00 00 00 04 00 00 00 01 00 00 00 05 00 00 00 09 00 00 00 02 00 00 00) */;
}