yield retun 0 と yield retun null でGCが発生するから yield retun null にしろ、という記事などがありますが、なぜGCAllocが発生してしまうのかを見てみましょう。
どのような展開がされるのかを確認するために使用するのは、SharpLabです。
yield return null
using System;
using System.Collections;
using System.Collections.Generic;
public class C {
public IEnumerator M()
{
yield return null;
}
}
yield retun 0
using System;
using System.Collections;
using System.Collections.Generic;
public class C {
public IEnumerator M()
{
yield return 0;
}
}
それぞれを展開すると、以下のようになります。
yield return null
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;
[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
public class C
{
[CompilerGenerated]
private sealed class <M>d__0 : IEnumerator<object>, IDisposable, IEnumerator
{
private int <>1__state;
private object <>2__current; // 値はObject型
object IEnumerator<object>.Current
{
[DebuggerHidden]
get
{
return <>2__current;
}
}
object IEnumerator.Current
{
[DebuggerHidden]
get
{
return <>2__current;
}
}
[DebuggerHidden]
public <M>d__0(int <>1__state)
{
this.<>1__state = <>1__state;
}
[DebuggerHidden]
void IDisposable.Dispose()
{
}
private bool MoveNext()
{
int num = <>1__state;
if (num != 0)
{
if (num != 1)
{
return false;
}
<>1__state = -1;
return false;
}
<>1__state = -1;
<>2__current = null; // ここが違う
<>1__state = 1;
return true;
}
bool IEnumerator.MoveNext()
{
//ILSpy generated this explicit interface implementation from .override directive in MoveNext
return this.MoveNext();
}
[DebuggerHidden]
void IEnumerator.Reset()
{
throw new NotSupportedException();
}
}
[IteratorStateMachine(typeof(<M>d__0))]
public IEnumerator M()
{
return new <M>d__0(0);
}
}
yield return 0
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;
[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
public class C
{
[CompilerGenerated]
private sealed class <M>d__0 : IEnumerator<object>, IDisposable, IEnumerator
{
private int <>1__state;
private object <>2__current; // 値はObject型
object IEnumerator<object>.Current
{
[DebuggerHidden]
get
{
return <>2__current;
}
}
object IEnumerator.Current
{
[DebuggerHidden]
get
{
return <>2__current;
}
}
[DebuggerHidden]
public <M>d__0(int <>1__state)
{
this.<>1__state = <>1__state;
}
[DebuggerHidden]
void IDisposable.Dispose()
{
}
private bool MoveNext()
{
int num = <>1__state;
if (num != 0)
{
if (num != 1)
{
return false;
}
<>1__state = -1;
return false;
}
<>1__state = -1;
<>2__current = 0; // ここが違う
<>1__state = 1;
return true;
}
bool IEnumerator.MoveNext()
{
//ILSpy generated this explicit interface implementation from .override directive in MoveNext
return this.MoveNext();
}
[DebuggerHidden]
void IEnumerator.Reset()
{
throw new NotSupportedException();
}
}
[IteratorStateMachine(typeof(<M>d__0))]
public IEnumerator M()
{
return new <M>d__0(0);
}
}
yield return 0 は boxingとunboxingが働いてしまう
上記のURLの「ボックス化変換の図」説明にあるように、スタックに配置した int 0 はboxingされることによってヒープへコピーされ、IEnumerator<object>を継承したクラス内のパラメータ代入時にGCAllocが発生してしまう、nullであれば、object型はclassなので発生しないという形で動作します。
ピンバック: コルーチン (Coroutines) [Unity] – Site-Builder.wiki