启用不同的方法，但使用相同的名称-C＃

Question

I realise that the title may be very confusing, so to explain what I would like to do, I will illustrate how I can easily solve the same problem in PHP.

Eg I give the user of my application a choice database types (ie MySQL, MSSQL, etc).

In my code I would like to have only one function for each operation rather than a function for each database as I explain below.

NOT

main.php

<?php
function mysql_get_users() {
  //Foo
}

function mssql_get_users() {
  //Foo
}

if($dbtype == "mysql") {
  echo mysql_get_users();
}

if($dbtype == "mssql") {
  echo mssql_get_users();
}

?>

BUT

main.php

<?php
if($dbtype == "mysql") {
  include("mysql.php");
}

if($dbtype == "mssql") {
  include("mssql.php");
}
echo get_users();
?>

mysql.php

<?php
function get_users() {
  //Foo
}
?>

mssql.php

<?php
function get_users() {
  //Foo
}
?>

Now I have the same problem, but in C# using Visual Studio 2010. Since I cannot do this:

 if(dbtype=="mysql") {
    public static MySqlConnection connect(String connectionString)
    {
        MySqlConnection connection = new MySqlConnection(connectionString);
        connection.Open();
        return connection;
    }
}

The big problem with the above is particularly that I cannot put an if-clause around a method(function), then I have to face the problems of the types which are unique to each database type amongst the fact that I have to include the returned connection variable in each method related to database function.

So, any thoughts on how to fix this and make it work like the PHP example above? I am neither an expert nor a newbie on C#, but I learn quite quickly.

Answer 1

You will want to research "interfaces" or "delegates".

Interfaces are more common across various languages (including C# and Java), and allow you to provide different implementations (through implementing classes) that all share the same interfaces, and can be used interchangeably.

Delegates are also a rather cool option, but are limited to one method - which may actually be what you're looking for here. (Compared to an interface, which defines one or more methods that exist as part of the interface).

Some additional references:

• Interfaces (C# Programming Guide): http://msdn.microsoft.com/en-us/library/ms173156.aspx
• Delegates tutorial: http://msdn.microsoft.com/en-us/library/aa288459%28v=vs.71%29.aspx
• When to Use Delegates Instead of Interfaces (C# Programming Guide): http://msdn.microsoft.com/en-us/library/ms173173.aspx

Answer 2

Personally, I think you need to approach the problem differently. This is where object-oriented programming is your friend. Basically, you would abstract out the database-specific stuff so the calling code knows nothing about which database it is using. If you were doing it from scratch, here is a very over-simplified example:

public interface IDatabase {
   void Connect(string ...);
   string Query(string ...);
}

public class MySQLDB : IDatabase {
   void Connect(string ...){
       MySqlConnection connection = new MySqlConnection(connectionString);
       ...
   }

    // same for query, etc
}

public class OracleDB : IDatabase {
   void Connect(string ...){
       // connect using Oracle library
       ...
   }

    // same for query, etc
}

So then, in your client code, you choose which type of database to instantiate, and then call the same functions regardless of what kind it is:

IDatabase db;
if (dbtype == "mysql")
   db = new MySqlDB();
else if (dbtype == "oracle")
   db = new OracleDB();

db.Connect(...);
...

Answer 3

public static IDbConnection connect(String connectionString)
{
    if ( dbtype == "mysql" )
    {
        MySqlConnection connection = new MySqlConnection(connectionString);
        connection.Open();
        return connection;
    }
    else if ( dbtype == "mssql" )
    {
        SqlConnection connection = new SqlConnection(connectionString);
        connection.Open();
        return connection;
    }
}

Answer 4

Maybe you can choose Factory Method or Strategy to do this. Basically you can create a interface for the work:

public interface ConnectionFactory {
    IDBConnection CreateConnection(string connectionString);
}

Then give them different implementation based on the type .