Answers:
只是将单个字段用于查找。
呼叫者没有Foo，它正在尝试获取一个。当然，您可以将Foo所有其他字段保留为空白，但这是临时的，但这仅适用于琐碎的数据结构。大多数对象具有不变性，大多数空对象方法会违反这些不变性，因此请避免使用。
现在或将来会在电线上（串行化/反序列化）进行处理吗？将单个ID类型优先于谁知道如何大的完整对象。
如果您正在寻找ID对其实体的类型安全性，那么也有代码解决方案。让我知道您是否需要一个例子。
编辑：扩展ID的类型安全性：
因此，让我们采用您的方法：
public Foo GetItem(int id) {}我们只希望id传入的整数用于Foo对象。有人可能会滥用它并传递某个Bar对象的整数ID，甚至只是手动输入812341。输入的类型不安全Foo。其次，即使您使用了传递对象的Foo版本，我也确信Foo有一个ID字段int，有人可以对其进行修改。最后，如果这些重载一起存在于一个类中，则只能使用方法重载，因为只有返回类型会有所不同。让我们重写一下此方法，以在C＃中看起来类型安全：
public Foo GetItem(IntId<Foo> id) {}因此，我介绍了一个名为class的类IntId，其中包含一个通用部分。在这种情况下，我只希望int与关联Foo。我不能只是裸照地传递，int也不能不IntId<Bar>小心将其分配给它。因此，下面是我编写这些类型安全标识符的方式。这样做，请注意该实际的底层的操作int是只在您的数据访问层。上面的任何内容都只能看到强类型，并且不能（直接）访问其内部intID。它应该没有理由。
IModelId.cs接口：
namespace GenericIdentifiers
{
using System.Runtime.Serialization;
using System.ServiceModel;
/// <summary>
/// Defines an interface for an object's unique key in order to abstract out the underlying key
/// generation/maintenance mechanism.
/// </summary>
/// <typeparam name="T">The type the key is representing.</typeparam>
[ServiceContract]
public interface IModelId<T> where T : class
{
/// <summary>
/// Gets a string representation of the domain the model originated from.
/// </summary>
/// <value>The origin.</value>
[DataMember]
string Origin
{
[OperationContract]get;
}
/// <summary>
/// The model instance identifier for the model object that this <see cref="IModelId{T}"/> refers to.
/// Typically, this is a database key, file name, or some other unique identifier.
/// <typeparam name="TKeyDataType">The expected data type of the identifier.</typeparam>
/// </summary>
/// <typeparam name="TKeyDataType">The expected data type of the identifier.</typeparam>
/// <returns>The unique key as the data type specified.</returns>
[OperationContract]
TKeyDataType GetKey<TKeyDataType>();
/// <summary>
/// Performs an equality check on the two model identifiers and returns <c>true</c> if they are equal; otherwise
/// <c>false</c> is returned. All implementations must also override the equal operator.
/// </summary>
/// <param name="obj">The identifier to compare against.</param>
/// <returns><c>true</c> if the identifiers are equal; otherwise <c>false</c> is returned.</returns>
[OperationContract]
bool Equals(IModelId<T> obj);
}
}ModelIdBase.cs基类：
namespace GenericIdentifiers
{
using System;
using System.Collections.Generic;
using System.Runtime.Serialization;
/// <summary>
/// Represents an object's unique key in order to abstract out the underlying key generation/maintenance mechanism.
/// </summary>
/// <typeparam name="T">The type the key is representing.</typeparam>
[DataContract(IsReference = true)]
[KnownType("GetKnownTypes")]
public abstract class ModelIdBase<T> : IModelId<T> where T : class
{
/// <summary>
/// Gets a string representation of the domain the model originated from.
/// </summary>
[DataMember]
public string Origin
{
get;
internal set;
}
/// <summary>
/// The model instance identifier for the model object that this <see cref="ModelIdBase{T}"/> refers to.
/// Typically, this is a database key, file name, or some other unique identifier.
/// </summary>
/// <typeparam name="TKeyDataType">The expected data type of the identifier.</typeparam>
/// <returns>The unique key as the data type specified.</returns>
public abstract TKeyDataType GetKey<TKeyDataType>();
/// <summary>
/// Performs an equality check on the two model identifiers and returns <c>true</c> if they are equal;
/// otherwise <c>false</c> is returned. All implementations must also override the equal operator.
/// </summary>
/// <param name="obj">The identifier to compare against.</param>
/// <returns>
/// <c>true</c> if the identifiers are equal; otherwise <c>false</c> is returned.
