C# 访问App.Config连接字符串时出现TypeInitializationException

我创建了一个静态实用程序类来简化对db的访问，当我运行代码“TypeInitializationException”时，遇到以下异常

来自异常的消息是

Message=The type initializer for 'VIServiceToolServiceLibrary.Database.DBUtility' threw an exception.

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使用下面的代码进行连接

var connectionString = ConfigurationManager.ConnectionStrings["vistConnectionString"].ConnectionString;
SqlConnection con = new SqlConnection(connectionString);
try { con.Open(); }

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我猜此行

private static string dbConnection=ConfigurationManager.ConnectionStrings[“vistConnectionString”]有一个错误。ConnectionString

coonnectionstring是正常的，因为它在硬编码时工作

Message=The type initializer for 'VIServiceToolServiceLibrary.Database.DBUtility' threw an exception.

var connectionString = ConfigurationManager.ConnectionStrings["vistConnectionString"].ConnectionString;
SqlConnection con = new SqlConnection(connectionString);
try { con.Open(); }