实体框架在运行时更改连接

我有一个 web API 项目，它引用了我的模型和 DAL 程序集。用户会看到一个登录屏幕，在那里他可以选择不同的数据库。

我按如下方式构建连接字符串:

    public void Connect(Database database)
{
//Build an SQL connection string
SqlConnectionStringBuilder sqlString = new SqlConnectionStringBuilder()
{
DataSource = database.Server,
InitialCatalog = database.Catalog,
UserID = database.Username,
Password = database.Password,
};


//Build an entity framework connection string
EntityConnectionStringBuilder entityString = new EntityConnectionStringBuilder()
{
Provider = database.Provider,
Metadata = Settings.Default.Metadata,
ProviderConnectionString = sqlString.ToString()
};
}

首先，我实际上如何更改数据上下文的连接？

其次，因为这是一个 web API 项目，所以连接字符串(设置在以上每次登录时)是否在整个用户交互过程中持久存在，还是应该每次都将其传递给我的数据上下文？

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小开

Add multiple connection strings in your web.config or app.config.

Then you can get them as a string like :

System.Configuration.ConfigurationManager.
ConnectionStrings["entityFrameworkConnection"].ConnectionString;

Then use the string to set :

Provider
Metadata
ProviderConnectionString

It is better explained here :

Read connection string from web.config

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string _connString = "metadata=res://*/Model.csdl|res://*/Model.ssdl|res://*/Model.msl;provider=System.Data.SqlClient;provider connection string="data source=localhost;initial catalog=DATABASE;persist security info=True;user id=sa;password=YourPassword;multipleactiveresultsets=True;App=EntityFramework"";


EntityConnectionStringBuilder ecsb = new EntityConnectionStringBuilder(_connString);
ctx = new Entities(_connString);

You can get the connection string from the web.config, and just set that in the EntityConnectionStringBuilder constructor, and use the EntityConnectionStringBuilder as an argument in the constructor for the context.

Cache the connection string by username. Simple example using a couple of generic methods to handle adding/retrieving from cache. 

private static readonly ObjectCache cache = MemoryCache.Default;


// add to cache
AddToCache<string>(username, value);


// get from cache


string value = GetFromCache<string>(username);
if (value != null)
{
// got item, do something with it.
}
else
{
// item does not exist in cache.
}




public void AddToCache<T>(string token, T item)
{
cache.Add(token, item, DateTime.Now.AddMinutes(1));
}


public T GetFromCache<T>(string cacheKey) where T : class
{
try
{
return (T)cache[cacheKey];
}
catch
{
return null;
}
}
小开

DbContext has a constructor overload that accepts the name of a connection string or a connection string itself. Implement your own version and pass it to the base constructor:

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

Then simply pass the name of a configured connection string or a connection string itself when you instantiate your DbContext

var context = new MyDbContext( "..." );
小开
最佳答案

A bit late on this answer but I think there's a potential way to do this with a neat little extension method. We can take advantage of the EF convention over configuration plus a few little framework calls.

Anyway, the commented code and example usage:

extension method class:

public static class ConnectionTools
{
// all params are optional
public static void ChangeDatabase(
this DbContext source,
string initialCatalog = "",
string dataSource = "",
string userId = "",
string password = "",
bool integratedSecuity = true,
string configConnectionStringName = "")
/* this would be used if the
*  connectionString name varied from
*  the base EF class name */
{
try
{
// use the const name if it's not null, otherwise
// using the convention of connection string = EF contextname
// grab the type name and we're done
var configNameEf = string.IsNullOrEmpty(configConnectionStringName)
? source.GetType().Name
: configConnectionStringName;


// add a reference to System.Configuration
var entityCnxStringBuilder = new EntityConnectionStringBuilder
(System.Configuration.ConfigurationManager
.ConnectionStrings[configNameEf].ConnectionString);


// init the sqlbuilder with the full EF connectionstring cargo
var sqlCnxStringBuilder = new SqlConnectionStringBuilder
(entityCnxStringBuilder.ProviderConnectionString);


// only populate parameters with values if added
if (!string.IsNullOrEmpty(initialCatalog))
sqlCnxStringBuilder.InitialCatalog = initialCatalog;
if (!string.IsNullOrEmpty(dataSource))
sqlCnxStringBuilder.DataSource = dataSource;
if (!string.IsNullOrEmpty(userId))
sqlCnxStringBuilder.UserID = userId;
if (!string.IsNullOrEmpty(password))
sqlCnxStringBuilder.Password = password;


// set the integrated security status
sqlCnxStringBuilder.IntegratedSecurity = integratedSecuity;


// now flip the properties that were changed
source.Database.Connection.ConnectionString
= sqlCnxStringBuilder.ConnectionString;
}
catch (Exception ex)
{
// set log item if required
}
}
}

basic usage:

// assumes a connectionString name in .config of MyDbEntities
var selectedDb = new MyDbEntities();
// so only reference the changed properties
// using the object parameters by name
selectedDb.ChangeDatabase
(
initialCatalog: "name-of-another-initialcatalog",
userId: "jackthelady",
password: "nomoresecrets",
dataSource: @".\sqlexpress" // could be ip address 120.273.435.167 etc
);

I know you already have the basic functionality in place, but thought this would add a little diversity.

