C# Generics – Why Generic Interface Cannot Implement Dynamic Type

cdynamicdynamic-typinggenericsinterfaces

If it possible

IList <dynamic> = new List <dynamic>;

class A <T> { }

class B: A <dynamic> { }

Why it is not possible to do

class U: IEnumerable <dynamic> {}

Best Answer

This is not allowed, as Chris Burrows (who helped create and implement dynamic) explains:

Well, for one thing, it doesn’t actually give you anything that you didn’t already have. The first thing and the second thing are already there if you implemented IEnumerable<object>. In that case, you still would have been able to define GetEnumerator the way we did, and you still can convert C to IEnumerable<dynamic> (again, because of the structural conversions). Think of it this way: if anyone ever looks directly at your type C, they are never going to “see” what interfaces you implement. They only “see” them when they cast, and at that point, your IEnumerable<dynamic> didn’t do them any good.

That’s a fine reason, but you might respond, why not let me do this anyway? Why impose this limitation that seems artificial? Good question. I encountered this for the first time when I was trying to get the compiler to emit these things, and I realized very quickly that there was no where for me to emit the [Dynamic] attribute that we use to mark dynamic types. The metadata team reported that a reading of the CLI spec seemed to indicate that the tables for interface implementations and custom attributes might have permitted it, but anyway no one we know of has ever done this, and it would have been effort expended. We have priorities and a limited budget of time, and this didn’t make the cut.

Related Solutions

C# – Implement Generic DataSet Builder with C#

Here is a link describing this approach.

Below is my resultant class based on Dan's suggestion

using System;
using System.Data;
using System.Data.Common;

namespace CustomDataAccess{

public class DataSetBuilder
{

    #region Properties

    private DataSet _DataSet;

    public DataSet DataSet { get { return _DataSet; } }

    #endregion

    #region Constructors

    public DataSetBuilder()
    {
        this._DataSet = new DataSet();
    }

    public DataSetBuilder(string DataSetName)
    {
        this._DataSet = new DataSet(DataSetName);
    }

    public DataSetBuilder(DataSet DataSet)
    {
        this._DataSet = DataSet;
    }

    #endregion

    #region Public Methods

    public DataSetBuilder InsertTables(DataTable Table)
    {
        this._DataSet.Tables.Add(Table);

        return this;
    }

    public DataSetBuilder InsertTables(string DbProviderName, string ConnectionString, string TableName, string CommandText)
    {
        System.Data.Common.DbDataAdapter adapter = Create_Adapter(DbProviderName, ConnectionString);

        Fill_Adapter(adapter, TableName, CommandText);

        adapter.SelectCommand.Connection.Close();

        return this;
    }

    public DataSetBuilder InsertTables(string DbProviderName, string ConnectionString, string[] TableName, string[] CommandText)
    {
        if (TableName.Length != CommandText.Length)
        {
            throw new Exception("Error: Must provide a table name for each command.");
        }

        System.Data.Common.DbDataAdapter adapter = Create_Adapter(DbProviderName, ConnectionString);

        for (int i = 0; i < TableName.Length; i++)
        {

            Fill_Adapter(adapter, TableName[i], CommandText[i]);
        }

        adapter.SelectCommand.Connection.Close();

        return this;
    }

    public void AddRelations(string ParentTable, string PrimaryKey, string ChildTable, string ForeignKey, bool NestingRule)
    {
        Add_Relations(ParentTable, PrimaryKey, ChildTable, ForeignKey, NestingRule);
    }

    public void AddRelations(string[] ParentTable, string[] PrimaryKey, string[] ChildTable, string[] ForeignKey, bool[] NestingRule)
    {

        for (int i = 0; i < ParentTable.Length; i++)
        {
            Add_Relations(ParentTable[i], PrimaryKey[i], ChildTable[i], ForeignKey[i], NestingRule[i]);
        }
    }

    #endregion

    #region Private Methods

    private System.Data.Common.DbDataAdapter Create_Adapter(string DbProviderName, string ConnectionString)
    {
        DbProviderFactory dbFactory = System.Data.Common.DbProviderFactories.GetFactory(DbProviderName);

        System.Data.Common.DbConnection connection = dbFactory.CreateConnection();

        connection.ConnectionString = ConnectionString;

        connection.Open();

        System.Data.Common.DbCommand command = dbFactory.CreateCommand();

        command.Connection = connection;

        System.Data.Common.DbDataAdapter adapter = dbFactory.CreateDataAdapter();

        adapter.SelectCommand = command;

        return adapter;
    }

    private void Fill_Adapter(System.Data.Common.DbDataAdapter Adapter, string TableName, string CommandText)
    {
        Adapter.SelectCommand.CommandText = CommandText;

        Adapter.Fill(_DataSet, TableName);
    }

    private void Add_Relations(string ParentTable, string PrimaryKey, string ChildTable, string ForeignKey, bool NestingRule)
    {
        DataColumn pk = _DataSet.Tables[ParentTable].Columns[PrimaryKey];

        DataColumn fk = _DataSet.Tables[ChildTable].Columns[ForeignKey];

        DataRelation relation = _DataSet.Relations.Add(pk, fk);

        relation.Nested = NestingRule;
    }

    #endregion

}}

C# – Make Return Type an Interface – Problem with Initialization

An implementation of an interface is the realization of that interface. Realization is being implemented within the class. Just return an instance of the class (A or B) which realizes the return-type interface.

interface IA{}   
class A: IA{}

class UseIA
{
  public IA DesiredMethod()
  {
    return new A();
  }
}

Best Answer

Related Solutions

C# – Implement Generic DataSet Builder with C#

C# – Make Return Type an Interface – Problem with Initialization

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