WinForm 上的 System.Configuration.ConfigurationErrorsException

我得到一个

“ConnectionString”引发了“System.Configuration.ConfigurationErrorsException”类型的异常

每次我尝试运行我的 WinForm 时。
知道如何解决吗？

这是我的 C# 代码：

public class DB
{
    /// <summary>
    /// 
    /// </summary>
    public static string ConnectionString
    {
        get
        {
            string connStr = ConfigurationManager.ConnectionStrings["Stoica Mihai"].ConnectionString;

            OracleConnectionStringBuilder sb = new OracleConnectionStringBuilder(connStr);
            //sb.ApplicationName = ApplicationName ?? sb.ApplicationName;
            //sb.ConnectTimeout = (ConnectionTimeout > 0) ? ConnectionTimeout : sb.ConnectTimeout;

            return sb.ToString();
        }
    }
    /// <summary>
    /// Returns an opened connection to the database
    /// </summary>
    /// <returns></returns>
    public static OracleConnection GetSqlConnection()
    {
        OracleConnection conn = new OracleConnection(ConnectionString);
        conn.Open();
        return conn;

    }
    /// <summary>
    /// Override the connection timeout
    /// </summary>
    public static int ConnectionTimeout { get; set; }
    /// <summary>
    /// Used to override the name of the application
    /// </summary>
    public static string ApplicationName { get; set; }
}