小开

The created class is 'partial'!

public partial class Database1Entities1 : DbContext
{
public Database1Entities1()
: base("name=Database1Entities1")
{
}

... and you call it like this:

using (var ctx = new Database1Entities1())
{
#if DEBUG
ctx.Database.Log = Console.Write;
#endif

so, you need only create a partial own class file for original auto-generated class (with same class name!) and add a new constructor with connection string parameter, like Moho's answer before.

After it you able to use parametrized constructor against original. :-)

example:

using (var ctx = new Database1Entities1(myOwnConnectionString))
{
#if DEBUG
ctx.Database.Log = Console.Write;
#endif
小开

In my case I'm using the ObjectContext as opposed to the DbContext so I tweaked the code in the accepted answer for that purpose.

public static class ConnectionTools
{
public static void ChangeDatabase(
this ObjectContext source,
string initialCatalog = "",
string dataSource = "",
string userId = "",
string password = "",
bool integratedSecuity = true,
string configConnectionStringName = "")
{
try
{
// use the const name if it's not null, otherwise
// using the convention of connection string = EF contextname
// grab the type name and we're done
var configNameEf = string.IsNullOrEmpty(configConnectionStringName)
? Source.GetType().Name
: configConnectionStringName;


// add a reference to System.Configuration
var entityCnxStringBuilder = new EntityConnectionStringBuilder
(System.Configuration.ConfigurationManager
.ConnectionStrings[configNameEf].ConnectionString);


// init the sqlbuilder with the full EF connectionstring cargo
var sqlCnxStringBuilder = new SqlConnectionStringBuilder
(entityCnxStringBuilder.ProviderConnectionString);


// only populate parameters with values if added
if (!string.IsNullOrEmpty(initialCatalog))
sqlCnxStringBuilder.InitialCatalog = initialCatalog;
if (!string.IsNullOrEmpty(dataSource))
sqlCnxStringBuilder.DataSource = dataSource;
if (!string.IsNullOrEmpty(userId))
sqlCnxStringBuilder.UserID = userId;
if (!string.IsNullOrEmpty(password))
sqlCnxStringBuilder.Password = password;


// set the integrated security status
sqlCnxStringBuilder.IntegratedSecurity = integratedSecuity;


// now flip the properties that were changed
source.Connection.ConnectionString
= sqlCnxStringBuilder.ConnectionString;
}
catch (Exception ex)
{
// set log item if required
}
}
}
小开 
Linq2SQLDataClassesDataContext db = new Linq2SQLDataClassesDataContext();


var query = from p in db.SyncAudits orderby p.SyncTime descending select p;
Console.WriteLine(query.ToString());

try this code...

小开

Jim Tollan's answer works great, but I got the Error: Keyword not supported 'data source'. To solve this problem I had to change this part of his code:

// add a reference to System.Configuration
var entityCnxStringBuilder = new EntityConnectionStringBuilder
(System.Configuration.ConfigurationManager
.ConnectionStrings[configNameEf].ConnectionString);

to this:

// add a reference to System.Configuration
var entityCnxStringBuilder = new EntityConnectionStringBuilder
{
ProviderConnectionString = new  SqlConnectionStringBuilder(System.Configuration.ConfigurationManager
.ConnectionStrings[configNameEf].ConnectionString).ConnectionString
};

I'm really sorry. I know that I should't use answers to respond to other answers, but my answer is too long for a comment :(

小开

I wanted to have multiple datasources in the app config. So after setting up a section in the app.config i swaped out the datasource and then pass it into the dbcontext as the connection string. 

//Get the key/value connection string from app config
var sect = (NameValueCollection)ConfigurationManager.GetSection("section");
var val = sect["New DataSource"].ToString();


//Get the original connection string with the full payload
var entityCnxStringBuilder = new EntityConnectionStringBuilder(ConfigurationManager.ConnectionStrings["OriginalStringBuiltByADO.Net"].ConnectionString);


//Swap out the provider specific connection string
entityCnxStringBuilder.ProviderConnectionString = val;


//Return the payload with the change in connection string.
return entityCnxStringBuilder.ConnectionString;

This took me a bit to figure out. I hope it helps someone out. I was making it way too complicated. before this.

小开

I have two extension methods to convert the normal connection string to the Entity Framework format. This version working well with class library projects without copying the connection strings from app.config file to the primary project. This is VB.Net but easy to convert to C#.

