Linq.Where
代码示例：
public class Student
{
public string Name { get; set; }
public int Age { get; set; }
}
class Program
{
static void Main(string[] args)
{
List<Student> students = new List<Student>
{
new Student{Name = "xiaoming",Age = 11 },
new Student{Name = "xiaohong",Age = 12 },
new Student{Name = "xiaohuang",Age = 13 },
}；
foreach (var item in students.Where(x=>x.Name == "xiaoming"))
{
Console.WriteLine($"年龄：{item.Age}");
}
Console.ReadKey();
}
}
结果： 11
解析Where源码
在解析源码前，笔者通过多次Debug，列出了方法调用间的顺序
System.Linq.Enumerable.MoveNext()会不断调用System.Collections.Generic.MoveNext()，当System.Collections.Generic.MoveNext()返回True时，System.Linq.Enumerable.MoveNext()会调用System.String.Equals()去判断数据是否匹配（笔者认为这里的调用是因为我定义的Name是string类型，不同的类型可能会有不同的判断方法，后续待验证）
笔者认为重点代码是以下这些：
namespace System.Linq
{
private sealed partial class WhereEnumerableIterator<TSource> : Iterator<TSource>
{
private readonly IEnumerable<TSource> _source;
private readonly Func<TSource, bool> _predicate;
private IEnumerator<TSource>? _enumerator;
public WhereEnumerableIterator(IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
Debug.Assert(source != null);
Debug.Assert(predicate != null);
_source = source;
_predicate = predicate;
}
public override bool MoveNext()
{
switch (_state)
{
case 1:
_enumerator = _source.GetEnumerator();
_state = 2;
goto case 2;
case 2:
while (_enumerator.MoveNext())
{
TSource item = _enumerator.Current;
if (_predicate(item))
{
_current = item;
return true;
}
}
Dispose();
break;
}
return false;
}
}
}
namespace System.Collections.Generic
{
// Implements a variable-size List that uses an array of objects to store the
// elements. A List has a capacity, which is the allocated length
// of the internal array. As elements are added to a List, the capacity
// of the List is automatically increased as required by reallocating the
// internal array.
//
[DebuggerTypeProxy(typeof(ICollectionDebugView<>))]
[DebuggerDisplay("Count = {Count}")]
[Serializable]
[TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
public class List<T> : IList<T>, IList, IReadOnlyList<T>
{
public Enumerator GetEnumerator() => new Enumerator(this);
public struct Enumerator : IEnumerator<T>, IEnumerator
{
private readonly List<T> _list;
private int _index;
private readonly int _version;
private T? _current;
internal Enumerator(List<T> list)
{
_list = list;
_index = 0;
_version = list._version;
_current = default;
}
public void Dispose()
{
}
public bool MoveNext()
{
List<T> localList = _list;
if (_version == localList._version && ((uint)_index < (uint)localList._size))
{
_current = localList._items[_index];
_index++;
return true;
}
return MoveNextRare();
}
}
}
}
在System.Linq.WhereEnumerableIterator.MoveNext()中，_state会等于1，这会调用GetEnumerator()方法拿到当前数组的所有数据，接着再将_state设置成2，会执行System.Collections.Generic.MoveNext()方法拿到数组中的第一个元素，与Linq中传入的数据进行匹配，如果匹配成功，则返回true，如果匹配失败，则继续调用System.Collections.Generic.MoveNext()方法拿到数组中的第二个元素，以此类推，直到System.Linq.WhereEnumerableIterator.MoveNext()返回true为止。
附截取的部分调用源码
System.Linq.Enumerable
public static partial class Enumerable
{
public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
if (source == null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}
if (predicate == null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
}
if (source is Iterator<TSource> iterator)
{
return iterator.Where(predicate);
}
if (source is TSource[] array)
{
return array.Length == 0 ?
Empty<TSource>() :
new WhereArrayIterator<TSource>(array, predicate);
}
if (source is List<TSource> list)
{
return new WhereListIterator<TSource>(list, predicate);
}
return new WhereEnumerableIterator<TSource>(source, predicate);
}
// <summary>
/// An iterator that filters each item of a <see cref="List{TSource}"/>.
/// </summary>
/// <typeparam name="TSource">The type of the source list.</typeparam>
private sealed partial class WhereListIterator<TSource> : Iterator<TSource>
{
private readonly List<TSource> _source;
private readonly Func<TSource, bool> _predicate;
private List<TSource>.Enumerator _enumerator;
public WhereListIterator(List<TSource> source, Func<TSource, bool> predicate)
{
Debug.Assert(source != null);
Debug.Assert(predicate != null);
_source = source;
_predicate = predicate;
}
public override bool MoveNext()
{
switch (_state)
{
case 1:
_enumerator = _source.GetEnumerator();
_state = 2;
goto case 2;
case 2:
while (_enumerator.MoveNext())
{
TSource item = _enumerator.Current;
if (_predicate(item))
{
_current = item;
return true;
}
}
Dispose();
break;
}
return false;
}
System.StartupHookProvider
using System;
using System.Diagnostics;
using System.Diagnostics.Tracing;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.Reflection;
using System.Runtime.Loader;
namespace System
{
internal static class StartupHookProvider
{
private const string StartupHookTypeName = "StartupHook";
private const string InitializeMethodName = "Initialize";
private const string DisallowedSimpleAssemblyNameSuffix = ".dll";
private static bool IsSupported => AppContext.TryGetSwitch("System.StartupHookProvider.IsSupported", out bool isSupported) ? isSupported : true;
private struct StartupHookNameOrPath
{
public AssemblyName AssemblyName;
public string Path;
}
// Parse a string specifying a list of assemblies and types
// containing a startup hook, and call each hook in turn.
