Question

我试图从类中的循环返回一个数组，但是这样做时我收到错误“NullReferenceException is Unhandled”

我可以返回一个没有任何问题的字符串，但是返回一个数组会带来这个错误。

这是我的代码：

class OleDB
{
    string[] test;

    public string [] readexcel()
    {

        //Below runs the OleDB connections which essentially turns excel into an Access database.

        string conn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:/Users/nlongmore/Documents/Ebay Click and Collect/Ebay MIP/MIP - UK - General Metadata.xlsx;Extended Properties=\"Excel 12.0;HDR=YES;\"";
        //Parameters for OleDB  
        using (OleDbConnection connection = new OleDbConnection(conn))
        {   //Opens the connection
            connection.Open();
            OleDbCommand command = new OleDbCommand("select Value from [Domestic Shipping Service$]", connection); //Searches the column Value from the Worksheet DSS

            using (OleDbDataReader dr = command.ExecuteReader())
            { //Reads in the data
                for (int x = 0; x < 23; x++)
                {

                    while (dr.Read())
                    {
                       var details = Convert.ToString(dr[0]);
                       test[x] = details;   
                    }
                }
                connection.Close();       
            }
        }
        return test;
    }
}

任何建议都会很棒。我还在学习很多关于C＃的知识，所以可能错过了一些完全明显的东西。如果是这种情况，请道歉。

Answer 1

您不创建实例。你只是在宣布考试; 您需要创建一个实例：

string[] test = new string[sizeOfTheArray];

但是，您不需要将test声明为私有字段。

顺便说一下：你的代码没有任何意义。您覆盖test[x]的次数与dr.Read()一样多true。

for (int x = 0; x < 23; x++)
{

    while (dr.Read())
    {
       var details = Convert.ToString(dr[0]);
       test[x] = details;   
    }
}

如果while循环运行大约10次，则会丢失9个值。您只是存储最后一个值。

如果您不期望这种行为，我建议您使用List<string>。

class OleDB   
{
    public string [] readexcel()
    {

        List<string> test = new List<string>();
        //Below runs the OleDB connections which essentially turns excel into an Access database.

        string conn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:/Users/nlongmore/Documents/Ebay Click and Collect/Ebay MIP/MIP - UK - General Metadata.xlsx;Extended Properties=\"Excel 12.0;HDR=YES;\"";
        //Parameters for OleDB  
        using (OleDbConnection connection = new OleDbConnection(conn))
        {   //Opens the connection
            connection.Open();
            OleDbCommand command = new OleDbCommand("select Value from [Domestic Shipping Service$]", connection); //Searches the column Value from the Worksheet DSS

            using (OleDbDataReader dr = command.ExecuteReader())
            { //Reads in the data
                for (int x = 0; x < 23; x++)
                {

                    while (dr.Read())
                    {
                       var details = Convert.ToString(dr[0]);
                       test.Add(details); 
                    }
                }
                connection.Close();       
            }
        }
        return test.ToArray();
    }
}

Answer 2

您没有在任何地方初始化数组，它的默认值为NULL。

string[] test = new string[10];

会准备你的数组包含10个元素。但是，这是不灵活的。更好地使用可变的东西，比如List.

using System.Collections.Generic;

List<string> test = new List<string>();

...

test.Add(details);

...

return test.ToArray();

顺便说一句：您的原始代码执行循环，然后循环数据读取器。因此，您将多次覆盖test[x]的内容。我怀疑这不是故意的。

Answer 3

你需要新的＆＃39;声明数组时

string[] test = new string[24];

Answer 4

您将test声明为成员变量，但未对其进行初始化，因此默认值为null。

在任何情况下，您都不希望返回成员变量，而是将其定义为局部变量。

class OleDB   
{

    public string [] readexcel()
    {
        string[] test = new string[23];
        //Below runs the OleDB connections which essentially turns excel into an Access database.

        string conn = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:/Users/nlongmore/Documents/Ebay Click and Collect/Ebay MIP/MIP - UK - General Metadata.xlsx;Extended Properties=\"Excel 12.0;HDR=YES;\"";
        //Parameters for OleDB  
        using (OleDbConnection connection = new OleDbConnection(conn))
        {   //Opens the connection
            connection.Open();
            OleDbCommand command = new OleDbCommand("select Value from [Domestic Shipping Service$]", connection); //Searches the column Value from the Worksheet DSS

            using (OleDbDataReader dr = command.ExecuteReader())
            { //Reads in the data
                for (int x = 0; x < 23; x++)
                {

                    while (dr.Read())
                    {
                       var details = Convert.ToString(dr[0]);
                       test[x] = details;   
                    }
                }
                connection.Close();       
            }
        }
        return test;
    }
}

访问数组时出现NullReferenceException

4 个答案: