c#成员变量作用域

在我的VSTO插件和一些成员变量中有三/四个函数:

成员变量:

private Dictionary<string, string> clientDict;
private Dictionary<string, string> clientHistoryDict;

处理这些变量的函数:

public void generateClientDict()
{
        clientDict.Add("alcatel-lucent.com", "Alcatel-Lucent");
        clientDict.Add("emerson.com", "Emerson");
        clientDict.Add("ericsson.com", "Ericsson");
        clientDict.Add("fortress-technologies.com", "Fortress Technologies");
        clientDict.Add("genesys.com", "Genesys");
        clientDict.Add("hitachi.com", "Hitachi Data Systems");
        clientDict.Add("hp.com", "Hewlett Packard");
        clientDict.Add("lg.com", "LG Electronics"); 
        clientDict.Add("samsung.com", "Samsung");
        clientDict.Add("sap.com", "SAP");
        clientDict.Add("tellabs.com", "Tellabs");
        clientDict.Add("thiel-audio.com", "Thiel Audio");
        clientDict.Add("xerox.com", "Xerox");
        clientDict.Add("zebra.com", "Zebra Technologies");
        clientHistoryDict.Add("3com.com", "3Com- CommWorks");
        clientHistoryDict.Add("3m.com", "3M");
        clientHistoryDict.Add("abis.com", "ABIS");
        clientHistoryDict.Add("acxiom.com", "Acxiom");
        clientHistoryDict.Add("ajusa.com", "AJ-USA");
        clientHistoryDict.Add("akamai.com", "Akamai Technologies");
        clientHistoryDict.Add("alcatel-lucent.com", "Alcatel-Lucent");
        clientHistoryDict.Add("avaya.com", "Avaya");
        clientHistoryDict.Add("beckmancoulter.com", "Beckman Coulter");
        clientHistoryDict.Add("bellsouth.com", "BellSouth");
        clientHistoryDict.Add("bridgevine.com", "Bridgevine");
        clientHistoryDict.Add("casio.com", "Casio");
        clientHistoryDict.Add("cca.com", "CCA");
        clientHistoryDict.Add("ccs.com", "CCS");
        clientHistoryDict.Add("centurylink.com", "CenturyLink");
        clientHistoryDict.Add("chinatelecom.com", "China Telecom");
        clientHistoryDict.Add("cisco.com", "Cisco");
        clientHistoryDict.Add("comcast.com", "Comcast");
        clientHistoryDict.Add("comodo.com", "Comodo");
        clientHistoryDict.Add("comverge.com", "Comverge");
        clientHistoryDict.Add("coriant.com", "Coriant (Spin off from Tellabs)");
        clientHistoryDict.Add("daneelectric.com", "Dane Electric");
        clientHistoryDict.Add("dell.com", "Dell");
        clientHistoryDict.Add("disney.com", "Disney");
        clientHistoryDict.Add("siemens.com", "Efficient Networks- Siemens");
        clientHistoryDict.Add("emc.com", "EMC");
        clientHistoryDict.Add("emergentcommunications.com", "Emergent Communications");
        clientHistoryDict.Add("emerson.com", "Emerson");
        clientHistoryDict.Add("epson.com", "Epson");
        clientHistoryDict.Add("ericsson.com", "Ericsson");
        clientHistoryDict.Add("exigen.com", "Exigen Services");
        clientHistoryDict.Add("frbny.com", "Federal Reverse Bank of New York");
        clientHistoryDict.Add("hometeamsports.com", "Fox Home Team Sports");
        clientHistoryDict.Add("freemansoundlabs.com", "Freeman Sound Labs");
        clientHistoryDict.Add("genesys.com", "Genesys");
        clientHistoryDict.Add("here.com", "HERE, a Nokia Company");
        clientHistoryDict.Add("hp.com", "Hewlett Packard");
        clientHistoryDict.Add("hitachi.com", "Hitachi Data Systems");
        clientHistoryDict.Add("intel.com", "Intel");
        clientHistoryDict.Add("lg.com", "LG Electronics");
        clientHistoryDict.Add("samsung.com", "Samsung");
        clientHistoryDict.Add("sap.com", "SAP");
        clientHistoryDict.Add("subway.com", "Subway");
        clientHistoryDict.Add("tellabs.com", "Tellabs");
        clientHistoryDict.Add("thiel-audio.com", "Thiel Audio");
        clientHistoryDict.Add("xerox.com", "Xerox");
        clientHistoryDict.Add("zebra.com", "Zebra Technologies");
}