Public Module Extensions


<Extension>
Public Function ToEntityConnectionString(ByRef sqlClientConnStr As String, ByVal modelFileName As String, Optional ByVal multipleActiceResultSet As Boolean = True)
Dim sqlb As New SqlConnectionStringBuilder(sqlClientConnStr)
Return ToEntityConnectionString(sqlb, modelFileName, multipleActiceResultSet)
End Function


<Extension>
Public Function ToEntityConnectionString(ByRef sqlClientConnStrBldr As SqlConnectionStringBuilder, ByVal modelFileName As String, Optional ByVal multipleActiceResultSet As Boolean = True)
sqlClientConnStrBldr.MultipleActiveResultSets = multipleActiceResultSet
sqlClientConnStrBldr.ApplicationName = "EntityFramework"


Dim metaData As String = "metadata=res://*/{0}.csdl|res://*/{0}.ssdl|res://*/{0}.msl;provider=System.Data.SqlClient;provider connection string='{1}'"
Return String.Format(metaData, modelFileName, sqlClientConnStrBldr.ConnectionString)
End Function


End Module

After that I create a partial class for DbContext:

Partial Public Class DlmsDataContext


Public Shared Property ModelFileName As String = "AvrEntities" ' (AvrEntities.edmx)


Public Sub New(ByVal avrConnectionString As String)
MyBase.New(CStr(avrConnectionString.ToEntityConnectionString(ModelFileName, True)))
End Sub


End Class

Creating a query:

Dim newConnectionString As String = "Data Source=.\SQLEXPRESS;Initial Catalog=DB;Persist Security Info=True;User ID=sa;Password=pass"


Using ctx As New DlmsDataContext(newConnectionString)
' ...
ctx.SaveChanges()
End Using
小开

For both SQL Server and SQLite Databases, use:

_sqlServerDBsContext = new SqlServerDBsContext(new DbContextOptionsBuilder<SqlServerDBsContext>().UseSqlServer("Connection String to SQL DB").Options);

For SQLite, make sure Microsoft.EntityFrameworkCore.Sqlite is installed, then the connection string is simply "'DataSource='+ the file name".

_sqliteDBsContext = new SqliteDBsContext(new DbContextOptionsBuilder<SqliteDBsContext>().UseSqlite("Connection String to SQLite DB").Options);
小开

well if you are working with EFCore, Then You can do something like to create a new connection string: In Your Context File (For Sqlite)

public biorevContext(string connectionString) : base(GetOptions(connectionString))
{
this.Database.EnsureCreated();
}
private static DbContextOptions GetOptions(string connectionString)
{
return SqliteDbContextOptionsBuilderExtensions.UseSqlite(new DbContextOptionsBuilder(), connectionString).Options;
}

For MySql:

 public biorevContext(string connectionString) : base(GetOptions(connectionString))
{
this.Database.EnsureCreated();
}
private static DbContextOptions GetOptions(string connectionString)
{
return MySQLDbContextOptionsExtensions.UseMySQL(new DbContextOptionsBuilder(), connectionString).Options;
}

For Sql:

    public biorevContext(string connectionString) : base(GetOptions(connectionString))
{
this.Database.EnsureCreated();
}
private static DbContextOptions GetOptions(string connectionString)
{
return SqlServerDbContextOptionsExtensions.UseSqlServer(new DbContextOptionsBuilder(), connectionString).Options;
}

and Then You can use it like this:

        var context = new biorevContext("connectionString");
小开

You can do this on-the-fly with an IDbConnectionInterceptor. This has the advantage of allowing you to work with a standard connection string and not the Entity Client version, and also not having to modify the auto-generated context classes in an EDMX model, or using overloaded constructors. It just works!

We use this, for instance, to replace a tokenized connection string with a password from a secrets vault.

First, implement the interface. I'm only showing one of the many interface methods that will need to be implemented. In this case, I'm implementing ConnectionStringGetting, and leaving all other method bodies empty:

public class SecretsDbConnectionInterceptor : IDbConnectionInterceptor
{
public void ConnectionStringGetting(DbConnection connection, DbConnectionInterceptionContext<string> interceptionContext)
{
var originalConnectionString = connection.ConnectionString;
try
{
connection.ConnectionString = /* Build your new connection string */;
}
catch (Exception e)
{
connection.ConnectionString = originalConnectionString;
Trace.WriteLine(e.Message);
}
}
    

// ... Many other methods here; leave them empty
}

You can wire this up via your .config file; just add an <interceptor /> to the existing <entityFramework /> node with your new inteceptor's fully qualified type name:

  <entityFramework>
<interceptors>
<interceptor type="Foo.Bar.SecretsDbConnectionInterceptor, Foo.Bar" />
</interceptors>
...
</entityFramework>

Or, my personal preference, you can wire it up via code. It is equivalent to the config version. Ideally this would go in an Application_Startup in a service/UI project, or towards the top of Main in a console app, because it must run before you attempt to establish any new DbContexts:

DbInterception.Add(new Foo.Bar.SecretsDbConnectionInterceptor());

When you configure via code, you could pass parameters to your interceptor constructor, or use DI.

Note: the interceptor code runs every time you create a new instance of any DbContext in your application, so beware of performance impacts. You could implement some caching strategy within your interceptor, or make it a singleton instance with a context name/connection string mapping, or something smart like that.


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