private static void ProcessStartupHooks()
{
if (!IsSupported)
return;
// Initialize tracing before any user code can be called if EventSource is enabled.
if (EventSource.IsSupported)
{
System.Diagnostics.Tracing.RuntimeEventSource.Initialize();
}
string? startupHooksVariable = AppContext.GetData("STARTUP_HOOKS") as string;
if (startupHooksVariable == null)
{
return;
}
ReadOnlySpan<char> disallowedSimpleAssemblyNameChars = stackalloc char[4]
{
Path.DirectorySeparatorChar,
Path.AltDirectorySeparatorChar,
' ',
','
};
// Parse startup hooks variable
string[] startupHookParts = startupHooksVariable.Split(Path.PathSeparator);
StartupHookNameOrPath[] startupHooks = new StartupHookNameOrPath[startupHookParts.Length];
for (int i = 0; i < startupHookParts.Length; i++)
{
string startupHookPart = startupHookParts[i];
if (string.IsNullOrEmpty(startupHookPart))
{
// Leave the slot in startupHooks empty (nulls for everything). This is simpler than shifting and resizing the array.
continue;
}
if (Path.IsPathFullyQualified(startupHookPart))
{
startupHooks[i].Path = startupHookPart;
}
else
{
// The intent here is to only support simple assembly names, but AssemblyName .ctor accepts
// lot of other forms (fully qualified assembly name, strings which look like relative paths and so on).
// So add a check on top which will disallow any directory separator, space or comma in the assembly name.
for (int j = 0; j < disallowedSimpleAssemblyNameChars.Length; j++)
{
if (startupHookPart.Contains(disallowedSimpleAssemblyNameChars[j]))
{
throw new ArgumentException(SR.Format(SR.Argument_InvalidStartupHookSimpleAssemblyName, startupHookPart));
}
}
if (startupHookPart.EndsWith(DisallowedSimpleAssemblyNameSuffix, StringComparison.OrdinalIgnoreCase))
{
throw new ArgumentException(SR.Format(SR.Argument_InvalidStartupHookSimpleAssemblyName, startupHookPart));
}
try
{
// This will throw if the string is not a valid assembly name.
startupHooks[i].AssemblyName = new AssemblyName(startupHookPart);
}
catch (Exception assemblyNameException)
{
throw new ArgumentException(SR.Format(SR.Argument_InvalidStartupHookSimpleAssemblyName, startupHookPart), assemblyNameException);
}
}
}
// Call each hook in turn
foreach (StartupHookNameOrPath startupHook in startupHooks)
{
#pragma warning disable IL2026 // suppressed in ILLink.Suppressions.LibraryBuild.xml
CallStartupHook(startupHook);
#pragma warning restore IL2026
}
}
}
}
System.MulticastDelegate
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.Serialization;
namespace System
{
[ClassInterface(ClassInterfaceType.None)]
[ComVisible(true)]
public abstract class MulticastDelegate : Delegate
{
// This is set under 2 circumstances
// 1. Multicast delegate
// 2. Wrapper delegate
private object? _invocationList; // Initialized by VM as needed
private IntPtr _invocationCount;
#pragma warning disable IDE0060
[System.Diagnostics.DebuggerNonUserCode]
private void CtorClosed(object target, IntPtr methodPtr)
{
if (target == null)
ThrowNullThisInDelegateToInstance();
this._target = target;
this._methodPtr = methodPtr;
}
}
}
System.String
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Buffers;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text.Unicode;
using Internal.Runtime.CompilerServices;
namespace System
{
public partial class String
{
// Determines whether two Strings match.
public static bool Equals(string? a, string? b)
{
if (object.ReferenceEquals(a, b))
{
return true;
}
if (a is null || b is null || a.Length != b.Length)
{
return false;
}
return EqualsHelper(a, b);
}
}
}
System.Collections.Generic.List
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
namespace System.Collections.Generic
{
// Implements a variable-size List that uses an array of objects to store the
// elements. A List has a capacity, which is the allocated length
// of the internal array. As elements are added to a List, the capacity
// of the List is automatically increased as required by reallocating the
// internal array.
//
[DebuggerTypeProxy(typeof(ICollectionDebugView<>))]
[DebuggerDisplay("Count = {Count}")]
[Serializable]
[TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
public class List<T> : IList<T>, IList, IReadOnlyList<T>
{
public Enumerator GetEnumerator() => new Enumerator(this);
}
}
参考链接
Enumerable 类 (System.Linq) | Microsoft Learn
IEnumerable 接口 (System.Collections.Generic) | Microsoft Learn
使用 Visual Studio |调试 .NET 并 ASP.NET 核心源代码微软文档 (microsoft.com)