/// </returns>
public abstract bool Equals(IModelId<T> obj);
protected static IEnumerable<Type> GetKnownTypes()
{
return new[] { typeof(IntId<T>), typeof(GuidId<T>) };
}
}
}IntId.cs：
namespace GenericIdentifiers
{
// System namespaces
using System;
using System.Diagnostics;
using System.Globalization;
using System.Runtime.Serialization;
/// <summary>
/// Represents an abstraction of the database key for a Model Identifier.
/// </summary>
/// <typeparam name="T">The expected owner data type for this identifier.</typeparam>
[DebuggerDisplay("Origin={Origin}, Integer Identifier={Id}")]
[DataContract(IsReference = true)]
public sealed class IntId<T> : ModelIdBase<T> where T : class
{
/// <summary>
/// Gets or sets the unique ID.
/// </summary>
/// <value>The unique ID.</value>
[DataMember]
internal int Id
{
get;
set;
}
/// <summary>
/// Implements the operator ==.
/// </summary>
/// <param name="intIdentifier1">The first Model Identifier to compare.</param>
/// <param name="intIdentifier2">The second Model Identifier to compare.</param>
/// <returns>
/// <c>true</c> if the instances are equal; otherwise <c>false</c> is returned.
/// </returns>
public static bool operator ==(IntId<T> intIdentifier1, IntId<T> intIdentifier2)
{
return object.Equals(intIdentifier1, intIdentifier2);
}
/// <summary>
/// Implements the operator !=.
/// </summary>
/// <param name="intIdentifier1">The first Model Identifier to compare.</param>
/// <param name="intIdentifier2">The second Model Identifier to compare.</param>
/// <returns>
/// <c>true</c> if the instances are equal; otherwise <c>false</c> is returned.
/// </returns>
public static bool operator !=(IntId<T> intIdentifier1, IntId<T> intIdentifier2)
{
return !object.Equals(intIdentifier1, intIdentifier2);
}
/// <summary>
/// Performs an implicit conversion from <see cref="IntId{T}"/> to <see cref="System.Int32"/>.
/// </summary>
/// <param name="id">The identifier.</param>
/// <returns>The result of the conversion.</returns>
public static implicit operator int(IntId<T> id)
{
return id == null ? int.MinValue : id.GetKey<int>();
}
/// <summary>
/// Performs an implicit conversion from <see cref="System.Int32"/> to <see cref="IntId{T}"/>.
/// </summary>
/// <param name="id">The identifier.</param>
/// <returns>The result of the conversion.</returns>
public static implicit operator IntId<T>(int id)
{
return new IntId<T> { Id = id };
}
/// <summary>
/// Determines whether the specified <see cref="T:System.Object"/> is equal to the current
/// <see cref="T:System.Object"/>.
/// </summary>
/// <param name="obj">The <see cref="T:System.Object"/> to compare with the current
/// <see cref="T:System.Object"/>.</param>
/// <returns>true if the specified <see cref="T:System.Object"/> is equal to the current
/// <see cref="T:System.Object"/>; otherwise, false.</returns>
/// <exception cref="T:System.NullReferenceException">The <paramref name="obj"/> parameter is null.</exception>
public override bool Equals(object obj)
{
return this.Equals(obj as IModelId<T>);
}
/// <summary>
/// Serves as a hash function for a particular type.
/// </summary>
/// <returns>
/// A hash code for the current <see cref="T:System.Object"/>.
/// </returns>
public override int GetHashCode()
{
unchecked
{
var hash = 17;
hash = (23 * hash) + (this.Origin == null ? 0 : this.Origin.GetHashCode());
return (31 * hash) + this.GetKey<int>().GetHashCode();
}
}
/// <summary>
/// Returns a <see cref="System.String"/> that represents this instance.
/// </summary>
/// <returns>
/// A <see cref="System.String"/> that represents this instance.
/// </returns>
public override string ToString()
{
return this.Origin + ":" + this.GetKey<int>().ToString(CultureInfo.InvariantCulture);
}
/// <summary>
/// Performs an equality check on the two model identifiers and returns <c>true</c> if they are equal;
/// otherwise <c>false</c> is returned. All implementations must also override the equal operator.
/// </summary>
/// <param name="obj">The identifier to compare against.</param>
/// <returns>
/// <c>true</c> if the identifiers are equal; otherwise <c>false</c> is returned.
