Passing string into a Constructor of DbContext with a Base class is not setting the value

问题

I have decide out of a business need to make a change to my DbContext class

Previously my code looked like this:

public class RPMContext : DbContext
{
    public RPMContext() : base(ConnectionString()) { }

    private static string ConnectionString(string connection)
    {
        conn = ConfigurationManager.ConnectionStrings["LH_RPMContext"].ConnectionString;
        return conn;
    }
}

Above "Works" but I need to be able to PASS in a string in which I'm able to set the static method ConnectionString to OTHER specified connection strings

So Thus my NEW code started down a path of NOT use base

public class RPMContext : DbContext
{

    public RPMContext(string environment)
    {
        ConnectionString(environment);
    }  

    private static string ConnectionString(string connection)
    {

        string conn;

        if (connection == "LH")
        {
            conn = ConfigurationManager.ConnectionStrings["LH_RPMContext"].ConnectionString;
        }
        else 
        {
          // dev , qa  etc..
        }

        return conn;
    }


}

PROBLEMS

I suppose this are several issue in which I'm not understanding that

I need to be "setting the DbContext in which the base does that?
Do I NEED base for what I am doing?
Would :this() be of benefit?

Previously I would instantiate with private RPMContext db = new RPMContext();

Now RPMContext("LH") and whatever string I wish /want to use.

Obviously with the current code, I'm getting "use of unassigned local variable conn"

回答1:

You are giving yourself unnecessary work.

DbContext already has a constructor that takes a full connection string or the name of the connection string in your config file.

public class RPMContext : DbContext {    
    public RPMContext(string nameOrConnectionString)
        :base(nameOrConnectionString) {
    }  
}

if for example you have the following in your config file

<appSettings>
    <add key="Environment" value="LH" />
</appSettings>
<connectionStrings>
    <add name="LH_RPMContext" connectionString="..." providerName="System.Data.EntityClient" />
    <add name="DEV_RPMContext" connectionString="..." providerName="System.Data.EntityClient" />
    <add name="QA_RPMContext" connectionString="..." providerName="System.Data.EntityClient" />
</connectionStrings>

You can create the DbContext you want by providing the name connection you want to use.

var db = new RPMContext("name=LH_RPMContext");
var dev_db = new RPMContext("name=DEV_RPMContext");
var qa_db = new RPMContext("name=QA_RPMContext");

or... based on some of your previous posts, you can do something like this...

public class RPMContext : DbContext {

    public RPMContext()
        :this(string.Format("name={0}_RPMContext",ConfigurationManager.AppSettings["Environment"])){}

    public RPMContext(string nameOrConnectionString)
        :base(nameOrConnectionString) {
    }  
}

来源：https://stackoverflow.com/questions/37537771/passing-string-into-a-constructor-of-dbcontext-with-a-base-class-is-not-setting

标签

.net

entity-framework