实体框架DBContext和动态ConnectionString

我想在Windows应用程序中动态地使用EF 4进行连接。当尝试busar记录时，给出错误:关键字不支持:'provider'。

代码:var nota = db.NotasFiscais.Find(28406);

代码如下

主应用程序

[STAThread]
static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    NF.DataBase.Global.Conexao = new System.Data.EntityClient.EntityConnectionStringBuilder();
    NF.DataBase.Global.Conexao.Provider = "System.Data.SqlClient";
    NF.DataBase.Global.Conexao.ProviderConnectionString = "Data Source=source;Initial Catalog=base;User ID=sa; Password=pass;";
    Application.Run(new Form1());
}

DBContext

public class ERPContext : DbContext
{
    public DbSet<NotasFiscais> NotasFiscais { get; set; }
    public DbSet<NFEmissores> NFEmissores { get; set; }
    public DbSet<Cidades> Cidades { get; set; }
    public ERPContext()
        : base(NF.DataBase.Global.Conexao.ToString())
    {
        Database.SetInitializer<ERPContext>(null);
    }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<NotasFiscais>().ToTable("NotasFiscais");
        modelBuilder.Entity<NFEmissores>().ToTable("NFEmissores");
        modelBuilder.Entity<Cidades>().ToTable("Cidades");
    }
}

字体:

private void button1_Click(object sender, EventArgs e)
{
    var db = new ERPContext();
    var nota = db.NotasFiscais.Find(28406);
    ...
}

  Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    NF.DataBase.Global.Conexao = new System.Data.EntityClient.EntityConnectionStringBuilder();
    NF.DataBase.Global.Conexao.Provider = "System.Data.SqlClient";
    NF.DataBase.Global.Conexao.ProviderConnectionString = "Data Source=source;Initial Catalog=base;User ID=user; Password=pass;";

  Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    NF.DataBase.Global.Conexao = new System.Data.EntityClient.EntityConnectionStringBuilder();
// not provider
    NF.DataBase.Global.Conexao.ProviderConnectionString = "Data Source=source;Initial Catalog=base;User ID=user; Password=pass;";