/// </returns>
public override bool Equals(IModelId<T> obj)
{
if (obj == null)
{
return false;
}
return (obj.Origin == this.Origin) && (obj.GetKey<int>() == this.GetKey<int>());
}
/// <summary>
/// The model instance identifier for the model object that this <see cref="ModelIdBase{T}"/> refers to.
/// Typically, this is a database key, file name, or some other unique identifier.
/// </summary>
/// <typeparam name="TKeyDataType">The expected data type of the identifier.</typeparam>
/// <returns>The unique key as the data type specified.</returns>
public override TKeyDataType GetKey<TKeyDataType>()
{
return (TKeyDataType)Convert.ChangeType(this.Id, typeof(TKeyDataType), CultureInfo.InvariantCulture);
}
/// <summary>
/// Generates an object from its string representation.
/// </summary>
/// <param name="value">The value of the model's type.</param>
/// <returns>A new instance of this class as it's interface containing the value from the string.</returns>
internal static ModelIdBase<T> FromString(string value)
{
if (value == null)
{
throw new ArgumentNullException("value");
}
int id;
var originAndId = value.Split(new[] { ":" }, StringSplitOptions.None);
if (originAndId.Length != 2)
{
throw new ArgumentOutOfRangeException("value", "value must be in the format of Origin:Identifier");
}
return int.TryParse(originAndId[1], NumberStyles.None, CultureInfo.InvariantCulture, out id)
? new IntId<T> { Id = id, Origin = originAndId[0] }
: null;
}
}
}并且，出于完整性考虑，我还为GUID实体GuidId.cs编写了一个：
namespace GenericIdentifiers
{
// System namespaces
using System;
using System.Diagnostics;
using System.Globalization;
using System.Runtime.Serialization;
/// <summary>
/// Represents an abstraction of the database key for a Model Identifier.
/// </summary>
/// <typeparam name="T">The expected owner data type for this identifier.</typeparam>
[DebuggerDisplay("Origin={Origin}, GUID={Id}")]
[DataContract(IsReference = true)]
public sealed class GuidId<T> : ModelIdBase<T> where T : class
{
/// <summary>
/// Gets or sets the unique ID.
/// </summary>
/// <value>The unique ID.</value>
[DataMember]
internal Guid Id
{
get;
set;
}
/// <summary>
/// Implements the operator ==.
/// </summary>
/// <param name="guidIdentifier1">The first Model Identifier to compare.</param>
/// <param name="guidIdentifier2">The second Model Identifier to compare.</param>
/// <returns>
/// <c>true</c> if the instances are equal; otherwise <c>false</c> is returned.
/// </returns>
public static bool operator ==(GuidId<T> guidIdentifier1, GuidId<T> guidIdentifier2)
{
return object.Equals(guidIdentifier1, guidIdentifier2);
}
/// <summary>
/// Implements the operator !=.
/// </summary>
/// <param name="guidIdentifier1">The first Model Identifier to compare.</param>
/// <param name="guidIdentifier2">The second Model Identifier to compare.</param>
/// <returns>
/// <c>true</c> if the instances are equal; otherwise <c>false</c> is returned.
/// </returns>
public static bool operator !=(GuidId<T> guidIdentifier1, GuidId<T> guidIdentifier2)
{
return !object.Equals(guidIdentifier1, guidIdentifier2);
}
/// <summary>
/// Performs an implicit conversion from <see cref="GuidId{T}"/> to <see cref="System.Guid"/>.
/// </summary>
/// <param name="id">The identifier.</param>
/// <returns>The result of the conversion.</returns>
public static implicit operator Guid(GuidId<T> id)
{
return id == null ? Guid.Empty : id.GetKey<Guid>();
}
/// <summary>
/// Performs an implicit conversion from <see cref="System.Guid"/> to <see cref="GuidId{T}"/>.
/// </summary>
/// <param name="id">The identifier.</param>
/// <returns>The result of the conversion.</returns>
public static implicit operator GuidId<T>(Guid id)
{
return new GuidId<T> { Id = id };
}
/// <summary>
/// Determines whether the specified <see cref="T:System.Object"/> is equal to the current
/// <see cref="T:System.Object"/>.
/// </summary>
/// <param name="obj">The <see cref="T:System.Object"/> to compare with the current
/// <see cref="T:System.Object"/>.</param>
/// <returns>true if the specified <see cref="T:System.Object"/> is equal to the current
/// <see cref="T:System.Object"/>; otherwise, false.</returns>
/// <exception cref="T:System.NullReferenceException">The <paramref name="obj"/> parameter is null.</exception>
public override bool Equals(object obj)
{
return this.Equals(obj as IModelId<T>);
}
/// <summary>
/// Serves as a hash function for a particular type.
