c# - 在 CSharpMigrationCodeGenerator 中使用自定义 MigrationOperation

Question

我想为 CSharpMigrationCodeGenerator 创建我自己的迁移操作，因此我创建了我自己的从 MigrationOperation 派生的迁移操作。

public class CustomMigrationOperation : MigrationOperation
{
    public CustomMigrationOperation() : base(null)
    {
        IsDestructiveChange = false;
    }

    public override bool IsDestructiveChange { get; }

    public override MigrationOperation Inverse => null;
}

以及自定义的 CSharpMigrationCodeGenerator。

public class CustomCSharpMigrationCodeGenerator : CSharpMigrationCodeGenerator
{
    public override ScaffoldedMigration Generate(string migrationId, IEnumerable<MigrationOperation> operations, string sourceModel, string targetModel,
        string @namespace, string className)
    {
        var extendedOperations = operations.ToList();
        extendedOperations.Add(new CustomMigrationOperation());

        return base.Generate(migrationId, extendedOperations, sourceModel, targetModel, @namespace, className);
    }

    protected virtual void Generate(CustomMigrationOperation operation, IndentedTextWriter writer)
    {
        // do something
    }
}

当我在程序包管理器控制台中运行 Add-Migration 脚本时，出现以下异常:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: The best overloaded method match for 'System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator.Generate(System.Data.Entity.Migrations.Model.AddColumnOperation, System.Data.Entity.Migrations.Utilities.IndentedTextWriter)' has some invalid arguments
   at CallSite.Target(Closure , CallSite , CSharpMigrationCodeGenerator , Object , IndentedTextWriter )
   at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid3[T0,T1,T2](CallSite site, T0 arg0, T1 arg1, T2 arg2)
   at System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator.<>c__DisplayClass35.<Generate>b__27(Object o)
   at System.Data.Entity.Utilities.IEnumerableExtensions.Each[T](IEnumerable`1 ts, Action`1 action)
   at System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator.Generate(IEnumerable`1 operations, String namespace, String className)
   at System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator.Generate(String migrationId, IEnumerable`1 operations, String sourceModel, String targetModel, String namespace, String className)
   at Custom.Model.CustomCSharpMigrationCodeGenerator`1.Generate(String migrationId, IEnumerable`1 operations, String sourceModel, String targetModel, String namespace, String className) in 
   at System.Data.Entity.Migrations.DbMigrator.Scaffold(String migrationName, String namespace, Boolean ignoreChanges)
   at System.Data.Entity.Migrations.Design.MigrationScaffolder.Scaffold(String migrationName, Boolean ignoreChanges)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Scaffold(MigrationScaffolder scaffolder)
   at System.Data.Entity.Migrations.Design.ToolingFacade.ScaffoldRunner.Run()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Scaffold(String migrationName, String language, String rootNamespace, Boolean ignoreChanges)
   at System.Data.Entity.Migrations.AddMigrationCommand.Execute(String name, Boolean force, Boolean ignoreChanges)
   at System.Data.Entity.Migrations.AddMigrationCommand.<>c__DisplayClass2.<.ctor>b__0()
   at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
The best overloaded method match for 'System.Data.Entity.Migrations.Design.CSharpMigrationCodeGenerator.Generate(System.Data.Entity.Migrations.Model.AddColumnOperation, System.Data.Entity.Migrations.Utilities.IndentedTextWriter)' has some invalid arguments

在对 Github 上的 EntityFramework 6 代码进行一些研究后，我找到了应该负责调用生成器的生成函数的行:

.Each<dynamic>(o => Generate(o, writer));

CSharpMigrationCodeGenerator on Github

我不明白为什么 C# 没有调用适当的生成函数。

解决方法是派生自 AddColumnOperation 并覆盖 Generate(AddColumnOperation operation, ...) 并在那里专门处理 CustomMigrationOperation。但这与其说是一个有用的解决方案，不如说是一种技巧。

Answer 1

简短的回答是 DLR 无法确定方法。

作为概念证明的示例，这里有一个简化的变体控制台应用程序，它显示了完全相同的行为。

using MicrosoftGenerator;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AddMigrationRunner {
    class Program {

        static void Main(string[] args) {
            var generator = (Generator) new Something.Deep.Somewhere.MyCustomGenerator(); // reverse casted simply to dictate the "angle" from which we are coming.