现在这个函数与成员变量一起工作。(所有这些都在同一类)。但是这些函数没有:

public void populateClientDict(SqlConnection conn)
{
        //Dictionary<string, string> clientDict = new Dictionary<string, string>();  If I don't add this I get an error
        try
        {
            using (conn)
            {
                SqlCommand command = new SqlCommand(
                  @"SELECT ClientDirName, ClientEmailDomain FROM ClientTable;",
                  conn);
                conn.Open();
                SqlDataReader reader = command.ExecuteReader();
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        string clientDir = reader.GetString(0);
                        string clientEmail = reader.GetString(1);
                        clientDict.Add(clientEmail, clientDir);
                    }
                }
                else
                {
                    MessageBox.Show("No rows found in ClientTable", "Rows Not Found", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
                reader.Close();
            }
        }
        catch (InvalidOperationException ex)
        {
            MessageBox.Show(String.Format("Exception while accessing ClientTable: {0}", ex), "Exception", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        catch (SqlException ex)
        {
            MessageBox.Show(String.Format("Exception while accessing ClientTable: {0}", ex), "Exception", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
}
public void populateClientHistoryDict(SqlConnection conn)
{
        //Dictionary<string, string> clientHistoryDict = new Dictionary<string, string>(); if I don't add this I get an error
        try
        {
            using (conn)
            {
                SqlCommand command = new SqlCommand(
                  @"SELECT ClientDirName, ClientEmailDomain FROM ClientHistoryTable;",
                  conn);
                conn.Open();
                SqlDataReader reader = command.ExecuteReader();
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        string clientDir = reader.GetString(0);
                        string clientEmail = reader.GetString(1);
                        clientHistoryDict.Add(clientEmail, clientDir);
                    }
                }
                else
                {
                    MessageBox.Show("No rows found in ClientHistoryTable", "Rows Not Found", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }
                reader.Close();
            }
        }
        catch (InvalidOperationException ex)
        {
            MessageBox.Show(String.Format("Exception while accessing ClientHistoryTable: {0}", ex), "Exception", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        catch (SqlException ex)
        {
            MessageBox.Show(String.Format("Exception while accessing ClientHistoryTable: {0}", ex), "Exception", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
}

正如我在函数的注释行中所写的那样，除非我在这些函数中声明字典，否则我会得到这个错误:

我得到了这一行的错误:

populateClientDict()中的clientDict.Add(clientEmail, clientDir);和populateClientHistoryDict()中的clientHistoryDict.Add(clientEmail, clientDir);

An exception of type 'System.NullReferenceException' occurred in Archive.dll but was not handled in user code
Additional information: Object reference not set to an instance of an object.

我有一种感觉，这个错误与函数的这一部分有关，它说:

while (reader.Read())
{
clientDir = reader.GetString(0);
string clientEmail = reader.GetString(1);
clientDict.Add(clientEmail, clientDir);
}

ID为第一列，clientDir为第二列，clientEmail为第三列。也许我用错了reader.GetString() ?我在某个地方读到，我可以做一些像reader.GetString["ClientDirName"] (ClientDirName是列名)，但我不确定正确的事情是什么。

这会导致错误吗?如果是这样，我如何正确访问第二和第三列(假设ID是第一列)以避免此错误。

如果不是，还有什么可以引起它?

我已经尝试了大量的组合，正如我所说的，如果我在函数中移动字典实例化，它就会起作用，但我不认为这解决了实际问题。

感谢您的帮助

private Dictionary<string, string> clientDict=new Dictionary<string, string>();
private Dictionary<string, string> clientHistoryDict=new Dictionary<string, string>();