c# - 应该使用 ToEntity 而不是类型转换吗？

Question

Xrm Sdk defines一个ToEntity<T>方法。我一直用它从 CRM 获取我早期绑定(bind)的实体。今天我正在审查一些代码，看到实体刚刚被转换:

var contact = service.Retrieve("contact", id, new ColumnSet()) as Contact;

我什至不知道这是可能的。甚至还需要 ToEntity 调用吗？

Answer 1

顺便说一句，这不是特定的转换，它是一种转换，转换的行为与直接转换略有不同。

As

You can use the as operator to perform certain types of conversions between compatible reference types or nullable types...The as operator is like a cast operation. However, if the conversion isn't possible, as returns null instead of raising an exception.

我假设您的 Contact 是由 CrmSvcUtil 创建的类，例如public partial class Contact : Microsoft.Xrm.Sdk.Entity，service.Retrieve 是 IOrganizationService.Retrieve其返回类型为 Entity。

Contact 是基类 Entity 的派生类。您不能将基类转换为更具体的派生类(请参阅 Is it possible to assign a base class object to a derived class reference with an explicit typecast in C#? )。如果您尝试执行从 Entity 到 Contact 的强制转换，您会遇到异常，并且转换会返回空对象。

包含来自 CrmSvcUtil 的 GeneratedCode 的示例，但与 CRM 没有实际连接。

var entity = new Entity();

Console.WriteLine($"Type of local entity: {entity.GetType()}");

Console.WriteLine($"Local entity as Contact is null? {entity as Contact == null}");

输出:

Type of local entity: Microsoft.Xrm.Sdk.Entity
Local entity as Contact is null? True

鉴于 Retrieve 返回一个 Entity，它不能被转换为 Contact，你的代码行 ( var contact = service.Retrieve("contact", id, new ColumnSet()) as Contact;) 还能用吗？

嗯，这很神奇。显然，如果您在应用程序中包含来自 CrmSvcUtil 的 GeneratedCode，则 Retrieve 函数会返回特定的派生类，而不是通用的 Entity。

包含来自 CrmSvcUtil 的生成代码的示例:

CrmServiceClient service = new CrmServiceClient(ConfigurationManager.ConnectionStrings["Crm"].ConnectionString);

Contact c = new Contact()
{
    LastName = "Test"
};

Guid contactId = service.Create(c);

var response = service.Retrieve("contact", contactId, new ColumnSet());

Console.WriteLine($"Type of response from CRM: {response.GetType()}");

Console.WriteLine($"Response from CRM as contact is null? {response as Contact == null}");

输出:

Type of response from CRM: Contact
Response from CRM as contact is null? False

不包含生成代码的示例:

CrmServiceClient service = new CrmServiceClient(ConfigurationManager.ConnectionStrings["Crm"].ConnectionString);

Entity c = new Entity("contact");
c["lastname"] = "Test";

Guid contactId = service.Create(c);

var response = service.Retrieve("contact", contactId, new ColumnSet());

Console.WriteLine($"Type of response: {response.GetType()}");

输出:

Type of response: Microsoft.Xrm.Sdk.Entity

回到你的问题。如果你在你的项目中包含生成的代码，假设 Retrieve 返回一个 Contact 无论如何你可以做一个简单的转换(例如 (Contact)service .Retrieve(...)) 或转换 (as)。就 ToEntity 的作用而言，它实际上并没有进行强制转换或转换。它创建一个新对象并执行浅拷贝等一些事情。因此，如果满足您的需要，请使用它，但没有它您也可以逃脱。

反编译代码:

public T ToEntity<T>() where T : Entity
{
    if (typeof(T) == typeof(Entity))
    {
        Entity entity = new Entity();
        this.ShallowCopyTo(entity);
        return entity as T;
    }
    if (string.IsNullOrWhiteSpace(this._logicalName))
    {
        throw new NotSupportedException("LogicalName must be set before calling ToEntity()");
    }
    string text = null;
    object[] customAttributes = typeof(T).GetCustomAttributes(typeof(EntityLogicalNameAttribute), true);
    if (customAttributes != null)
    {
        object[] array = customAttributes;
        int num = 0;
        if (num < array.Length)
        {
            EntityLogicalNameAttribute entityLogicalNameAttribute = (EntityLogicalNameAttribute)array[num];
            text = entityLogicalNameAttribute.LogicalName;
        }
    }
    if (string.IsNullOrWhiteSpace(text))
    {
        throw new NotSupportedException("Cannot convert to type that is does not have EntityLogicalNameAttribute");
    }
    if (this._logicalName != text)
    {
        throw new NotSupportedException(string.Format(CultureInfo.InvariantCulture, "Cannot convert entity {0} to {1}", new object[]
        {
            this._logicalName,
            text
        }));
    }
    T t = (T)((object)Activator.CreateInstance(typeof(T)));
    this.ShallowCopyTo(t);
    return t;
}