            // PMC:> Add-Migration MyMigrationName
            generator.AddMigration("MyMigrationName"); 
        }
    }
}
namespace MicrosoftGenerator {
    // meant to be a rough "clone" of: https://github.com/mono/entityframework/blob/master/src/EntityFramework/Migrations/Design/CSharpMigrationCodeGenerator.cs
    public class Generator {
        private List<MigrationOperation> operations = new List<MigrationOperation>() { };

        public Generator() {
            operations.Add(new AddColumnOperation());
        }
        public void AddMigration(string name) {
            Generate(name, operations);
        }
        public virtual void Generate(string migrationId, IEnumerable<MigrationOperation> operations) {
            Console.WriteLine("Up() method generating..");
            operations.Each<dynamic>(o => Generate(o));
        }


        protected void Generate(AddColumnOperation op) {
            Console.WriteLine("<AddColumnOperation> execute.");
        }
    }

    public class AddColumnOperation : MigrationOperation {}
    public class MigrationOperation {}


 //https://github.com/mono/entityframework/blob/master/src/Common/IEnumerableExtensions.cs
    public static class MicrosoftUtilities {
        public static void Each<T>(this IEnumerable<T> ts, Action<T> action) {
            foreach (var t in ts) {
                action(t);
            }
        }

    }
}

namespace Something.Deep.Somewhere {
    public class MyCustomGenerator : Generator {
        public MyCustomGenerator() : base() {

        }
        public override void Generate(string migrationId, IEnumerable<MigrationOperation> operations) {
            var extendedOperations = operations.ToList();
            extendedOperations.Add(new MyCustomOperation());

            base.Generate(migrationId, extendedOperations);
        }

        protected void Generate(MyCustomOperation op) {
            Console.WriteLine("<MyCustomOperation> execute.");
        }
    }
    public class MyCustomOperation : MigrationOperation {

    }
}

因此，通过将自定义的 .Each Action 执行器拆分为一个 foreach，我们可以看到，如果编码得当，它确实可以正常工作。

            //operations.Each<dynamic>(o => Generate(o));
            foreach(var op in operations) {
                Type op_type = op.GetType();
                var actionT = typeof (Action<>).MakeGenericType(op_type);
                var methodInfo = this.GetType().GetMethod("Generate", BindingFlags.NonPublic | BindingFlags.Instance, null, new Type[] { op_type }, null);
                methodInfo.Invoke(this, new object[]{ op });
            }

我现在无法解释为什么具有有效对象和实例的委托(delegate)操作无法自行解析。我想这是一个很好的例子，说明为什么你不应该使用 dynamic以一种懒惰的方式。 咳嗽 @microsoft。

如果不修改 EntityFramework 源代码，我看不到任何解决此问题的方法。dynamic的使用没关系，如果他们不使用运行时绑定(bind)扩展方法来执行该方法作为硬 <type>参数化 Action ；因为如果它被保存为 dynamic它将保持运行时绑定(bind)。事实证明(它有效!)如果你修改他们的根 Generate像这样的方法(按照上面的示例):

        public virtual void Generate(string migrationId, IEnumerable<MigrationOperation> operations) {
            Console.WriteLine("Up() method generating..");
            operations.Each<dynamic>(o => ((dynamic)this).Generate(o));
        }

注意 ((dynamic)this)类型转换前缀。附言我从 here 中收集了这种决心.

所以 3 件事:

1. 感谢您发布这个，这正是我所需要的
2. 抱歉，我没有解决方案
3. 希望在此处明确输入此 Intel 后，有头脑的人可以向我们展示一种绕过此问题的方法，而无需在 Microsoft 帐户上开票。

所以如果你想像我们一样保持代码干净，我能想到的唯一解决方案是不调用 base.Generate() , 而不是实现其源代码的复制/粘贴；这样你就可以覆盖他们的扩展方法 .Each调用 delegate .这对于 nuget 更新和不知道自己在做什么的 future 开发人员来说显然是有问题的。

人必须先弄脏才能干净。我有点不喜欢这一点。