从部分类c＃调用Base实体构造函数

Question

Consider the below scenario,

I have entity created by DB First approach,

// auto-generated class
public partial DBEntity : DBContext
{
   public DBEntity ()
        : base("name=DBEntity ") // gets connection string from api.config
    {
    }
}

I have a situation where I need to build and pass the connection string at times, So I created this,

public partial class DBEntity 
{
    public DBEntity (String connectionString)
    {
        if (string.IsNullOrEmpty(connectionString))
            new DBEntity ();
        else
            base(connectionString); // this one fails
    }
}

What would be the best way to

1. call the parameterless entity constructor if connection string is empty?
2. call the base constructor from the parametrized constructor if conn string is not empty and achieve what happens below,

public DBEntity (String connectionString) : base (connectionString)

I will always be doing a new DBEntity(string)

Answer 1

You can not change the reference of the object being created in constructor. In another words when you call a constructor the object is initialized in memory and you are just setting it up.

Instead you should use a static method which allows you to control the return type. You can put it in another class like DbEntityFactory and call DbEntityFactory.Create or put it in your DbEntity class and call DbEntity.Create .

public static DBEntity Create(String connectionString)
{
    if (string.IsNullOrWhiteSpace(connectionString)) //use this instead of IsNullOrEmpty
        return new DBEntity();
    else
        return new DBEntity(connectionString); 
}

Answer 2

You can't delay the call to the base constructor. The base constructor always fires first. This is to prevent you have a half-initialized class.

I think you should create two constructors if you want to deviate the call to the base class. Or, as dotctor mentions: create a factory class. If that will work may depend on the need to use generics and initialize instances from those generic types.

Answer 3

At the moment you are allowing a null or empty string as a value for connectionString , and I think that is wrong.

If you want to use the constructor that has an argument connectionString , then you have to supply a valid value. If you do it this way, then the whole problem goes away, since that constructor doesn't have the duality anymore.

Answer 4

按照“构造函数中没有逻辑”这个简单的规则，我将提供两个构造函数 - 一个有一个，一个没有参数。