如何用另一个 class 更改 static 字符串值？

Question

I have class DatabaseObjects which have public static string fields. I have a filename and ConnectionStringExcel in this class. Code is as-

class DatabaseObjects
{
    public static string filename = "";
    public static string ConnectionStringExcel = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" +
                                       filename + "; Extended Properties= 'Excel 12.0 Xml;HDR=YES;'";
}

I want to set the filename by the OpenDialogBox in frmStudents class on button Import Click event. And this filename given by user should be added to the ConnectionStringExcel (Which is in other class).

Code of frmStudents Class

 public partial class frmStudents : Form
{
private void btnImport_Click(object sender, EventArgs e)
    {
        OpenFileDialog openFileExcel = new OpenFileDialog();
        openFileExcel.Filter = "Excel Files | *.xlsx; *.xls; *.xlsm";
        openFileExcel.Title = "Select an Excel File";
        if (openFileExcel.ShowDialog() == DialogResult.Cancel || openFileExcel.FileName.Equals(""))
            return;
        DatabaseObjects.filename = openFileExcel.FileName;
        using(OleDbConnection connExcel = new OleDbConnection(DatabaseObjects.ConnectionStringExcel))
        {
            string queryExcel = "SELECT * FROM [Six$]";
            using (OleDbCommand commandExcel = new OleDbCommand(queryExcel,connExcel))
            {
                connExcel.Open();
            }
        }
    }
}

When the user select the file in OpenDialogBox , the filename string get the value correctly. But filename string value is not combining in ConnectionStringExcel . The Data Source value remains empty. How to resolve this?

If I remove static keyword from filename and ConnectionStringExcel , it is giving error that filename A field initializer cannot reference the non-static field . What now?

Answer 1

ConnectionStringExcel is initialized using filename but it won't track future changes.

You can convert ConnectionStringExcel into a readonly property with this getter

public static string ConnectionStringExcel => @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" +
                                       filename + "; Extended Properties= 'Excel 12.0 Xml;HDR=YES;'";

This will cause the string to be constructed with every call to ConnectionStringExcel

EDIT

If you're using an old version of the .net framework you can use

public static string ConnectionStringExcel 
{
   get 
   { 
      return @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" +
                                       filename + "; Extended Properties= 'Excel 12.0 Xml;HDR=YES;'"; 
   } 
}

Answer 2

You can create a function to build the GetConnectionStringExcel . Also, I recommend you to use the CheckFileExists in OpenFileDialog object, it will check if the selected file exist. Here is a code snip.

public static class DatabaseObjects
{
    public static string FileName;

    public static string GetConnectionStringExcel()
    {
        return GetConnectionStringExcel(FileName);
    }

    public static string GetConnectionStringExcel(string filename)
    {
        return @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" +
               filename + "; Extended Properties= 'Excel 12.0 Xml;HDR=YES;'";
    }
}

public partial class frmStudents : Form
{
    private void btnImport_Click(object sender, EventArgs e)
    {
        var openFileExcel = new OpenFileDialog
        {
            Filter = "Excel Files | *.xlsx; *.xls; *.xlsm",
            Title = "Select an Excel File",
            CheckFileExists = true
        };
        if (openFileExcel.ShowDialog() == DialogResult.Cancel)
            return;

        DatabaseObjects.FileName = openFileExcel.FileName;
        using (var connExcel = new OleDbConnection(DatabaseObjects.GetConnectionStringExcel()))
        {
            string queryExcel = "SELECT * FROM [Six$]";
            using (var commandExcel = new OleDbCommand(queryExcel, connExcel))
            {
                connExcel.Open();
            }
        }
    }
}