/// </summary>
/// <returns>
/// A hash code for the current <see cref="T:System.Object"/>.
/// </returns>
public override int GetHashCode()
{
unchecked
{
var hash = 17;
hash = (23 * hash) + (this.Origin == null ? 0 : this.Origin.GetHashCode());
return (31 * hash) + this.GetKey<Guid>().GetHashCode();
}
}
/// <summary>
/// Returns a <see cref="System.String"/> that represents this instance.
/// </summary>
/// <returns>
/// A <see cref="System.String"/> that represents this instance.
/// </returns>
public override string ToString()
{
return this.Origin + ":" + this.GetKey<Guid>();
}
/// <summary>
/// Performs an equality check on the two model identifiers and returns <c>true</c> if they are equal;
/// otherwise <c>false</c> is returned. All implementations must also override the equal operator.
/// </summary>
/// <param name="obj">The identifier to compare against.</param>
/// <returns>
/// <c>true</c> if the identifiers are equal; otherwise <c>false</c> is returned.
/// </returns>
public override bool Equals(IModelId<T> obj)
{
if (obj == null)
{
return false;
}
return (obj.Origin == this.Origin) && (obj.GetKey<Guid>() == this.GetKey<Guid>());
}
/// <summary>
/// The model instance identifier for the model object that this <see cref="ModelIdBase{T}"/> refers to.
/// Typically, this is a database key, file name, or some other unique identifier.
/// </summary>
/// <typeparam name="TKeyDataType">The expected data type of the identifier.</typeparam>
/// <returns>The unique key as the data type specified.</returns>
public override TKeyDataType GetKey<TKeyDataType>()
{
return (TKeyDataType)Convert.ChangeType(this.Id, typeof(TKeyDataType), CultureInfo.InvariantCulture);
}
/// <summary>
/// Generates an object from its string representation.
/// </summary>
/// <param name="value">The value of the model's type.</param>
/// <returns>A new instance of this class as it's interface containing the value from the string.</returns>
internal static ModelIdBase<T> FromString(string value)
{
if (value == null)
{
throw new ArgumentNullException("value");
}
Guid id;
var originAndId = value.Split(new[] { ":" }, StringSplitOptions.None);
if (originAndId.Length != 2)
{
throw new ArgumentOutOfRangeException("value", "value must be in the format of Origin:Identifier");
}
return Guid.TryParse(originAndId[1], out id) ? new GuidId<T> { Id = id, Origin = originAndId[0] } : null;
}
}
}Origin属性：它很像SQL Server用语中的架构。您可能Foo在会计软件中使用了一个软件，而Foo在人力资源中使用了另一个软件，并且在数据访问层中几乎没有其他字段可以区分它们。或者，如果您没有冲突，请像我一样忽略它。
我当然同意你的结论。由于某些原因，首选传递ID：
Foo仅为id 创建对象意味着创建错误的值。有人可能会犯一个错误并使用这些值。int在整个平台范围内，可以在所有现代语言中本地声明。要Foo通过方法调用者创建对象，您可能需要创建一个复杂的数据结构（如json对象）。我认为按照Ben Voigt的建议，在对象的标识符上进行查找是明智的。
但是，请记住，对象标识符的类型可能会更改。这样，我将为每个商品创建一个标识符类，并且仅允许通过这些标识符的这些实例查找商品。请参见以下示例：
public class Item
{
public class ItemId
{
public int Id { get; set;}
}
public ItemId Id; { get; set; }
}
public interface Service
{
Item GetItem(ItemId id);
}我使用了封装，但是您也可以Item从继承ItemId。
这样，如果您的ID类型沿途变化，则无需在Item类或GetItem方法的签名中进行任何更改。仅在服务的实现中，您才需要更改代码（无论如何，这是唯一在所有情况下都会更改的东西）
这取决于您的方法。
通常，对于Get methods，传递id parameter并取回对象是常识。在进行更新时，或者SET methods您将发送整个对象以进行设置/更新。
在某些其他情况下method is passing search parameters（您作为单个原始类型的集合）来检索一组结果，那么使用use a container to hold搜索参数可能是明智的。如果长期需要更改参数数量，这很有用。因此，您would not need可以更改signature of your method, add or remove parameter in all over the places。