c# - 为什么密封类不允许是泛型类型参数？

Question

我有:

public abstract class DbManager<ConnectionType>
    where ConnectionType : class, IDbConnection, new() {...}

现在我分别有:

public class Db2Manager : DbManager<iDB2Connection>      // OK, NO problem
public class OleDbManager : DbManager<OleDbConnection>   // OK, NO problem
public class SqlServerManager : DbManager<SqlConnection> // OK, NO problem
public class SsasManager : DbManager<AdomdConnection>    // NOK, YES, a PROBLEM!

这是为什么？因为在命名空间 Microsoft.AnalysisServices.AdomdClient 中我们有一个密封类:

// Assembly Microsoft.AnalysisServices.AdomdClient.dll, v2.0.50727
using System;
using System.ComponentModel;
using System.Data;

namespace Microsoft.AnalysisServices.AdomdClient
{
    public sealed class AdomdConnection : Component, IDbConnection, 
                                                     IDisposable, ICloneable
    {
        public AdomdConnection();

a) 为什么限制使用密封类作为泛型参数(!参数，而不是约束!)？
b) 如何解决这个问题？
c*) 密封该类是个好主意吗？

附言。也许您有很多理由认为这个通用 DbManager 不是一个好主意，但是，无论如何，我只想了解是否有可能在通用参数中解决这个密封类...谢谢!

PPS。错误信息:

'Microsoft.AnalysisServices.AdomdClient.AdomdConnection' must be a non-abstract type with a public parameterless constructor in order to use it as parameter in the generic type or method.

另一条错误消息指定:

The type 'Microsoft.AnalysisServices.AdomdClient.AdomdConnection' cannot be used as type parameter in the generic type or method there is no boxing conversion or type parameter conversion from 'Microsoft.AnalysisServices.AdomdClient.AdomdConnection' to 'System.Data.IDbConnection'

AdomdConnection 有一个无参数公共(public)构造函数。

Answer 1

我相信密封类没有这样的限制。以下代码编译正常，因此可能还有其他问题。

public abstract class DbManager<T> where T : class, IDbConnection, new()
{
}

public class ConcreteDbManagerUsingSealedClass : DbManager<MySealedConnection>
{
}

public sealed class MySealedConnection : IDbConnection
{
    public IDbTransaction BeginTransaction(IsolationLevel il)
    {
        throw new NotImplementedException();
    }

    public IDbTransaction BeginTransaction()
    {
        throw new NotImplementedException();
    }

    public void ChangeDatabase(string databaseName)
    {
        throw new NotImplementedException();
    }

    public void Close()
    {
        throw new NotImplementedException();
    }

    public string ConnectionString
    {
        get
        {
            throw new NotImplementedException();
        }
        set
        {
            throw new NotImplementedException();
        }
    }

    public int ConnectionTimeout
    {
        get { throw new NotImplementedException(); }
    }

    public IDbCommand CreateCommand()
    {
        throw new NotImplementedException();
    }

    public string Database
    {
        get { throw new NotImplementedException(); }
    }

    public void Open()
    {
        throw new NotImplementedException();
    }

    public ConnectionState State
    {
        get { throw new NotImplementedException(); }
    }

    public void Dispose()
    {
        throw new NotImplementedException();
    }
}